Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
The EPR study of Mn2+ ion doped KBr and VO2+ ion doped KH2PO4 under high pressure Ümit Ceylan ⇑, Recep Tapramaz Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, 55139 Samsun, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
2+ doped KBr and VO2+ doped KH2PO4 single crystals grown in high pressure reactor was studied. The A and g values of the paramagnetic centers and the electronic transitions were determined. Using the values determined from spectra, structures and molecular orbital parameters were found.
Mn
a r t i c l e
i n f o
Article history: Received 23 June 2014 Received in revised form 1 February 2015 Accepted 5 February 2015 Available online xxxx Keywords: High-pressure reactor EPR UV–Vis Argon atmosphere Oxygen atmosphere
a b s t r a c t In previous works under open atmosphere at various temperatures KBr host crystal never accepted paramagnetic Mn2+ ion in its lattice, but KH2PO4 host crystal accepted VO2+ ion in its lattice where K+ ion substituted with VO2+. In a series of works using high-pressure reactor at various pressures and temperatures under different gas atmospheres, Mn2+ ion was inserted in KBr host, and VO2+ ion was inserted in KH2PO4 host in sites different from those observed in previous works. The best results were obtained for Mn2+ doped KBr at pressure of 100 bars and 50 °C under Argon atmosphere, and for VO2+ doped KH2PO4 at pressure of 100 bars and temperature of 50 °C under oxygen atmosphere. The structural parameters of paramagnetic species formed were determined by EPR spectroscopy and optical transitions were determined by UV–Vis spectroscopy. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Potassium dihydrogen phosphate-like (KDP-like) ferroelectric structures in crystalline form show nonlinear optical properties and have widespread use in optic technology. Some impurities or defects doped in these host crystals like transition metal ions, ionic holes, small chemical groups etc. at trace amounts can change physical properties at different rates and these properties are evaluated in different applications [1–3]. Insertion of impurities and determinations of changes in physical properties of KBr and KDP lattices by means of different ⇑ Corresponding author. Tel.: +90 505 313 51 57. E-mail address: [email protected] (Ü. Ceylan).
techniques, among them optical techniques are pioneering, have been reported in large amount of paper [4–16]. In order to purify matters and to observe doped trace amounts of particular impurities and to investigate three-dimensional spatial structures and especially anisotropic properties, single crystalline samples are needed. Under atmospheric pressure and at room temperature, large number of studies has done on various impurities doped host crystals [17–20]. KBr type crystals, in this context, are evaluated mainly to form color centers in them by exposing to ionizing radiation, inserting some ionic impurities [21]. Although substitution of K+ with Mn2+ is probable, it could not be achieved under open atmosphere at various temperatures. In this work the experiment was carried out in a high-pressure reactor as explained in Experimental
http://dx.doi.org/10.1016/j.saa.2015.02.019 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Ü. Ceylan, R. Tapramaz, The EPR study of Mn2+ ion doped KBr and VO2+ ion doped KH2PO4 under high pressure, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2015.02.019
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Ü. Ceylan, R. Tapramaz / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx
section. Technologically important crystalline KDP, on the other hand, accepts VO2+ ion as impurity under open atmosphere and room temperature and the EPR study was published [16], in this work the experiment was repeated using high-pressure reactor similar to processes applied for KBr crystal, explained above. High-pressure reactors are mainly used in chemical processes to produce high quality materials like polymers. However a few studies were found on EPR spectroscopic investigation of structures with impurities doped under high pressures and relatively high temperatures under some inert gas atmospheres [22–25]. Experimental Sample preparation Polycrystalline KBr and KDP were obtained commercially from Merck. Both samples, KBr and KDP, investigated in this study were prepared in similar way. Approximately %3 MnSO4 and VOSO4 was added as impurity to KBr and KDP respectively at each trial. Both samples were dissolved in triply distilled water until saturation at experiment temperatures; for instance for the reactor temperature of 90 °C, the solution was saturated at 90 °C, etc. For temperatures above boiling point of water at open atmosphere, some extra amount of compound was added into solution to be dissolved in reactor by magnetic stirrer. Prepared saturated or oversaturated solutions were put separately in reactor and a specific gas was applied at specific pressures for each trial. Gases used were normal air, pure N2, CO2, O2 and Ar, and pressures applied were between 1 and 150 atmospheres with 25 atmosphere steps and temperatures were between room temperature and 160 °C with 30 °C steps. The solutions were stirred during slow heating and when experiment temperature was reached stirring was stopped and the reactor was left for slow cooling down to room temperature for 12 h. After cooling period wellshaped single crystals were chosen spectral studies. Clear EPR spectra were obtained for Mn2+ doped KBr at 100 atmospheres and 70 °C, and for VO2+ doped KDP at 100 atmospheres and 50 °C. KBr crystallizes in face centered cubic (fcc) system with unit cell parameter of a = 6.602 Å [26]. KDP crystallizes in tetragonal symmetry. Space group is 42 m and unit cell parameters are a = b = 7.4529 Å, c = 6.9751 Å. There are 4 molecules in unit cell [27].
temperatures. Insertion could be made as indicated in Experimental section. EPR spectra of Mn2+ doped KBr single crystals were recorded at room temperature by rotating the single crystals around three mutually perpendicular crystalline axes. Spectra gave nearly isotropic spectra at all orientations as expected from cubic symmetry. Fig. 1 shows a sample EPR spectrum of Mn2+ ion doped KBr single crystal obtained at random orientation together with simulated spectrum and Fig. 2 shows powder spectrum which indicates that a small anisotropy in hyperfine splitting and g value. Mn2+ ion is in 2d5 state with total electron spin of S = 5/2. The nuclear spin is also 5/2. This spin system should give 30 lines in EPR spectra containing 5 zero field splitting groups of 6 hyperfine splitting. All splitting values and g value show small anisotropy. The average g and hyperfine values are g = 2.00 and a = 9.7 mT respectively. Hyperfine and g values are compatible with 3d5 electron configuration of Mn2+ ion and with previous works [29]. Zero field splitting (ZFS) values which reflect the symmetry of the crystal filed surrounding Mn2+ ion were determined via simulation and possible maximum values were determined to be D = 4.59 mT and E = 1.31 mT and higher values distort spectra. Small ZFS and small anisotropy indicates slightly distorted cubic environmental symmetry as expected from KBr and similar lattices. During the crystallization process some K+ ions substitutes with Mn2+ ion compensating the charge with Br atoms in the neighborhood. Fig. 3 shows optical absorption spectrum of this crystal. Three transitions were observed at 443 nm near UV region, 646 near IR region and 836 nm in near IR region. The transitions are very weak because of very low concentration of substituted Mn2+ ions in one hand, and low spin electronic transitions in the other hand. It is well known than KBr has no vibrational transition between wide range 30,000 and 500 cm1 including UV, visible and IR regions, and therefore it is used make pellets in IR spectroscopy to dilute samples. The weak transitions must be due to Mn2+ impurities because pure MnBr2 samples give electronic transitions in 360– 435 nm region [30]. In Fig. 3 none of the transitions fit to pure MnBr2 because the structure is fixed in a KBr lattice and the
EPR spectra EPR spectra were recorded on a Varian E109 Century Series X band EPR spectrometer having a rectangular cavity. Single crystals were glued on a Lucite pillar attached to a goniometer graded in 1°. The crystals were rotated in three mutually perpendicular planes with 10° intervals. The g value corrections were made using a dpph sample (g = 2.0036). Simulations of powder spectra of both compounds were made using Bruker’s WINEPR software [28]. Optical absorption spectra The optical absorption spectra of Mn2+ doped KBr and VO2+ doped KDP single crystals were recorded on X-Rite Color I5 spectrometer in UV–Vis (360–750 nm) region at room temperature. The (750–900 nm) range of spectra was recorded by using Pharmacia LKB-Ultrospec III spectrometer. Results and discussions Mn2+ ion doped KBr Previous works made on KBr showed that Mn2+ ion could not be inserted into KBr lattice under atmospheric pressure at various
Fig. 1. (a) EPR spectrum of Mn+2 ion doped KBr single crystal at a random orientation under room temperature, and (b) simulation.
Please cite this article in press as: Ü. Ceylan, R. Tapramaz, The EPR study of Mn2+ ion doped KBr and VO2+ ion doped KH2PO4 under high pressure, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2015.02.019
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Fig. 2. Powder crystal EPR spectrum of Mn2+ ion doped KBr at room temperature.
3
Fig. 4. EPR spectrum at ca plane with magnetic field at a axis of VO+2 ion doped KDP single crystal at room temperature. The spectrum is the simplest one among the spectra taken at other orientation of the single crystal.
Fig. 5. EPR spectrum of VO+2 ion doped KDP powder crystal at room temperature.
Fig. 3. Optical absorption spectrum of Mn2+ ion doped KBr single crystal.
transitions shift to visible region or defects may give some transitions in visible and near IR regions. IR spectra of MnBr2 gives mainly stretching frequencies in far IR region between 3300 and 220 cm1 (3030 and 45,454 nm) and hence are not seen here. VO2+ ion doped KH2PO4 EPR spectroscopic study of VO+2 ion doped KDP single crystals under normal atmospheric pressure and at room temperature was previously reported [16]. In that study single crystal spectra were resolved and four magnetically equivalent paramagnetic sites were determined. With the crystallization process explained in Experimental section however, the crystals grown under highpressure and at 50 °C in reactor, at least eight VO2+ sites were observed. Fig. 4 shows EPR spectra ca planes of unit cell with magnetic field parallel to a axes and powder EPR spectrum is shown in Fig. 5. Both spectra indicate that all paramagnetic species in the single crystal are magnetically equivalent. In order to solve the spectra all line positions of the spectra taken at three mutually perpendicular planes were drawn against rotation angles by means of software written for this purpose [16]. The numbers of lines were however, large and unresolvable
and it was impossible to resolve. At some specific orientations about 60 lines could be counted. Since the only paramagnetic specie in the host is VO2+ the number of lines must be 64 corresponding to eight VO2+ sites. Because of unresolvability of single crystal spectra, experimental EPR parameters were obtained from powder spectrum given in Fig. 5 to be g k ¼ 1:920, g ? ¼ 1:945, Ak ¼ 17:9 mT (167.3 104 cm1) and A? ¼ 6:5 mT (38.3 104 cm1) giving isotropic values as g iso ¼ 1:937 and Aiso ¼ 10:3 mT (60.7 104 cm1). The g values obviously indicate highly axial symmetry and are comparable to the values in the previous paper [16] but both parallel and perpendicular hyperfine values are about 1 mT less indicating VO2+ centered complexes are somehow different. The values fit to distorted octahedral toward square pyramidal complex structure nearly in C4v symmetry. In KDP lattice K+ ion substitutes with VO2+ ion compensating negative charge deficiency with 5 oxygen atoms of PO3 4 groups in the ligand positions where sixth position is occupied by O2 ion along axial direction where distortion takes place. Hamiltonian of the paramagnetic center can be given as,
H ¼ g k bHz Sz þ g ? bðHx Sx þ Hy Sy Þ þ Ak Sz Iz þ A? ðSx Ix þ Sy Iy Þ
ð1Þ
for electronic spin 1/2 and nuclear spin 7/2 where b is Bohr magneton and g k and g ? , Ak and A? are parallel and perpendicular component of g and hyperfine Hx ; Hy and Hz are magnetic field components, Sx ; Sy and Sz and Ix ; Iy and Iz are components of
Please cite this article in press as: Ü. Ceylan, R. Tapramaz, The EPR study of Mn2+ ion doped KBr and VO2+ ion doped KH2PO4 under high pressure, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2015.02.019
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Conclusion In previous works KBr and KH2PO4 (KDP) hosts did not accept Mn2+ and VO2+ ions respectively under open atmosphere and room temperature condition. In this work, however, we doped these ions in high-pressure reactor under O2 and Ar atmosphere at 100 bars. The optimal temperatures were 50 and 70 °C for KBr and KDP hosts. Doped single crystals were grown in reactor. EPR spectra of Mn2+ doped KBr single crystal gives isotropic Mn2+ spectra. Hyperfine, g and zero field (ZFS) splitting parameters were determined from spectra and simulation. K+ ion substitutes with Mn2+ ion by compensating charge deficiency via Br+ ions around. KBr is as well-known inactive in UV, visible and IR regions, but after Mn2+ doping it giver very weak transitions in visible and near IR regions. In a previous work VO2+ ions was doped in KDP single crystal and magnetically four VO2+ sites were determined. In high-pressure reactor however, VO2+ ions settle in different locations giving eight magnetically equivalent sites. Hyperfine and g values were measured from EPR spectra and molecular orbital parameters were calculated using transition values obtained from optical spectrum.
Fig. 6. Optical absorbtion spectrum of VO2+ ion doped KDP single crystal.
Table 1 Electronic transitions obtained from optical absorption spectrum of VO2+ doped KDP prepared in high-pressure reactor. Transition
Region
Wavelength (nm)
Frequency (cm
D? ¼ 2B2g 2Eg Dk ¼ 2B2g 2B1g D ¼ 2B2g 2A1g
Visible Visible IR
490 680 830
20,408 14,706 12,048
1
)
Acknowledgments This work was financially supported by the BAP, Ondokuz Mayıs University (Samsun) (Project numbers: PYO. FEN. 1904.09.017 and FEN 1901-10.001). References
electronic and nuclear spins. Nuclear Zeeman and other terms with small effect were neglected. Optical absorption spectrum given in Fig. 6 shows mainly three transitions originating d–d transitions of V4+ ion in 3d1 state in accordance with Ballhausen–Gray schema [31]. The transitions are very broad because of the sample is solid. The values are given in Table 1 together with allowed transition levels. The molecular orbital coefficients and parameters were calculated using the expressions [32]
Ak ¼ P
A? ¼ P
3 7
2 7
11 ðg g ? Þ 14 e
j b2 þ ðg e g k Þ
gk ¼ ge 1
4 7
j b2 þ ðg e g k Þ ðg e g ? Þ
! 4kb21 b22 ; Dk
g? ¼ ge 1
c2 kb22
ð2aÞ ð2bÞ
constant and has the value for V
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
! ð2cÞ
D?
[20] [21] [22]
here P is dipolar hyperfine constant and k is spin orbit-coupling 4+
[1] [2] [3] [4] [5] [6] [7] [8] [9]
1
ion as 170 cm
b21
2
and c are ion-
ic degrees of r and p bonds in equatorial plane, and b22 is the ionic degree of the bond between V4+ and O2 bond and must be close to unity for completely ionic bond as accepted here. Calculations using experimental values give j ¼ 0:76; P ¼ 116 104 cm1 ; b21 ¼ 0:89; c2 ¼ 0:34 and b22 ¼ 0:99 as expected. The value of b21 is close to unity and c2 is close to zero indicating in plane r bond is highly ionic and in plane p bond is highly covalent [33].
[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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Please cite this article in press as: Ü. Ceylan, R. Tapramaz, The EPR study of Mn2+ ion doped KBr and VO2+ ion doped KH2PO4 under high pressure, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2015.02.019
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
The EPR study of Mn2+ ion doped KBr and VO2+ ion doped KH2PO4 under high pressure Ümit Ceylan ⇑, Recep Tapramaz Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, 55139 Samsun, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
2+ doped KBr and VO2+ doped KH2PO4 single crystals grown in high pressure reactor was studied. The A and g values of the paramagnetic centers and the electronic transitions were determined. Using the values determined from spectra, structures and molecular orbital parameters were found.
Mn
a r t i c l e
i n f o
Article history: Received 23 June 2014 Received in revised form 1 February 2015 Accepted 5 February 2015 Available online xxxx Keywords: High-pressure reactor EPR UV–Vis Argon atmosphere Oxygen atmosphere
a b s t r a c t In previous works under open atmosphere at various temperatures KBr host crystal never accepted paramagnetic Mn2+ ion in its lattice, but KH2PO4 host crystal accepted VO2+ ion in its lattice where K+ ion substituted with VO2+. In a series of works using high-pressure reactor at various pressures and temperatures under different gas atmospheres, Mn2+ ion was inserted in KBr host, and VO2+ ion was inserted in KH2PO4 host in sites different from those observed in previous works. The best results were obtained for Mn2+ doped KBr at pressure of 100 bars and 50 °C under Argon atmosphere, and for VO2+ doped KH2PO4 at pressure of 100 bars and temperature of 50 °C under oxygen atmosphere. The structural parameters of paramagnetic species formed were determined by EPR spectroscopy and optical transitions were determined by UV–Vis spectroscopy. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Potassium dihydrogen phosphate-like (KDP-like) ferroelectric structures in crystalline form show nonlinear optical properties and have widespread use in optic technology. Some impurities or defects doped in these host crystals like transition metal ions, ionic holes, small chemical groups etc. at trace amounts can change physical properties at different rates and these properties are evaluated in different applications [1–3]. Insertion of impurities and determinations of changes in physical properties of KBr and KDP lattices by means of different ⇑ Corresponding author. Tel.: +90 505 313 51 57. E-mail address: [email protected] (Ü. Ceylan).
techniques, among them optical techniques are pioneering, have been reported in large amount of paper [4–16]. In order to purify matters and to observe doped trace amounts of particular impurities and to investigate three-dimensional spatial structures and especially anisotropic properties, single crystalline samples are needed. Under atmospheric pressure and at room temperature, large number of studies has done on various impurities doped host crystals [17–20]. KBr type crystals, in this context, are evaluated mainly to form color centers in them by exposing to ionizing radiation, inserting some ionic impurities [21]. Although substitution of K+ with Mn2+ is probable, it could not be achieved under open atmosphere at various temperatures. In this work the experiment was carried out in a high-pressure reactor as explained in Experimental
http://dx.doi.org/10.1016/j.saa.2015.02.019 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Ü. Ceylan, R. Tapramaz, The EPR study of Mn2+ ion doped KBr and VO2+ ion doped KH2PO4 under high pressure, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2015.02.019
2
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section. Technologically important crystalline KDP, on the other hand, accepts VO2+ ion as impurity under open atmosphere and room temperature and the EPR study was published [16], in this work the experiment was repeated using high-pressure reactor similar to processes applied for KBr crystal, explained above. High-pressure reactors are mainly used in chemical processes to produce high quality materials like polymers. However a few studies were found on EPR spectroscopic investigation of structures with impurities doped under high pressures and relatively high temperatures under some inert gas atmospheres [22–25]. Experimental Sample preparation Polycrystalline KBr and KDP were obtained commercially from Merck. Both samples, KBr and KDP, investigated in this study were prepared in similar way. Approximately %3 MnSO4 and VOSO4 was added as impurity to KBr and KDP respectively at each trial. Both samples were dissolved in triply distilled water until saturation at experiment temperatures; for instance for the reactor temperature of 90 °C, the solution was saturated at 90 °C, etc. For temperatures above boiling point of water at open atmosphere, some extra amount of compound was added into solution to be dissolved in reactor by magnetic stirrer. Prepared saturated or oversaturated solutions were put separately in reactor and a specific gas was applied at specific pressures for each trial. Gases used were normal air, pure N2, CO2, O2 and Ar, and pressures applied were between 1 and 150 atmospheres with 25 atmosphere steps and temperatures were between room temperature and 160 °C with 30 °C steps. The solutions were stirred during slow heating and when experiment temperature was reached stirring was stopped and the reactor was left for slow cooling down to room temperature for 12 h. After cooling period wellshaped single crystals were chosen spectral studies. Clear EPR spectra were obtained for Mn2+ doped KBr at 100 atmospheres and 70 °C, and for VO2+ doped KDP at 100 atmospheres and 50 °C. KBr crystallizes in face centered cubic (fcc) system with unit cell parameter of a = 6.602 Å [26]. KDP crystallizes in tetragonal symmetry. Space group is 42 m and unit cell parameters are a = b = 7.4529 Å, c = 6.9751 Å. There are 4 molecules in unit cell [27].
temperatures. Insertion could be made as indicated in Experimental section. EPR spectra of Mn2+ doped KBr single crystals were recorded at room temperature by rotating the single crystals around three mutually perpendicular crystalline axes. Spectra gave nearly isotropic spectra at all orientations as expected from cubic symmetry. Fig. 1 shows a sample EPR spectrum of Mn2+ ion doped KBr single crystal obtained at random orientation together with simulated spectrum and Fig. 2 shows powder spectrum which indicates that a small anisotropy in hyperfine splitting and g value. Mn2+ ion is in 2d5 state with total electron spin of S = 5/2. The nuclear spin is also 5/2. This spin system should give 30 lines in EPR spectra containing 5 zero field splitting groups of 6 hyperfine splitting. All splitting values and g value show small anisotropy. The average g and hyperfine values are g = 2.00 and a = 9.7 mT respectively. Hyperfine and g values are compatible with 3d5 electron configuration of Mn2+ ion and with previous works [29]. Zero field splitting (ZFS) values which reflect the symmetry of the crystal filed surrounding Mn2+ ion were determined via simulation and possible maximum values were determined to be D = 4.59 mT and E = 1.31 mT and higher values distort spectra. Small ZFS and small anisotropy indicates slightly distorted cubic environmental symmetry as expected from KBr and similar lattices. During the crystallization process some K+ ions substitutes with Mn2+ ion compensating the charge with Br atoms in the neighborhood. Fig. 3 shows optical absorption spectrum of this crystal. Three transitions were observed at 443 nm near UV region, 646 near IR region and 836 nm in near IR region. The transitions are very weak because of very low concentration of substituted Mn2+ ions in one hand, and low spin electronic transitions in the other hand. It is well known than KBr has no vibrational transition between wide range 30,000 and 500 cm1 including UV, visible and IR regions, and therefore it is used make pellets in IR spectroscopy to dilute samples. The weak transitions must be due to Mn2+ impurities because pure MnBr2 samples give electronic transitions in 360– 435 nm region [30]. In Fig. 3 none of the transitions fit to pure MnBr2 because the structure is fixed in a KBr lattice and the
EPR spectra EPR spectra were recorded on a Varian E109 Century Series X band EPR spectrometer having a rectangular cavity. Single crystals were glued on a Lucite pillar attached to a goniometer graded in 1°. The crystals were rotated in three mutually perpendicular planes with 10° intervals. The g value corrections were made using a dpph sample (g = 2.0036). Simulations of powder spectra of both compounds were made using Bruker’s WINEPR software [28]. Optical absorption spectra The optical absorption spectra of Mn2+ doped KBr and VO2+ doped KDP single crystals were recorded on X-Rite Color I5 spectrometer in UV–Vis (360–750 nm) region at room temperature. The (750–900 nm) range of spectra was recorded by using Pharmacia LKB-Ultrospec III spectrometer. Results and discussions Mn2+ ion doped KBr Previous works made on KBr showed that Mn2+ ion could not be inserted into KBr lattice under atmospheric pressure at various
Fig. 1. (a) EPR spectrum of Mn+2 ion doped KBr single crystal at a random orientation under room temperature, and (b) simulation.
Please cite this article in press as: Ü. Ceylan, R. Tapramaz, The EPR study of Mn2+ ion doped KBr and VO2+ ion doped KH2PO4 under high pressure, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2015.02.019
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Fig. 2. Powder crystal EPR spectrum of Mn2+ ion doped KBr at room temperature.
3
Fig. 4. EPR spectrum at ca plane with magnetic field at a axis of VO+2 ion doped KDP single crystal at room temperature. The spectrum is the simplest one among the spectra taken at other orientation of the single crystal.
Fig. 5. EPR spectrum of VO+2 ion doped KDP powder crystal at room temperature.
Fig. 3. Optical absorption spectrum of Mn2+ ion doped KBr single crystal.
transitions shift to visible region or defects may give some transitions in visible and near IR regions. IR spectra of MnBr2 gives mainly stretching frequencies in far IR region between 3300 and 220 cm1 (3030 and 45,454 nm) and hence are not seen here. VO2+ ion doped KH2PO4 EPR spectroscopic study of VO+2 ion doped KDP single crystals under normal atmospheric pressure and at room temperature was previously reported [16]. In that study single crystal spectra were resolved and four magnetically equivalent paramagnetic sites were determined. With the crystallization process explained in Experimental section however, the crystals grown under highpressure and at 50 °C in reactor, at least eight VO2+ sites were observed. Fig. 4 shows EPR spectra ca planes of unit cell with magnetic field parallel to a axes and powder EPR spectrum is shown in Fig. 5. Both spectra indicate that all paramagnetic species in the single crystal are magnetically equivalent. In order to solve the spectra all line positions of the spectra taken at three mutually perpendicular planes were drawn against rotation angles by means of software written for this purpose [16]. The numbers of lines were however, large and unresolvable
and it was impossible to resolve. At some specific orientations about 60 lines could be counted. Since the only paramagnetic specie in the host is VO2+ the number of lines must be 64 corresponding to eight VO2+ sites. Because of unresolvability of single crystal spectra, experimental EPR parameters were obtained from powder spectrum given in Fig. 5 to be g k ¼ 1:920, g ? ¼ 1:945, Ak ¼ 17:9 mT (167.3 104 cm1) and A? ¼ 6:5 mT (38.3 104 cm1) giving isotropic values as g iso ¼ 1:937 and Aiso ¼ 10:3 mT (60.7 104 cm1). The g values obviously indicate highly axial symmetry and are comparable to the values in the previous paper [16] but both parallel and perpendicular hyperfine values are about 1 mT less indicating VO2+ centered complexes are somehow different. The values fit to distorted octahedral toward square pyramidal complex structure nearly in C4v symmetry. In KDP lattice K+ ion substitutes with VO2+ ion compensating negative charge deficiency with 5 oxygen atoms of PO3 4 groups in the ligand positions where sixth position is occupied by O2 ion along axial direction where distortion takes place. Hamiltonian of the paramagnetic center can be given as,
H ¼ g k bHz Sz þ g ? bðHx Sx þ Hy Sy Þ þ Ak Sz Iz þ A? ðSx Ix þ Sy Iy Þ
ð1Þ
for electronic spin 1/2 and nuclear spin 7/2 where b is Bohr magneton and g k and g ? , Ak and A? are parallel and perpendicular component of g and hyperfine Hx ; Hy and Hz are magnetic field components, Sx ; Sy and Sz and Ix ; Iy and Iz are components of
Please cite this article in press as: Ü. Ceylan, R. Tapramaz, The EPR study of Mn2+ ion doped KBr and VO2+ ion doped KH2PO4 under high pressure, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2015.02.019
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Ü. Ceylan, R. Tapramaz / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx
Conclusion In previous works KBr and KH2PO4 (KDP) hosts did not accept Mn2+ and VO2+ ions respectively under open atmosphere and room temperature condition. In this work, however, we doped these ions in high-pressure reactor under O2 and Ar atmosphere at 100 bars. The optimal temperatures were 50 and 70 °C for KBr and KDP hosts. Doped single crystals were grown in reactor. EPR spectra of Mn2+ doped KBr single crystal gives isotropic Mn2+ spectra. Hyperfine, g and zero field (ZFS) splitting parameters were determined from spectra and simulation. K+ ion substitutes with Mn2+ ion by compensating charge deficiency via Br+ ions around. KBr is as well-known inactive in UV, visible and IR regions, but after Mn2+ doping it giver very weak transitions in visible and near IR regions. In a previous work VO2+ ions was doped in KDP single crystal and magnetically four VO2+ sites were determined. In high-pressure reactor however, VO2+ ions settle in different locations giving eight magnetically equivalent sites. Hyperfine and g values were measured from EPR spectra and molecular orbital parameters were calculated using transition values obtained from optical spectrum.
Fig. 6. Optical absorbtion spectrum of VO2+ ion doped KDP single crystal.
Table 1 Electronic transitions obtained from optical absorption spectrum of VO2+ doped KDP prepared in high-pressure reactor. Transition
Region
Wavelength (nm)
Frequency (cm
D? ¼ 2B2g 2Eg Dk ¼ 2B2g 2B1g D ¼ 2B2g 2A1g
Visible Visible IR
490 680 830
20,408 14,706 12,048
1
)
Acknowledgments This work was financially supported by the BAP, Ondokuz Mayıs University (Samsun) (Project numbers: PYO. FEN. 1904.09.017 and FEN 1901-10.001). References
electronic and nuclear spins. Nuclear Zeeman and other terms with small effect were neglected. Optical absorption spectrum given in Fig. 6 shows mainly three transitions originating d–d transitions of V4+ ion in 3d1 state in accordance with Ballhausen–Gray schema [31]. The transitions are very broad because of the sample is solid. The values are given in Table 1 together with allowed transition levels. The molecular orbital coefficients and parameters were calculated using the expressions [32]
Ak ¼ P
A? ¼ P
3 7
2 7
11 ðg g ? Þ 14 e
j b2 þ ðg e g k Þ
gk ¼ ge 1
4 7
j b2 þ ðg e g k Þ ðg e g ? Þ
! 4kb21 b22 ; Dk
g? ¼ ge 1
c2 kb22
ð2aÞ ð2bÞ
constant and has the value for V
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
! ð2cÞ
D?
[20] [21] [22]
here P is dipolar hyperfine constant and k is spin orbit-coupling 4+
[1] [2] [3] [4] [5] [6] [7] [8] [9]
1
ion as 170 cm
b21
2
and c are ion-
ic degrees of r and p bonds in equatorial plane, and b22 is the ionic degree of the bond between V4+ and O2 bond and must be close to unity for completely ionic bond as accepted here. Calculations using experimental values give j ¼ 0:76; P ¼ 116 104 cm1 ; b21 ¼ 0:89; c2 ¼ 0:34 and b22 ¼ 0:99 as expected. The value of b21 is close to unity and c2 is close to zero indicating in plane r bond is highly ionic and in plane p bond is highly covalent [33].
[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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Please cite this article in press as: Ü. Ceylan, R. Tapramaz, The EPR study of Mn2+ ion doped KBr and VO2+ ion doped KH2PO4 under high pressure, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/10.1016/j.saa.2015.02.019
