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Energetics of disordered and ordered rare earth oxide-stabilized bismuth oxide ionic conductors Tien B. Tranab and Alexandra Navrotsky*a Rare-earth stabilized bismuth oxides are known for their excellent ionic conductivity at intermediate temperatures. However, previous studies have shown that their conductivity deteriorates during extended heat treatments at 500–600 1C, although the fluorite phase is maintained. In this study, the enthalpies of formation of quenched and aged ytterbia- and dysprosia-stabilized bismuth oxides were measured using high-temperature oxide melt solution calorimetry in 3Na2O–4MoO3 solvent at 702 1C.
Received 28th October 2013, Accepted 9th December 2013
While a modest energy difference (2 to 3 kJ mol1) drives the kinetically slow aging transformation in
DOI: 10.1039/c3cp54553a
dysprosia-stabilized system. Although the small magnitude of the exothermic ordering energy suggests
the ytterbia-stabilized system at moderate dopant contents, no energetic driving force is detectable in the extensive short range ordering in both the quenched and aged samples, the anion configuration specific
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to the aged samples is nevertheless responsible for the significant decrease in conductivity.
Introduction Fluorite-structured bismuth oxide (BiO1.5) materials are excellent oxygen ion conductors in the intermediate temperature (IT, 700–800 1C) range. Since the fluorite structure is only stable from 732 1C to the melting point (825 1C) in pure bismuth oxide, phase stabilization has been accomplished by metal oxide doping, particularly with yttrium or rare earth (RE) oxides.1–3 Despite a large body of research, the application of bismuth oxide systems remains limited due to the high vapor pressure and reducibility of BiO1.5 under operating conditions,4 as well as some undesirable effects of doping on conductivity.5–7 Specifically, while stabilization in terms of persistence of the fluorite phase increases with increasing dopant content, ionic conductivity decreases. Another limiting issue in the use of stabilized bismuth oxides is the still somewhat ill-understood ‘‘aging’’ phenomenon. In stabilized bismuth oxides, low temperature phase transformations can cause a dramatic decrease in conductivity. However, it has also been observed that even without a phase transformation detectable by X-ray diffraction, conductivity may gradually decrease over a period of tens to hundreds of hours at temperatures around 500–600 1C.7 The rate at which conductivity decays during aging has been related to dopant cation radius and polarizability, as well as dopant content.7–9 In ErO1.5-stabilized
a
Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, CA 95616, USA. E-mail: [email protected] b Department of Mechanical Engineering, University of the Pacific, Stockton, CA 95211, USA
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BiO1.5, for example, the rate and the total degree of conductivity decay were ameliorated at high dopant contents.7 Jiang and Wachsman found that, in BiO1.5 stabilized by 25 mol% REO1.5 (RE = Dy, Ho, Er, Tm, Yb, and Y), initial conductivity as a function of temperature is nearly independent of the specific rare earth dopant.5–8 However, conductivity at 500 1C decayed more aggressively with decreasing cation radius. Taking s(0) as initial conductivity, and s(t) as conductivity at some time t, s(t)/s(0) after 80 hours at 500 1C was 0.8 for 25 mol% DyO1.5-stabilized BiO1.5 (DSB), and 0.1 for 25 mol% YbO1.5-stabilized BiO1.5 (YbSB). Dy3+ and Yb3+ were, respectively, the most and least polarizable dopant cations studied. To characterize the aging behavior, a time constant, t, was defined as: b t sðtÞ ¼ sð0Þ þ ðsð0Þ sðtÞÞ exp (1) t where b is a dimensionless parameter. It was seen that 25 DSB exhibits a time constant of 171 hours, which is 44 times larger than the time constant of 25YbSB at 500 1C, although the ionic radii of Dy3+ and Yb3+ differ by only 4%.7 Apparently, DSB is kinetically more stable against aging than YbSB. Neutron diffraction studies by Boyapati et al. have also shown that the fraction of anions displaced from the 8c to the 32f sites during aging was lowest in DSB, and highest in YbSB.10 Considering the previous findings, the objective of this study is to determine the effect of dopant type and concentration on the energetics of aging in REO1.5-stabilized bismuth oxide. We have already determined the formation enthalpy of quenched dysprosia-stabilized bismuth oxide, and computed the thermodynamic effect of this modest stabilization on bismuth oxide vapor pressure and reducibility.11 In the current
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work, we investigate the formation enthalpy of d-phase BiO1.5 stabilized by the oxide of a less polarizable rare earth cation—Yb3+. Additionally we study, for the first time, the enthalpy difference between quenched and extended-time annealed samples to investigate the energetics of the aging phenomenon in stabilized bismuth oxides.
Materials and methods Synthesis Ytterbia-stabilized bismuth oxide (YbSB, YbxBi1xO1.5) powders with x = 0.10 to 0.60 were synthesized by a co-precipitation method. Since bismuth nitrate does not dissolve well in water,12 appropriate amounts of Bi(NO3)35H2O (MP Biomedicals, LLC) and Yb(NO3)35H2O (Strem Chemicals, Inc.) were dissolved into sufficient HNO3 to achieve a total metal concentration of 0.06 mol L1. The nitrate solution was then added drop-wise into a 50 vol% solution of NH4OH to precipitate the metal hydroxide. The precipitation was completed at room temperature under magnetic stirring. Additional NH4OH was added until a pH Z 11 was achieved. The precipitates were then centrifuged and rinsed 3 times with deionized water by agitation, centrifugation, and decantation. They were then dried at 80 1C overnight to obtain a white precursor. Finally, the precursors were calcined at 500 1C for 3 hours, followed by 1 hour at 750 1C, and then quenched to room temperature in air to produce nanopowders. To avoid surface energy effects, the nanopowders were pelletized, placed into capped alumina crucibles, and sealed into silica ampoules containing excess BiO1.5 powder. The powders were then coarsened at 750 1C for three days, quenched, and then ground with an agate mortar and pestle. Dysprosia-stabilized bismuth oxide (DSB, DyxBi1xO1.5) with x = 0.11 to 0.45 was prepared by solid state reaction as described by Tran and Navrotsky.11 For aging, bulk powders were pelletized and sealed into silica ampoules as previously described. The ampoules were heated at 550 1C for 170 hours (B1 week). Characterization X-ray diffraction (XRD) was completed using a Bruker-AXS D8 Advance diffractometer (Bruker-AXS, Inc.). The diffractometer was operated at 40 kV and 40 mA using Cu-Ka radiation. Data were acquired at 20 to 801 2y using a step size of 0.0161, and a collection time of 2 s step1. The sample was rotated at 15 rpm during data collection. Lattice parameters and crystallite sizes were calculated by whole pattern fitting as implemented in MDI Jade 6.1 (Materials Data, Inc.). Elemental analysis was conducted by wavelength dispersive spectroscopy (WDS) using a CAMECA SX-100 electron microprobe (CAMECA) operating at 15 kV and 20 nA, with a beam size of 1 mm. Powder samples were sintered and polished prior to analysis. Bi12GeO20 (C.M. Taylor Co.) and YbPO4 (Smithsonian Microbeam Standards) were used as standards, and at least ten measurements were made for each sample. Sample homogeneity was evaluated by X-ray mapping and electron backscatter imaging.
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Calorimetry High temperature oxide melt drop solution calorimetry was performed using a house-built isoperibol Tian-Calvet microcalorimeter as described by Navrotsky.13 Prior to calorimetry, powders previously stored in a desiccator were dried at 725 1C for one hour, and quenched to room temperature in air. For each measurement, two pellets weighing a total of B15 mg were simultaneously dropped into molten sodium molybdate (3Na2O–4MoO3) solvent at 702 1C. The calorimeter was calibrated using the heat content of high purity a-Al2O3. Oxygen gas was flushed through the calorimeter assembly at 55 mL min1 to maintain a constant atmosphere, and bubbled through the solvent at 7 mL min1 to aid dissolution and to prevent local solvent saturation. Heats of drop solution of the end-members have been previously published.11,14 Differential scanning calorimetry (DSC) was conducted in a Setaram LabSys Evo thermoanalyzer (Setaram, Cailure, France) using lidded platinum crucibles. The instrument was operated in TG/DSC mode (thermogravimetry/differential scanning calorimetry). Aged stabilized bismuth oxide samples (B50 mg, pelletized) were heated from 50 to 850 1C at 10 1C min1 in flowing oxygen (40 mL min1) for two cycles. The heat flow sensitivity was calibrated using high purity a-Al2O3, which was heat treated at 1500 1C overnight prior to the experiment. Temperature calibration was performed by melting a gold standard in an alumina crucible.
Results Quenched materials Compositional analysis of the ytterbia-stabilized bismuth oxide (YbSB) powders was completed by wavelength dispersive spectroscopy. Analysis of the DSB powders was published previously.11 The YbxBi1xO1.5 samples contained x = 0.10 to 0.60, and will be referred to by their dopant content in the form of 16YbSB for x = 0.16, 21YbSB for x = 0.21, etc. No impurity elements greater than 1% were detected, while all single-phase samples (x = 0.16 to 0.35) were determined to be homogeneous by electron backscatter imaging and X-ray dot mapping. By whole pattern fitting of the powder XRD data, the lattice parameters of the single-phase samples approximately follow Vegard’s law, consistent with the ionic radii of Bi3+ (1.11 Å) and Yb3+ (0.98 Å)15 (Fig. 1). At the lowest dopant levels, 10YbSB and 16YbSB were found to contain a secondary phase corresponding to C-type g-BiO1.5 (PDF #74-1375). At the highest dopant levels (x Z 0.44), a secondary phase corresponding to a C-type YbO1.5-rich solid solution (PDF #770458) was found in addition to d-BiO1.5. Similar to data for the DSB system, the drop solution enthalpy (DHds) decreases with increasing dopant content in the YbSB system (Fig. 2). The heat of formation of YbSB from the room temperature phases of the oxide end-members (DHf,ox) can be calculated using the thermochemical cycle in Table 1. To calculate the heat of formation from the cubic fluorite end-members (DHf,c), the thermochemical cycle is modified using the heat of transformation (DHt) of the end-members from their respective room temperature phases to the fluorite phase (Table 2). While DHt(a - d) was directly measured for BiO1.5,11 DHt(C - d) for YbO1.5 was calculated
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Fig. 1 (a) X-ray diffraction patterns of quenched YbSB at different dopant levels. (b) Lattice constants of quenched YbSB and DSB11 as determined by whole pattern fitting with trend-lines representing Vegard’s behavior.
Table 2 Experimental heats of drop solution of the room temperature end-member phases, heats of transformation from monoclinic BiO1.5 and C-type YbO1.5 to the cubic fluorite phase, and calculated heats of drop solution of the fluorite phases. Number of drop solution calorimetry measurements are presented in parentheses
BiO1.5 YbO1.5
DHds (RT phase) (kJ mol1)
DHt (kJ mol1)
DHds (fluorite phase) (kJ mol1)
5.21 0.53 (14)11 49.23 1.59 (9)14
16.39 0.4711 25.2 4.316
11.18 0.71 74.43 4.58
as constraints. For the YbSB system, O is 68.45 2.14 kJ mol1, compared to 72.99 1.43 kJ mol1 for the DSB system. Moreover, the enthalpy of formation from the oxide end-members (DHf,ox) can be expressed by, Fig. 2 The heats of drop solution of YbSB show deviation from ideal solution behavior.
DHf,ox(Bi1xRExO1.5) = (1 x)DHt(BiO1.5) + xDHt(REO1.5) + DHmix (3)
by Simoncic and Navrotsky by extrapolating the enthalpies of formation for the ZrO2–YbO1.5 and HfO2–YbO1.5 solid solution systems.16 Directly measuring DHt(C - d) for YbO1.5 is difficult since the transformation occurs at ultra-high temperatures (T > 2000 1C).16 The heats of formation, DHf,ox and DHf,c, are reported in Fig. 3 and Table 3. The heat of drop solution, DHds, deviates positively from linear behavior, indicating non-ideal solution behavior with energetic stabilization. A regular solution model may be applied, where:
Of course, DHmix is the heat of formation from the fluorite end-members, DHf,c, which is negative for both YbSB and DSB. Although the interaction parameters are not greatly different for DyO1.5 and YbO1.5 doping, it should be noted that DHf,ox never becomes exothermic for YbSB in the dopant range investigated, while it does for DSB at high dopant levels (x > 0.30). Thus, quenched YbSB is energetically unstable but entropy stabilized throughout its composition range and is expected to be thermodynamically unstable at low temperature.
DHmix = Ox(1 x)
(2)
The interaction parameter, O, may be determined from a quadratic fit of DHds, using DHds of the fluorite end-members
Aged materials Both YbSB and DSB samples were heat treated at 550 1C for 170 hours in sealed vials. At a dopant content of x = 0.22, YbSB
Table 1 Thermochemical cycle for determining YbSB heats of formation from the room temperature end-member phases. Here, c = cubic fluorite phase, C = C-type phase, and m = monoclinic phase
Reaction
DH
Bi1xYbxO1.5 (c, 25 1C) - xYbO1.5 (soln, 700 1C) + (1 x)BiO1.5 (soln, 700 1C) YbO1.5 (C, 25 1C) - YbO1.5 (soln, 700 1C) BiO1.5 (m, 25 1C) - BiO1.5 (soln, 700 1C) xYbO1.5 (C, 25 1C) + (1 x)BiO1.5 (m, 25 1C) - Bi1xYbxO1.5 (c, 25 1C)
DHds(1) DHds(2)14 DHds(3)11 DHf,ox(4)
DHf,ox(4) = DHds(1) + xDHds(2) + (1 x)DHds(3)
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Fig. 3 The heats of formation of YbSB from (a) the room temperature oxide end-members and (b) the fluorite end-members show regular solution behavior and stabilization by the dopant, YbO1.5.
Table 3 Heats of drop solution (DHds), heats of formation from the room temperature oxide end-members (DHf,ox), and heats of formation from the fluorite end-members (DHf,c) determined for YbSB by drop solution calorimetry in 3Na2O–4MoO3 solvent at 702 1Ca
x
DHds (kJ mol1)
DHf,ox (kJ mol1)
0.22 0.26 0.32 0.35 0.38
14.64 13.42 17.25 16.10 19.89
7.65 4.26 4.99 2.43 4.46
0.32 0.45 0.57 0.31 0.57
0.6 0.73 0.85 0.72 0.89
DHf,c (kJ mol1) 10.71 14.46 14.23 17.02 15.27
1.21 1.39 1.65 1.69 1.88
a
Compositions were determined by WDS, errors are reported as two standard deviations of the mean, and 8 measurements were taken for each sample.
undergoes a phase transformation from the quenched d-phase to a YbO1.5-doped tetragonal b-phase, as evidenced by XRD. Refined unit cell parameters, are a = 7.687 0.019 Å and c = 5.507 0.011 Å for the b-phase. A minor amount of a secondary phase was also present. Although the very low intensity and broadness of its peaks made certain phase identification difficult, this secondary phase is likely bcc g-BiO1.5. Transformation to the b-phase suggests that doping with 22 mol% ytterbia is not sufficient for stabilizing the d-phase for low-temperature (T o 700 1C) solid oxide fuel cell applications. At YbO1.5 dopant contents of x = 0.26–0.38, the X-ray diffraction patterns were unchanged before and after aging (Fig. 4). The transformation enthalpy (DHt) between the disordered phase (quenched samples) and the more ordered phase (aged samples) may be determined directly from the heats of drop solution (DHds). In order to avoid surface energy contributions to DHds, the quenched samples were coarsened at synthesis temperatures prior to final quenching. Back-scattered electron imaging shows that the particle sizes of the quenched samples was approximately 50–150 mm. For particles of this scale, it is expected that the specific surface area of the samples is sufficiently low for surface energy contributions to be considered negligible. Thus, DHt was not significantly affected by any further particle coarsening that may have occurred during the low-temperature aging. With DHt between 2 and 3 kJ mol1 for x = 0.26–0.32, the vacancy ordering transformation is energetically favorable, although only slightly,
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Fig. 4 XRD patterns for 32YbSB after quenching from 750 1C and aging at 550 1C for 170 hours are identical.
at moderate dopant concentrations. DSC shows that the transformation occurs between 700 and 800 1C for these samples (Fig. 5). However, at x = 0.35 and 0.38, only very small, broad humps are detectable in DSC, in agreement with the small enthalpy differences determined by drop solution calorimetry (Table 4). In the DSB system, at x = 0.11–0.23, the samples undergo phase transformations during aging. The resulting phases are summarized in Table 4. At the very lowest dopant content investigated, 11DSB could be quenched from synthesis temperatures to retain a fluorite structure. After aging at 550 1C for B1 week, the sample contained mainly a tetragonal b-phase coexisting with a minor amount of a rhombohedral e-phase. At x = 0.16–0.23, the DSB samples contained mixtures of the e-phase and d-phase, with increasing amounts of the d-phase present with increasing dopant content. DSC of these samples demonstrates that the e - d transformation occurs between 720 and 750 1C, depending on dopant content. Notably, the reverse transformation is not observed during cooling at 10 1C min1. For x = 0.30–0.45, XRD did not reveal any obvious structural changes between the quenched and aged samples. By drop solution calorimetry at 700 1C and DSC experiments up
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Fig. 5 DSC traces of aged (a) 32YbSB and (b) 30DSB during heating and cooling at a rate of 10 1C min1. A heat effect associated with the order–disorder transformation is observed in 32YbSB; however, the reverse transition is not observed during cooling due to its slow kinetics.
Table 4 Phases of YbSB and DSB after quenching from synthesis temperatures, and after aging at 550 1C for 170 hours. Heats of transformation, DHt, were determined from the difference of DHds of the quenched and aged specimens, respectively, in 3Na2O–4MoO3 solvent at 702 1C
YbSB
DSB 1
x
Quenched
Aged
DHt (kJ mol )
x
Quenched
Aged
DHt (kJ mol1)
0.22 0.26 0.32 0.35 0.38
d d d d d
d+b d d d d
— 2.37 2.79 0.02 0.52
0.11 0.16 0.23 0.30 0.45
d d d d d
b+e e+d d+e d d
— — — 0.97 0.98 0.23 1.06
0.71 0.69 0.46 0.72
to 875 1C, no changes in enthalpy, either from the order– disorder transformation, or from the e - d transformation, were observed for 30DSB and 45DSB (Table 4).
Discussion In the quenched YbxBi1xO1.5 solid solution range investigated, at x r 0.16, a C-type g-BiO1.5 phase was present in addition to the primary d-phase. On the other side of the fluorite stability range, at x Z 0.44, a secondary phase corresponding to a C-type YbO1.5 solid solution was found. Interestingly, these results do not agree with previous findings. In the YbSB system, Chen et al. reported that d-BiO1.5 persists from x = 0.15 to 0.35 after furnace cooling from synthesis temperatures.17 However, Zargarova et al.18 published an equilibrium phase diagram in which d-BiO1.5 spans x = 0.25–0.60. In each of these studies, the secondary phases found at dopant levels below the d-phase stability range differ, especially since the formation of the metastable tetragonal b- and body-centered cubic (I23) g-polymorphs in slightly doped BiO1.5 is highly dependent on cooling history.19 The formation of these metastable phases is described by Levin and Roth20 in detail. Regardless of these differences, in the current study the solubility limit of quenched YbSB fluorite ranges from x = 0.22 to 0.38, compared to x = 0.11 to 0.50 for quenched DSB.11 This is not surprising since Keller et al. have shown that solubility limits in fluorite systems increase with decreasing size mismatch between the host and dopant cations.21 Unlike other fluorite-structured electrolytes, 1/4 of the anion sites in bismuth oxide are inherently vacant. Gaining insight
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into the structure of stabilized bismuth oxides, as well as the structure of pure d-BiO1.5, proves to be challenging. Several models for the anion sublattice have been proposed, including the Gattow model which assumes partial 3/4 occupancy of each anion site,22 ´n model wherein vacancies are ordered along h111i23 and the Sille the very disordered Harwig model wherein anions are displaced from the 8c sites at (1/4, 1/4, 1/4) to (1/4 d, 1/4 d, 1/4 d), resulting in a 3/16 occupancy of the 32f sites.24 Jacobs and Mac ´naill analyzed well-known theories and experimental results, Do concluding that the d-phase is best-described by a combination of long-range disorder, short-range order, and a tendency for vacancies to favor sites along h111i.23 Another complication is positional disordering, or the displacement of anions away from the ideal 8c sites, as affected by the presence of dopant cations.25 In YbSB, as in DSB, both DHmix and O are negative. This energetic stabilization suggests the formation of cation– vacancy associates. By neutron diffraction, it has been shown that oxygen anions occupy the 8c and 32f sites in pure bismuth oxide,25 but the 48i sites are also occupied in YO1.5- and REO1.5stabilized bismuth oxides.10,25 Anions in the 48i sites, which are associated with doping, may be considered Frenkel interstitials. While these defects increase the vacancy concentration in the 8c sites per formula unit, in the case of 25% yttria-doped bismuth oxide, the vacancies are trapped in the vicinity of the yttrium dopant atoms.25 Thus, the concentration of mobile charge carriers decreases, and conductivity decreases. Similar to behavior in DSB, with increasing dopant content, the increasing deviation from ideal solution behavior mirrors the decrease in ionic conductivity in YbSB,7,11 and is at least in part due to the formation of cation–vacancy associates.
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Positional disordering becomes even more important in considering the ‘‘aging’’ phenomenon. In several studies, REO1.5-stabilized bismuth oxides were heat treated for hundreds of hours at T r 600 1C, below the temperature at which the cubicto-rhombohedral transformation is expected to occur.5,7,26,27 Aging, which is believed to be due to an order–disorder transformation of the anion sublattice, manifests as a decay in conductivity, and is quickly reversed during heating. However, our thermodynamic data suggest that the term ‘‘order–disorder’’ is somewhat misleading, since significant ordering already exists in the quenched phase. Although not distinguishable from the quenched d-structure by XRD due to weak scattering from oxygen anions, the transformation has been confirmed by TEM5,8 and neutron diffraction.8 The latter showed that only 1 out of 3 anions occupy the 8c tetrahedral sites in unaged 20 mol% erbia-stabilized bismuth oxide (20ESB), with the remaining anions distributed among the 32f and 48i sites. In contrast, after aging, almost no anions occupy the 8c sites, 2.61 out of 3 anions occupy the 32f sites, and the remaining anions occupy the 48i sites. Vacancy ordering on the 8c sites inhibits oxygen ion transport, which leads to a decay in conductivity.7 In the present study, for samples aged at 550 1C for 170 hours (B1 week), YbSB with x = 0.26–0.38 and DSB samples with x = 0.30–0.45 maintained the d-phase (Table 4). All calorimetric results are interpreted under the assumption that the order– disorder transformation in these materials represents equilibrium after the given heat treatment. This is a reasonable assumption based on previous studies.5–7 Using neutron diffraction, Boyapati et al. showed that aging is characterized by the displacement of anions from the 8c to the 32f sites, with little change in the occupancy of 48i sites.10 Of bismuth oxides stabilized by Y3+, Dy3+, Ho3+, E3+, and Yb3+, the greatest number of anion displacements to the 32f sites was observed in YbSB, which exhibits the most dramatic aging behavior, and least in DSB, which shows very little conductivity decay during aging. As a reflection of this difference in aging behavior, a definitive heat effect was measured for the order–disorder transformation in YbSB (x = 0.26–0.32) by DSC and drop solution calorimetry; however, no clear transformations were observed for DSB (x = 0.30–0.45). Also noteworthy is that, when fitted with a regular solution model, the interaction parameter (O) of the aged YbSB samples is more negative than that of the quenched samples (76.49 2.07 kJ mol1 vs. 68.45 2.14 kJ mol1, respectively), which suggests that the former is more ordered. It is not surprising that no evidence of aging was found in DSB with x = 0.30–0.45. Similar behavior has been observed in erbia stabilized bismuth oxide (ESB) at high dopant levels.9 By differential thermal analysis (DTA), it was found that aged ESB exhibits either an order–disorder transformation at T o 600 1C, or a e - d transformation between 600 and 700 1C, depending on dopant content and heat treatment.27 However, at dopant contents greater than x = 0.30, neither transformation is observed. In contrast, DSB exhibits a e - d transformation between 720 and 750 1C at x = 0.16–0.23, and no detectable transformations at x Z 0.30, when DHf,ox becomes exothermic.
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25DSB was found to age somewhat at 500 1C,5–8 however, it is possible that this decrease in conductivity was due to the formation of a small amount of the e-phase, or because the transformation is more extensive at 500 1C compared to the 550 1C aging temperature used in this study.7 In considering the effects of different dopant cations on bismuth oxide, we should note that the interaction parameter for the quenched DSB system is more negative than that for quenched YbSB. Thus the DSB system is more stabilized with respect to its room temperature oxides than YbSB. This finding reflects a difference in the degree of cation–vacancy (or cation– anion) association in the quenched phase, which is partially due to the greater polarizability of Dy3+ in comparison to Yb3+. Conductivity studies have shown that doping bismuth oxides with highly polarizable lanthanide cations results in greater stabilization against aging.27 Notably, the time constant for conductivity decay increases with increasing dopant polarizability. From calorimetry, the thermodynamic driving force for aging is 2 to 3 kJ mol1 in YbSB, but nearly negligible in DSB at relevant dopant levels for which the d-phase is preserved during aging. Although the heat effects measured by drop solution calorimetry were accompanied by large errors relative to the total magnitude of the heat effects, these errors provide acceptable uncertainties in the heat of transformation (see tabulated values in Table 4). Furthermore, the drop solution calorimetry results were supported by DSC traces, which showed clear heat effects for aged YbSB at low dopant contents upon heating, and no transformations in DSB. In summary, the negative interaction parameters of the quenched YbSB and DSB systems and the low transformation enthalpies for aging suggest that the aging phenomenon can be described by additional ordering on an already significantly ordered oxygen sublattice. Unlike other fluorites that see a decrease in conductivity with increased doping due to a dramatic increase in clustering,28,29 the total degree of increased ordering in aged stabilized bismuth oxides cannot completely explain the large extent of conductivity decay during aging. Rather, the thermodynamic data support that it is the occupation of specific anion sites, namely the exclusive occupation of the 32f and 48i sites, that is responsible for this conductivity decay, as was previously suggested by other investigators.7,8
Conclusion The thermochemistry of aging in rare earth oxide-stabilized bismuth oxide electrolytes was studied by calorimetry for the first time. Ytterbia- and dysprosia-stabilized systems were selected due to their contrasting aging behavior and the differing polarizabilities of the dopant cations. In the quenched materials, DSB is more stabilized with respect to its room temperature oxide end-members than YbSB. The deviation from ideal solution behavior in both systems is attributed to cation–vacancy association and positional disordering on the anion sublattice. This is supported by the heat of formation,
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which becomes more exothermic in parallel with a decrease in ionic conductivity with increasing dopant content. After aging at 550 1C for 170 hours, a small order–disorder enthalpy (2 to 3 kJ mol1) was measurable for YbSB at moderate dopant contents (x = 0.26–0.32) by DSC and oxide melt solution calorimetry. However, no obvious order–disorder transformation was observable in DSB samples for which the d-phase is preserved during the low temperature heat treatment. Taken together with previous findings by other authors, these results suggest that the tendency for aging decreases with increasing dopant content, and with an increase in the polarizability of the dopant cation.
Acknowledgements This work was performed with funding from the U.S. Department of Energy (Grant No. DE-FG02-03ER46053).
References 1 A. M. Azad, S. Larose and S. A. Akbar, J. Mater. Sci., 1994, 29, 4135–4151. ¨fe and F. Aldinger, 2 N. M. Sammes, G. A. Tompsett, H. Na J. Eur. Ceram. Soc., 1999, 19, 1801–1826. 3 L. Malavasi, C. A. J. Fisher and M. S. Islam, Chem. Soc. Rev., 2010, 39, 4370–4387. ¨fer, U. Guth, W. Go ¨pel and 4 P. Shuk, H.-D. Wiemho M. Greenblatt, Solid State Ionics, 1996, 89, 179–196. 5 N. Jiang, R. M. Buchanan, F. E. G. Henn, A. F. Marshall, D. A. Stevenson and E. D. Wachsman, Mater. Res. Bull., 1994, 29, 247–254. 6 N. Jiang, R. M. Buchanan, D. A. Stevenson, W. D. Nix, J.-Z. Li and J.-L. Yang, Mater. Lett., 1995, 22, 215–219. 7 N. Jiang and E. D. Wachsman, J. Am. Ceram. Soc., 1999, 82, 3057–3064. 8 E. D. Wachsman, S. Boyapati, M. J. Kaufman and N. Jiang, in Solid-State Ionic Devices: Proceedings of the International Symposium, ed. E. D. Wachsman, J. R. Akridge, M. Liu and N. Yamazoe, The Electrochemical Society, Inc., New Jersey, USA, 1999, pp. 42–81.
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9 E. D. Wachsman, G. R. Ball, N. Jiang and D. A. Stevenson, Solid State Ionics, 1992, 52, 213–218. 10 S. Boyapati, E. D. Wachsman and B. C. Chakoumakos, Solid State Ionics, 2001, 138, 293–304. 11 T. B. Tran and A. Navrotsky, Chem. Mater., 2012, 24, 4185–4191. 12 Y. Zhang, Z. Fang, M. Muhammed, K. V. Rao, V. Skumryev, H. Medelius and J. L. Costa, Phys. C, 1989, 157, 108–114. 13 A. Navrotsky, Phys. Chem. Miner., 1997, 24, 222–241. 14 S. V. Ushakov, K. B. Helean, A. Navrotsky and L. A. Boatner, J. Mater. Res., 2001, 16, 2623–2633. 15 R. D. Shannon and C. T. Prewitt, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1969, 25, 925–946. 16 P. Simoncic and A. Navrotsky, J. Am. Ceram. Soc., 2007, 90, 2143–2150. 17 X. L. Chen, F. F. Zhang, Y. M. Shen, J. K. Liang, W. H. Tang and Q. Y. Tu, J. Solid State Chem., 1998, 139, 398–403. 18 M. I. Zargarova, N. A. Akhmedova, E. S. Kuli-zade and N. M. Mustafaev, Zh. Neorg. Khim., 1995, 40, 1389–1391. 19 R. J. Betsch and W. B. White, Spectrochim. Acta, Part A, 1978, 34, 505–514. 20 E. M. Levin and R. S. Roth, J. Res. Natl. Bur. Stand., Sect. A, 1964, 68A, 189–195. 21 C. Keller, U. Berndt, H. Engerer and L. Leitner, J. Solid State Chem., 1972, 4, 453–465. 22 V. G. Gattow and H. Schroder, Z. Anorg. Allg. Chem., 1962, 318, 176–189. ´naill, Solid State Ionics, 23 P. W. M. Jacobs and D. A. Mac Do 1987, 23, 279–293. 24 H. A. Harwig, Z. Anorg. Allg. Chem., 1978, 444, 151–166. 25 I. Abrahams, X. Liu, S. Hull, S. T. Norberg, F. Krok, A. Kozanecka-Szmigiel, M. S. Islam and S. J. Stokes, Chem. Mater., 2010, 22, 4435–4445. 26 E. D. Wachsman, S. Boyapati, M. J. Kaufman and N. Jiang, J. Am. Ceram. Soc., 2000, 83, 1964–1968. 27 E. D. Wachsman, S. Boyapati and N. Jiang, Ionics, 2001, 7, 1–6. 28 H. J. Avila-Paredes, T. Shvareva, W. Chen, A. Navrotsky and S. Kim, Phys. Chem. Chem. Phys., 2009, 11, 8580–8585. 29 M. Aizenshtein, T. Y. Shvareva and A. Navrotsky, J. Am. Ceram. Soc., 2010, 93, 4142–4147.
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Energetics of disordered and ordered rare earth oxide-stabilized bismuth oxide ionic conductors Tien B. Tranab and Alexandra Navrotsky*a Rare-earth stabilized bismuth oxides are known for their excellent ionic conductivity at intermediate temperatures. However, previous studies have shown that their conductivity deteriorates during extended heat treatments at 500–600 1C, although the fluorite phase is maintained. In this study, the enthalpies of formation of quenched and aged ytterbia- and dysprosia-stabilized bismuth oxides were measured using high-temperature oxide melt solution calorimetry in 3Na2O–4MoO3 solvent at 702 1C.
Received 28th October 2013, Accepted 9th December 2013
While a modest energy difference (2 to 3 kJ mol1) drives the kinetically slow aging transformation in
DOI: 10.1039/c3cp54553a
dysprosia-stabilized system. Although the small magnitude of the exothermic ordering energy suggests
the ytterbia-stabilized system at moderate dopant contents, no energetic driving force is detectable in the extensive short range ordering in both the quenched and aged samples, the anion configuration specific
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to the aged samples is nevertheless responsible for the significant decrease in conductivity.
Introduction Fluorite-structured bismuth oxide (BiO1.5) materials are excellent oxygen ion conductors in the intermediate temperature (IT, 700–800 1C) range. Since the fluorite structure is only stable from 732 1C to the melting point (825 1C) in pure bismuth oxide, phase stabilization has been accomplished by metal oxide doping, particularly with yttrium or rare earth (RE) oxides.1–3 Despite a large body of research, the application of bismuth oxide systems remains limited due to the high vapor pressure and reducibility of BiO1.5 under operating conditions,4 as well as some undesirable effects of doping on conductivity.5–7 Specifically, while stabilization in terms of persistence of the fluorite phase increases with increasing dopant content, ionic conductivity decreases. Another limiting issue in the use of stabilized bismuth oxides is the still somewhat ill-understood ‘‘aging’’ phenomenon. In stabilized bismuth oxides, low temperature phase transformations can cause a dramatic decrease in conductivity. However, it has also been observed that even without a phase transformation detectable by X-ray diffraction, conductivity may gradually decrease over a period of tens to hundreds of hours at temperatures around 500–600 1C.7 The rate at which conductivity decays during aging has been related to dopant cation radius and polarizability, as well as dopant content.7–9 In ErO1.5-stabilized
a
Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, CA 95616, USA. E-mail: [email protected] b Department of Mechanical Engineering, University of the Pacific, Stockton, CA 95211, USA
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BiO1.5, for example, the rate and the total degree of conductivity decay were ameliorated at high dopant contents.7 Jiang and Wachsman found that, in BiO1.5 stabilized by 25 mol% REO1.5 (RE = Dy, Ho, Er, Tm, Yb, and Y), initial conductivity as a function of temperature is nearly independent of the specific rare earth dopant.5–8 However, conductivity at 500 1C decayed more aggressively with decreasing cation radius. Taking s(0) as initial conductivity, and s(t) as conductivity at some time t, s(t)/s(0) after 80 hours at 500 1C was 0.8 for 25 mol% DyO1.5-stabilized BiO1.5 (DSB), and 0.1 for 25 mol% YbO1.5-stabilized BiO1.5 (YbSB). Dy3+ and Yb3+ were, respectively, the most and least polarizable dopant cations studied. To characterize the aging behavior, a time constant, t, was defined as: b t sðtÞ ¼ sð0Þ þ ðsð0Þ sðtÞÞ exp (1) t where b is a dimensionless parameter. It was seen that 25 DSB exhibits a time constant of 171 hours, which is 44 times larger than the time constant of 25YbSB at 500 1C, although the ionic radii of Dy3+ and Yb3+ differ by only 4%.7 Apparently, DSB is kinetically more stable against aging than YbSB. Neutron diffraction studies by Boyapati et al. have also shown that the fraction of anions displaced from the 8c to the 32f sites during aging was lowest in DSB, and highest in YbSB.10 Considering the previous findings, the objective of this study is to determine the effect of dopant type and concentration on the energetics of aging in REO1.5-stabilized bismuth oxide. We have already determined the formation enthalpy of quenched dysprosia-stabilized bismuth oxide, and computed the thermodynamic effect of this modest stabilization on bismuth oxide vapor pressure and reducibility.11 In the current
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work, we investigate the formation enthalpy of d-phase BiO1.5 stabilized by the oxide of a less polarizable rare earth cation—Yb3+. Additionally we study, for the first time, the enthalpy difference between quenched and extended-time annealed samples to investigate the energetics of the aging phenomenon in stabilized bismuth oxides.
Materials and methods Synthesis Ytterbia-stabilized bismuth oxide (YbSB, YbxBi1xO1.5) powders with x = 0.10 to 0.60 were synthesized by a co-precipitation method. Since bismuth nitrate does not dissolve well in water,12 appropriate amounts of Bi(NO3)35H2O (MP Biomedicals, LLC) and Yb(NO3)35H2O (Strem Chemicals, Inc.) were dissolved into sufficient HNO3 to achieve a total metal concentration of 0.06 mol L1. The nitrate solution was then added drop-wise into a 50 vol% solution of NH4OH to precipitate the metal hydroxide. The precipitation was completed at room temperature under magnetic stirring. Additional NH4OH was added until a pH Z 11 was achieved. The precipitates were then centrifuged and rinsed 3 times with deionized water by agitation, centrifugation, and decantation. They were then dried at 80 1C overnight to obtain a white precursor. Finally, the precursors were calcined at 500 1C for 3 hours, followed by 1 hour at 750 1C, and then quenched to room temperature in air to produce nanopowders. To avoid surface energy effects, the nanopowders were pelletized, placed into capped alumina crucibles, and sealed into silica ampoules containing excess BiO1.5 powder. The powders were then coarsened at 750 1C for three days, quenched, and then ground with an agate mortar and pestle. Dysprosia-stabilized bismuth oxide (DSB, DyxBi1xO1.5) with x = 0.11 to 0.45 was prepared by solid state reaction as described by Tran and Navrotsky.11 For aging, bulk powders were pelletized and sealed into silica ampoules as previously described. The ampoules were heated at 550 1C for 170 hours (B1 week). Characterization X-ray diffraction (XRD) was completed using a Bruker-AXS D8 Advance diffractometer (Bruker-AXS, Inc.). The diffractometer was operated at 40 kV and 40 mA using Cu-Ka radiation. Data were acquired at 20 to 801 2y using a step size of 0.0161, and a collection time of 2 s step1. The sample was rotated at 15 rpm during data collection. Lattice parameters and crystallite sizes were calculated by whole pattern fitting as implemented in MDI Jade 6.1 (Materials Data, Inc.). Elemental analysis was conducted by wavelength dispersive spectroscopy (WDS) using a CAMECA SX-100 electron microprobe (CAMECA) operating at 15 kV and 20 nA, with a beam size of 1 mm. Powder samples were sintered and polished prior to analysis. Bi12GeO20 (C.M. Taylor Co.) and YbPO4 (Smithsonian Microbeam Standards) were used as standards, and at least ten measurements were made for each sample. Sample homogeneity was evaluated by X-ray mapping and electron backscatter imaging.
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Calorimetry High temperature oxide melt drop solution calorimetry was performed using a house-built isoperibol Tian-Calvet microcalorimeter as described by Navrotsky.13 Prior to calorimetry, powders previously stored in a desiccator were dried at 725 1C for one hour, and quenched to room temperature in air. For each measurement, two pellets weighing a total of B15 mg were simultaneously dropped into molten sodium molybdate (3Na2O–4MoO3) solvent at 702 1C. The calorimeter was calibrated using the heat content of high purity a-Al2O3. Oxygen gas was flushed through the calorimeter assembly at 55 mL min1 to maintain a constant atmosphere, and bubbled through the solvent at 7 mL min1 to aid dissolution and to prevent local solvent saturation. Heats of drop solution of the end-members have been previously published.11,14 Differential scanning calorimetry (DSC) was conducted in a Setaram LabSys Evo thermoanalyzer (Setaram, Cailure, France) using lidded platinum crucibles. The instrument was operated in TG/DSC mode (thermogravimetry/differential scanning calorimetry). Aged stabilized bismuth oxide samples (B50 mg, pelletized) were heated from 50 to 850 1C at 10 1C min1 in flowing oxygen (40 mL min1) for two cycles. The heat flow sensitivity was calibrated using high purity a-Al2O3, which was heat treated at 1500 1C overnight prior to the experiment. Temperature calibration was performed by melting a gold standard in an alumina crucible.
Results Quenched materials Compositional analysis of the ytterbia-stabilized bismuth oxide (YbSB) powders was completed by wavelength dispersive spectroscopy. Analysis of the DSB powders was published previously.11 The YbxBi1xO1.5 samples contained x = 0.10 to 0.60, and will be referred to by their dopant content in the form of 16YbSB for x = 0.16, 21YbSB for x = 0.21, etc. No impurity elements greater than 1% were detected, while all single-phase samples (x = 0.16 to 0.35) were determined to be homogeneous by electron backscatter imaging and X-ray dot mapping. By whole pattern fitting of the powder XRD data, the lattice parameters of the single-phase samples approximately follow Vegard’s law, consistent with the ionic radii of Bi3+ (1.11 Å) and Yb3+ (0.98 Å)15 (Fig. 1). At the lowest dopant levels, 10YbSB and 16YbSB were found to contain a secondary phase corresponding to C-type g-BiO1.5 (PDF #74-1375). At the highest dopant levels (x Z 0.44), a secondary phase corresponding to a C-type YbO1.5-rich solid solution (PDF #770458) was found in addition to d-BiO1.5. Similar to data for the DSB system, the drop solution enthalpy (DHds) decreases with increasing dopant content in the YbSB system (Fig. 2). The heat of formation of YbSB from the room temperature phases of the oxide end-members (DHf,ox) can be calculated using the thermochemical cycle in Table 1. To calculate the heat of formation from the cubic fluorite end-members (DHf,c), the thermochemical cycle is modified using the heat of transformation (DHt) of the end-members from their respective room temperature phases to the fluorite phase (Table 2). While DHt(a - d) was directly measured for BiO1.5,11 DHt(C - d) for YbO1.5 was calculated
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Fig. 1 (a) X-ray diffraction patterns of quenched YbSB at different dopant levels. (b) Lattice constants of quenched YbSB and DSB11 as determined by whole pattern fitting with trend-lines representing Vegard’s behavior.
Table 2 Experimental heats of drop solution of the room temperature end-member phases, heats of transformation from monoclinic BiO1.5 and C-type YbO1.5 to the cubic fluorite phase, and calculated heats of drop solution of the fluorite phases. Number of drop solution calorimetry measurements are presented in parentheses
BiO1.5 YbO1.5
DHds (RT phase) (kJ mol1)
DHt (kJ mol1)
DHds (fluorite phase) (kJ mol1)
5.21 0.53 (14)11 49.23 1.59 (9)14
16.39 0.4711 25.2 4.316
11.18 0.71 74.43 4.58
as constraints. For the YbSB system, O is 68.45 2.14 kJ mol1, compared to 72.99 1.43 kJ mol1 for the DSB system. Moreover, the enthalpy of formation from the oxide end-members (DHf,ox) can be expressed by, Fig. 2 The heats of drop solution of YbSB show deviation from ideal solution behavior.
DHf,ox(Bi1xRExO1.5) = (1 x)DHt(BiO1.5) + xDHt(REO1.5) + DHmix (3)
by Simoncic and Navrotsky by extrapolating the enthalpies of formation for the ZrO2–YbO1.5 and HfO2–YbO1.5 solid solution systems.16 Directly measuring DHt(C - d) for YbO1.5 is difficult since the transformation occurs at ultra-high temperatures (T > 2000 1C).16 The heats of formation, DHf,ox and DHf,c, are reported in Fig. 3 and Table 3. The heat of drop solution, DHds, deviates positively from linear behavior, indicating non-ideal solution behavior with energetic stabilization. A regular solution model may be applied, where:
Of course, DHmix is the heat of formation from the fluorite end-members, DHf,c, which is negative for both YbSB and DSB. Although the interaction parameters are not greatly different for DyO1.5 and YbO1.5 doping, it should be noted that DHf,ox never becomes exothermic for YbSB in the dopant range investigated, while it does for DSB at high dopant levels (x > 0.30). Thus, quenched YbSB is energetically unstable but entropy stabilized throughout its composition range and is expected to be thermodynamically unstable at low temperature.
DHmix = Ox(1 x)
(2)
The interaction parameter, O, may be determined from a quadratic fit of DHds, using DHds of the fluorite end-members
Aged materials Both YbSB and DSB samples were heat treated at 550 1C for 170 hours in sealed vials. At a dopant content of x = 0.22, YbSB
Table 1 Thermochemical cycle for determining YbSB heats of formation from the room temperature end-member phases. Here, c = cubic fluorite phase, C = C-type phase, and m = monoclinic phase
Reaction
DH
Bi1xYbxO1.5 (c, 25 1C) - xYbO1.5 (soln, 700 1C) + (1 x)BiO1.5 (soln, 700 1C) YbO1.5 (C, 25 1C) - YbO1.5 (soln, 700 1C) BiO1.5 (m, 25 1C) - BiO1.5 (soln, 700 1C) xYbO1.5 (C, 25 1C) + (1 x)BiO1.5 (m, 25 1C) - Bi1xYbxO1.5 (c, 25 1C)
DHds(1) DHds(2)14 DHds(3)11 DHf,ox(4)
DHf,ox(4) = DHds(1) + xDHds(2) + (1 x)DHds(3)
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Fig. 3 The heats of formation of YbSB from (a) the room temperature oxide end-members and (b) the fluorite end-members show regular solution behavior and stabilization by the dopant, YbO1.5.
Table 3 Heats of drop solution (DHds), heats of formation from the room temperature oxide end-members (DHf,ox), and heats of formation from the fluorite end-members (DHf,c) determined for YbSB by drop solution calorimetry in 3Na2O–4MoO3 solvent at 702 1Ca
x
DHds (kJ mol1)
DHf,ox (kJ mol1)
0.22 0.26 0.32 0.35 0.38
14.64 13.42 17.25 16.10 19.89
7.65 4.26 4.99 2.43 4.46
0.32 0.45 0.57 0.31 0.57
0.6 0.73 0.85 0.72 0.89
DHf,c (kJ mol1) 10.71 14.46 14.23 17.02 15.27
1.21 1.39 1.65 1.69 1.88
a
Compositions were determined by WDS, errors are reported as two standard deviations of the mean, and 8 measurements were taken for each sample.
undergoes a phase transformation from the quenched d-phase to a YbO1.5-doped tetragonal b-phase, as evidenced by XRD. Refined unit cell parameters, are a = 7.687 0.019 Å and c = 5.507 0.011 Å for the b-phase. A minor amount of a secondary phase was also present. Although the very low intensity and broadness of its peaks made certain phase identification difficult, this secondary phase is likely bcc g-BiO1.5. Transformation to the b-phase suggests that doping with 22 mol% ytterbia is not sufficient for stabilizing the d-phase for low-temperature (T o 700 1C) solid oxide fuel cell applications. At YbO1.5 dopant contents of x = 0.26–0.38, the X-ray diffraction patterns were unchanged before and after aging (Fig. 4). The transformation enthalpy (DHt) between the disordered phase (quenched samples) and the more ordered phase (aged samples) may be determined directly from the heats of drop solution (DHds). In order to avoid surface energy contributions to DHds, the quenched samples were coarsened at synthesis temperatures prior to final quenching. Back-scattered electron imaging shows that the particle sizes of the quenched samples was approximately 50–150 mm. For particles of this scale, it is expected that the specific surface area of the samples is sufficiently low for surface energy contributions to be considered negligible. Thus, DHt was not significantly affected by any further particle coarsening that may have occurred during the low-temperature aging. With DHt between 2 and 3 kJ mol1 for x = 0.26–0.32, the vacancy ordering transformation is energetically favorable, although only slightly,
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Fig. 4 XRD patterns for 32YbSB after quenching from 750 1C and aging at 550 1C for 170 hours are identical.
at moderate dopant concentrations. DSC shows that the transformation occurs between 700 and 800 1C for these samples (Fig. 5). However, at x = 0.35 and 0.38, only very small, broad humps are detectable in DSC, in agreement with the small enthalpy differences determined by drop solution calorimetry (Table 4). In the DSB system, at x = 0.11–0.23, the samples undergo phase transformations during aging. The resulting phases are summarized in Table 4. At the very lowest dopant content investigated, 11DSB could be quenched from synthesis temperatures to retain a fluorite structure. After aging at 550 1C for B1 week, the sample contained mainly a tetragonal b-phase coexisting with a minor amount of a rhombohedral e-phase. At x = 0.16–0.23, the DSB samples contained mixtures of the e-phase and d-phase, with increasing amounts of the d-phase present with increasing dopant content. DSC of these samples demonstrates that the e - d transformation occurs between 720 and 750 1C, depending on dopant content. Notably, the reverse transformation is not observed during cooling at 10 1C min1. For x = 0.30–0.45, XRD did not reveal any obvious structural changes between the quenched and aged samples. By drop solution calorimetry at 700 1C and DSC experiments up
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Fig. 5 DSC traces of aged (a) 32YbSB and (b) 30DSB during heating and cooling at a rate of 10 1C min1. A heat effect associated with the order–disorder transformation is observed in 32YbSB; however, the reverse transition is not observed during cooling due to its slow kinetics.
Table 4 Phases of YbSB and DSB after quenching from synthesis temperatures, and after aging at 550 1C for 170 hours. Heats of transformation, DHt, were determined from the difference of DHds of the quenched and aged specimens, respectively, in 3Na2O–4MoO3 solvent at 702 1C
YbSB
DSB 1
x
Quenched
Aged
DHt (kJ mol )
x
Quenched
Aged
DHt (kJ mol1)
0.22 0.26 0.32 0.35 0.38
d d d d d
d+b d d d d
— 2.37 2.79 0.02 0.52
0.11 0.16 0.23 0.30 0.45
d d d d d
b+e e+d d+e d d
— — — 0.97 0.98 0.23 1.06
0.71 0.69 0.46 0.72
to 875 1C, no changes in enthalpy, either from the order– disorder transformation, or from the e - d transformation, were observed for 30DSB and 45DSB (Table 4).
Discussion In the quenched YbxBi1xO1.5 solid solution range investigated, at x r 0.16, a C-type g-BiO1.5 phase was present in addition to the primary d-phase. On the other side of the fluorite stability range, at x Z 0.44, a secondary phase corresponding to a C-type YbO1.5 solid solution was found. Interestingly, these results do not agree with previous findings. In the YbSB system, Chen et al. reported that d-BiO1.5 persists from x = 0.15 to 0.35 after furnace cooling from synthesis temperatures.17 However, Zargarova et al.18 published an equilibrium phase diagram in which d-BiO1.5 spans x = 0.25–0.60. In each of these studies, the secondary phases found at dopant levels below the d-phase stability range differ, especially since the formation of the metastable tetragonal b- and body-centered cubic (I23) g-polymorphs in slightly doped BiO1.5 is highly dependent on cooling history.19 The formation of these metastable phases is described by Levin and Roth20 in detail. Regardless of these differences, in the current study the solubility limit of quenched YbSB fluorite ranges from x = 0.22 to 0.38, compared to x = 0.11 to 0.50 for quenched DSB.11 This is not surprising since Keller et al. have shown that solubility limits in fluorite systems increase with decreasing size mismatch between the host and dopant cations.21 Unlike other fluorite-structured electrolytes, 1/4 of the anion sites in bismuth oxide are inherently vacant. Gaining insight
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into the structure of stabilized bismuth oxides, as well as the structure of pure d-BiO1.5, proves to be challenging. Several models for the anion sublattice have been proposed, including the Gattow model which assumes partial 3/4 occupancy of each anion site,22 ´n model wherein vacancies are ordered along h111i23 and the Sille the very disordered Harwig model wherein anions are displaced from the 8c sites at (1/4, 1/4, 1/4) to (1/4 d, 1/4 d, 1/4 d), resulting in a 3/16 occupancy of the 32f sites.24 Jacobs and Mac ´naill analyzed well-known theories and experimental results, Do concluding that the d-phase is best-described by a combination of long-range disorder, short-range order, and a tendency for vacancies to favor sites along h111i.23 Another complication is positional disordering, or the displacement of anions away from the ideal 8c sites, as affected by the presence of dopant cations.25 In YbSB, as in DSB, both DHmix and O are negative. This energetic stabilization suggests the formation of cation– vacancy associates. By neutron diffraction, it has been shown that oxygen anions occupy the 8c and 32f sites in pure bismuth oxide,25 but the 48i sites are also occupied in YO1.5- and REO1.5stabilized bismuth oxides.10,25 Anions in the 48i sites, which are associated with doping, may be considered Frenkel interstitials. While these defects increase the vacancy concentration in the 8c sites per formula unit, in the case of 25% yttria-doped bismuth oxide, the vacancies are trapped in the vicinity of the yttrium dopant atoms.25 Thus, the concentration of mobile charge carriers decreases, and conductivity decreases. Similar to behavior in DSB, with increasing dopant content, the increasing deviation from ideal solution behavior mirrors the decrease in ionic conductivity in YbSB,7,11 and is at least in part due to the formation of cation–vacancy associates.
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Positional disordering becomes even more important in considering the ‘‘aging’’ phenomenon. In several studies, REO1.5-stabilized bismuth oxides were heat treated for hundreds of hours at T r 600 1C, below the temperature at which the cubicto-rhombohedral transformation is expected to occur.5,7,26,27 Aging, which is believed to be due to an order–disorder transformation of the anion sublattice, manifests as a decay in conductivity, and is quickly reversed during heating. However, our thermodynamic data suggest that the term ‘‘order–disorder’’ is somewhat misleading, since significant ordering already exists in the quenched phase. Although not distinguishable from the quenched d-structure by XRD due to weak scattering from oxygen anions, the transformation has been confirmed by TEM5,8 and neutron diffraction.8 The latter showed that only 1 out of 3 anions occupy the 8c tetrahedral sites in unaged 20 mol% erbia-stabilized bismuth oxide (20ESB), with the remaining anions distributed among the 32f and 48i sites. In contrast, after aging, almost no anions occupy the 8c sites, 2.61 out of 3 anions occupy the 32f sites, and the remaining anions occupy the 48i sites. Vacancy ordering on the 8c sites inhibits oxygen ion transport, which leads to a decay in conductivity.7 In the present study, for samples aged at 550 1C for 170 hours (B1 week), YbSB with x = 0.26–0.38 and DSB samples with x = 0.30–0.45 maintained the d-phase (Table 4). All calorimetric results are interpreted under the assumption that the order– disorder transformation in these materials represents equilibrium after the given heat treatment. This is a reasonable assumption based on previous studies.5–7 Using neutron diffraction, Boyapati et al. showed that aging is characterized by the displacement of anions from the 8c to the 32f sites, with little change in the occupancy of 48i sites.10 Of bismuth oxides stabilized by Y3+, Dy3+, Ho3+, E3+, and Yb3+, the greatest number of anion displacements to the 32f sites was observed in YbSB, which exhibits the most dramatic aging behavior, and least in DSB, which shows very little conductivity decay during aging. As a reflection of this difference in aging behavior, a definitive heat effect was measured for the order–disorder transformation in YbSB (x = 0.26–0.32) by DSC and drop solution calorimetry; however, no clear transformations were observed for DSB (x = 0.30–0.45). Also noteworthy is that, when fitted with a regular solution model, the interaction parameter (O) of the aged YbSB samples is more negative than that of the quenched samples (76.49 2.07 kJ mol1 vs. 68.45 2.14 kJ mol1, respectively), which suggests that the former is more ordered. It is not surprising that no evidence of aging was found in DSB with x = 0.30–0.45. Similar behavior has been observed in erbia stabilized bismuth oxide (ESB) at high dopant levels.9 By differential thermal analysis (DTA), it was found that aged ESB exhibits either an order–disorder transformation at T o 600 1C, or a e - d transformation between 600 and 700 1C, depending on dopant content and heat treatment.27 However, at dopant contents greater than x = 0.30, neither transformation is observed. In contrast, DSB exhibits a e - d transformation between 720 and 750 1C at x = 0.16–0.23, and no detectable transformations at x Z 0.30, when DHf,ox becomes exothermic.
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25DSB was found to age somewhat at 500 1C,5–8 however, it is possible that this decrease in conductivity was due to the formation of a small amount of the e-phase, or because the transformation is more extensive at 500 1C compared to the 550 1C aging temperature used in this study.7 In considering the effects of different dopant cations on bismuth oxide, we should note that the interaction parameter for the quenched DSB system is more negative than that for quenched YbSB. Thus the DSB system is more stabilized with respect to its room temperature oxides than YbSB. This finding reflects a difference in the degree of cation–vacancy (or cation– anion) association in the quenched phase, which is partially due to the greater polarizability of Dy3+ in comparison to Yb3+. Conductivity studies have shown that doping bismuth oxides with highly polarizable lanthanide cations results in greater stabilization against aging.27 Notably, the time constant for conductivity decay increases with increasing dopant polarizability. From calorimetry, the thermodynamic driving force for aging is 2 to 3 kJ mol1 in YbSB, but nearly negligible in DSB at relevant dopant levels for which the d-phase is preserved during aging. Although the heat effects measured by drop solution calorimetry were accompanied by large errors relative to the total magnitude of the heat effects, these errors provide acceptable uncertainties in the heat of transformation (see tabulated values in Table 4). Furthermore, the drop solution calorimetry results were supported by DSC traces, which showed clear heat effects for aged YbSB at low dopant contents upon heating, and no transformations in DSB. In summary, the negative interaction parameters of the quenched YbSB and DSB systems and the low transformation enthalpies for aging suggest that the aging phenomenon can be described by additional ordering on an already significantly ordered oxygen sublattice. Unlike other fluorites that see a decrease in conductivity with increased doping due to a dramatic increase in clustering,28,29 the total degree of increased ordering in aged stabilized bismuth oxides cannot completely explain the large extent of conductivity decay during aging. Rather, the thermodynamic data support that it is the occupation of specific anion sites, namely the exclusive occupation of the 32f and 48i sites, that is responsible for this conductivity decay, as was previously suggested by other investigators.7,8
Conclusion The thermochemistry of aging in rare earth oxide-stabilized bismuth oxide electrolytes was studied by calorimetry for the first time. Ytterbia- and dysprosia-stabilized systems were selected due to their contrasting aging behavior and the differing polarizabilities of the dopant cations. In the quenched materials, DSB is more stabilized with respect to its room temperature oxide end-members than YbSB. The deviation from ideal solution behavior in both systems is attributed to cation–vacancy association and positional disordering on the anion sublattice. This is supported by the heat of formation,
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which becomes more exothermic in parallel with a decrease in ionic conductivity with increasing dopant content. After aging at 550 1C for 170 hours, a small order–disorder enthalpy (2 to 3 kJ mol1) was measurable for YbSB at moderate dopant contents (x = 0.26–0.32) by DSC and oxide melt solution calorimetry. However, no obvious order–disorder transformation was observable in DSB samples for which the d-phase is preserved during the low temperature heat treatment. Taken together with previous findings by other authors, these results suggest that the tendency for aging decreases with increasing dopant content, and with an increase in the polarizability of the dopant cation.
Acknowledgements This work was performed with funding from the U.S. Department of Energy (Grant No. DE-FG02-03ER46053).
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