Research article Received: 18 October 2014,
Accepted: 28 December 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2855
Characterization and luminescence properties of CaMgSi2O6:Eu2+ blue phosphor P. Chandrakar, R. N. Baghel,* D. P. Bisen and B. P. Chandra ABSTRACT: A blue CaMgSi2O6:Eu2+ phosphor was prepared by the solid-state reaction method and the phosphor characterized in terms of crystal structure, particle size, photoluminescence (PL), thermoluminescence (TL) and mechanoluminescence (ML) properties using X-ray diffraction (XRD), transmission electron microscopy (TEM), PL spectroscopy, TLD reader and ML impact technique. The XRD result shows that phosphor is formed in a single phase and has a monoclinic structure with the space group C2/c. Furthermore, the PL excitation spectra of Eu2+-doped CaMgSi2O6 phosphor showed a strong band peak at 356 nm and the PL emission spectrum has a peak at 450 nm. The depths and frequency factors of trap centers were calculated using the TL glow curve by deconvolution method in which the trap depths were found to be 0.48 and 0.61 eV. The formation of CaMgSi2O6:Eu2+ phosphor was confirmed by Fourier transform infrared spectroscopy. The ML intensity increased linearly with the impact velocity of the piston used to deform the phosphor. It was shown that the local piezoelectricityinduced electron bombardment model is responsible for the ML emission. Finally, the optical properties of CaMgSi2O6:Eu2+ phosphors are discussed. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: CaMgSi2O6:Eu2+ phosphor; TEM; photoluminescence; thermoluminescence; mechanoluminescence; solid-state reaction synthesis
Introduction Over the last 10 years, aluminate-based rare earth (RE)-ion-doped phosphors have attracted a great deal of interest because of their excellent properties, e.g., high brightness, long duration, excellent photoresistance and environmental capability. However, the properties of these phosphors are decreased greatly on soaking in water for several hours, which limits their application in pigments, paints and other fields (1,2). Recently, silicate-based phosphors have attracted much attention because of their many advantages over sulfide and aluminate phosphors (3-8). CaMgSi2O6 is one of the pyroxene minerals (diopside) and is known to be a good bioactive material (9). Jiang et al. have prepared a long-lasting alkaline earth silicate, CaMgSi2O6 activated by Eu2+, Dy3+ and Nd3+, and have shown that the phosphor has good blue emission under UV illumination (10). A characteristic feature of the CaMgSi2O6:Eu2+ (CMS:Eu2+) blue light-emitting phosphor is that it is stable and its emission efficiency decreases less than that of BaMgSi2O6: Eu2+ (BAM:Eu2+), which is widely utilized as a blue light-emitting material of Plasma display panel (PDP) or as a rare gas lamp. However, it is known that the emission intensity of CMS:Eu2+ is lower than that of the BAM:Eu2+. Accordingly, there has been a demand for the development of a CMS:Eu2+ blue light-emitting phosphor with a high emission intensity. The pyroxene structure consists of single chains of SiO4 tetrahedra extending along the c-axis of the unit cell. Cameron and Papike (11) described two types of cation positions, designated as M1 and M2. Site M1 is smaller than M2 and lies between the apices of opposing tetrahedra forming an almost regular octahedron. Site M2 lies between the bases of the tetrahedral forming distorted six- or eight-fold sites. In the diopside, position M1 is occupied by Mg and position M2 is occupied by Ca.
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Optical transitions of divalent europium (4f 6–5d1) have been investigated in many phosphors. The spectra of Eu2+-doped compounds are caused by electric dipole transitions with parity allowed so that they occur with high transition probabilities. The emission colors vary from ultraviolet to red depending on the host lattice (12). Recently, the luminescent properties of solidstate reaction-derived CaMgSi2O6:Eu2+ on UV excitation have been reported. As an important family of luminescent materials, phosphate compounds have attracted more attention because of their excellent thermal and chemical stability (13). In this work, the blue-emitting CaMgSi2O6:Eu2+ phosphor was prepared by the solid-state reaction method. The excitation spectra, emission spectra, thermoluminescence (TL), Fourier transform infrared spectroscopy (FTIR) and mechanoluminescence (ML) of the crystalline properties were investigated. The morphology of the phosphor was been characterized.
Experimental Synthesis A sample with the general formula CaMgSi2O6:Eu2+ was prepared by the solid-state reaction method. The starting reagents were CaCO3, MgO, SiO2 and Eu2O3 with small quantities of H3BO3 added as flux. The raw materials were weighed stoichiometrically and mixed homogeneously in a mortar for 2 h. The mixture contained 2.5 mol% Eu2O3. The mixture was sintered * Correspondence to: R. N. Baghel, School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur- 492010 (C.G) India. E-mail: [email protected] School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur, India
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P. Chandrakar et al. at 1000 °C for 3 h under a reducing atmosphere, which was created using activated charcoal. Characterization For structure determination, powder X-ray diffraction (XRD) patterns of the samples were recorded using Panalytical Empyrean XRD with CuKα radiation (λ = 1.5418 Å). A transmission electron microscope (TECNAI G2 20 S-TWIN (IIT Roorkee)) was used to take the TEM images. The surface morphology of the sample was analyzed using a scanning electron microscope (Hitachi SU6600 (SEM centre, NIT Calicut Kerela)). Room temperature PL emissions of the powder samples were measured using a Shimadzu RF 5301 PC (M. S. University of Baroda, India) spectrofluorophotometer. The PL decay was measured using a home-made setup comprising a RCA 931 photomultiplier tube (PMT); after 2 min the UV-irradiated sample was placed in front of the PMT. TL glow curves were plotted between emitted TL intensity and the temperature of the sample with the help of TLD Reader (TL 1009I). ML was monitored using a home-made setup comprising a RCA 931 PMT positioned below the transparent lucite plate and connected to a storage oscilloscope (Scientific 300 MHz, SM 340 (Pt. R. S. University Raipur, India)) (14). The ML activity of
CaMgSi2O6:Eu2+ was determined for different impact velocities, ranging from 3.1305 to 1.9798 m/s. In each measurement, 4 mg of the phosphor was used. FTIR spectra were also recorded (IR-Prestige 21, Shimadzu) to evaluate the vibrational features of the sample.
Results and Discussion XRD analysis Figure 1 shows the crystal phase of the phosphor determined from the X-ray powder diffraction pattern with CuKα (λ = 1.5405 Å). Comparison of the recorded XRD patterns with the standard JCPDS card number 01-070-3482 showed good agreement. The XRD data were measured over a scattering angle range of 10° to 80°. The crystal has a monoclinic structure, with cell parameters of a = 9.7397 Å, b = 8.9174 Å, c = 5.2503 Å, α = 90°, β = 105.866 and γ = 90°, belonging to space group C2/c. The XRD pattern shows that the sample is single phased, which is consistent with JCPDS file no. 01-070-3482. The average particle size obtained from the XRD measurement is 45 nm.
TEM analysis of CaMgSi2O6:Eu2+ phosphors TEM image and SAED patterns of Eu2+-doped CaMgSi2O6 phosphors are shown in Fig. 2(a,b). The average particle size determined from the TEM images is ~ 45 nm. Particle sizes from TEM and XRD analysis are similar. The TEM analysis result confirms via the SAED pattern that the phosphor is polycrystalline, smooth and has uniform layers.
SEM analysis of CaMgSi2O6:Eu2+ phosphors
Figure 1. XRD of CaMgSi2O6:Eu
2+
phosphors.
The surface morphology of the synthesized Eu2+-doped CaMgSi2O6 phosphor was examined by SEM and it is shown in Fig. 3. From the SEM image, it can be observed that the prepared sample consists of particles 30–60 nm or more in size. In addition, there are some large aggregates caused by the high temperature heat treatment. Fig. 3(b) presents the EDX spectra of
Figure 2. TEM image and SAED patterns of CaMgSi2O6:Eu
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2+
Copyright © 2015 John Wiley & Sons, Ltd.
phosphors.
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Characterization and luminescence properties
Figure 3. SEM image (a) and EDX analysis (b) of CaMgSi2O6:Eu
the as-prepared phosphors and confirms doping of Eu2+ in the CaMgSi2O6 host. FTIR analysis The IR spectra of Eu2+-doped CaMgSi2O6 phosphor in the range 4000 to 400 cm1 are shown in Fig. 4. Vibration modes for the deformation of SiO2 at 489.92 and 632.35 cm1 and tetrahedral Si4+ lie at 682.80, 875.68, 975.98 and 1080.14 cm1. A typical feature of the IR spectrum is the observed influence of the vibration from the doping material Eu2+ in the CaMgSi2O6 host structure. The spectrum of CaMgSi2O6: Eu2+ shows a vibration band at 1654.92 and 1446.61 cm1, which indicate the influence of Eu2+. Eu2+ ions are expected to replace Ca2+ in the CaMgSi2O6 host because the ionic radii of Eu2+ and Ca2+ are 1.25 and 1.12 Å, respectively, and match closely (15). Eu2+ does not replace Mg2+ because the ionic radius of Mg2+ (0.72 Å) is far smaller than that of Eu2+ (16). The vibration modes at 1654.92 and 1446.61 cm1 represent the vibration modes of Mg2+ and Ca2+, respectively, in the CaMgSi2O6 host. When Eu2+ enters the lattice, it replaces Ca2+ in the CaMgSi2O6 host and occupies Ca2+ lattice sites due to distortion of the CaMgSi2O6 host crystal lattice (17). The original position of Ca2+ was replaced by Eu2+ and the original Ca2+ is located somewhere else. Therefore,
2+
phosphors.
the vibration mode of Ca2+ at 1446.61 cm1 is clearly observed with CaMgSi2O6:Eu2+. The vibration mode of Mg2+ was observed at 1654.92 cm1 because of the same of distortion in the CaMgSi2O6 host crystal lattice.
PL analysis Eu2+-doped phosphors showed strong absorption in the UV band and emitted a blue light when excited by UV light. The excitation and emission spectra of CaMgSi2O6:Eu2+ phosphor at room temperature are shown in Fig. 5. The excitation spectrum shows a broad absorption band at 200–400 nm in the UV range which is attributed to transition from the 4f 7 ground state of Eu2+ to the 4f 65d1 excited state. Fig. 5 shows the emission spectra of CaMgSi2O6:Eu2+ phosphor under 356 nm excitation. Under 356 nm excitation, the CaMgSi2O6:Eu2+ phosphor shows an intense and broad blue emission at 451 nm. The broadband emission centered at 451 nm can be assigned to the typical 4f 65d1 → 4f 7 transition of Eu2+, which is an allowed electrostatic dipole transition. No emission peaks of Eu3+ were observed in the spectra. This implies that the Eu3+ ions in the matrix had been completely reduced to Eu2+ in the reducing atmosphere.
400
Excitation spectra
356
Emission spectra 450
PL Intensity (a.u.)
2+
CaMgSi2O6:Eu
350 300 250 200 266
150 100 50 0 200
300
400
500
600
700
Wavelength (nm) Figure 4. FTIR spectra of CaMgSi2O6:Eu
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2+
phosphors.
Figure 5. Excitation and emission spectra of CaMgSi2O6:Eu
Copyright © 2015 John Wiley & Sons, Ltd.
2+
phosphor.
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P. Chandrakar et al. 60000
PL decay
CaMgSi2O6:Eu
2+
I ¼ A1 expð–t=τ 1 Þ þ A2 expð–t=τ 2 Þ
(1)
UV Radiation 1 (1 min) 2 (3 min) 3 (5 min) 4 (8 min)
4 2
40000
1
30000
20000
10000
where I is phosphorescence intensity, A1 and A2 are constants, t is time, and τ1 and τ2 are the decay times for the fast and slow decay exponential components, respectively. The fitting results are given in Table 1.
0 50
100
150
200
Figure 7. TL glow curve of CaMgSi2O6:Eu
TL is the emission of light from a solid, either inorganic, semiconductor or insulator in form, when it is heated after exposure to some radiation (18). Fig. 7 shows the TL glow curve of CaMgSi2O6:Eu2+ phosphors for different UV radiation times. In the present case, the TL intensity increases with increasing irradiation time up to 5 min and then decreases over time because, at a particular time, the population of trapped electrons in a metastable state reaches a maximum values. From the TL glow curve of CaMgSi2O6:Eu2+ it was observed that one broad peak exists at 151.14 °C, with another at a low temperature 80.04 °C. The corresponding depth of the trap at 80.04 °C should be shallow (low temperature). Shallow traps contribute to the initial intensity of the afterglow, and deep traps contribute to the longer duration. Therefore, TL data 120 2+
CaMgSi206:Eu
100
0.025
80
250
300
Temperature (Time in sec.)
TL analysis
Intensity (a.u.)
3
50000
TL Intensity (a.u.)
Figure 6 shows the PL decay curve of CaMgSi2O6:Eu phosphors irradiated with 254 nm light for 2 min. The phosphors show rapid decay and then long-lasting phosphorescence. The initial afterglow intensity of the phosphor was high. Decay times can be calculated using a curve fitting technique and the decay curves are fitted by the sum of two exponentials, as given below:
2+
2+
phosphor
reveal the presence of two different trapping levels in CaMgSi2O6:Eu2+ phosphor. The TL glow curve of CaMgSi2O6:Eu2+ phosphor is shown in Fig. 8. It was deconvoluted into two peaks representing curve (b) and (c). The two fitted peaks are centered around 80.04 and 151.14 °C, respectively. In order to determine the kinetic parameters of TL glow curves, Chen’s empirical formula was used. The geometrical factor μg was calculated as: μg ¼ ðT2 – Tm Þ=ðT2 – T1 Þ
(2)
where Tm is temperature corresponding to the maxima, whereas T1 and T2 correspond to the full-width at half maxima (FWHM). The μg of the two peaks were calculated and found to be 0.51 and 0.52, respectively, which are close to the value for the second-order peak (0.52). This shows that the two bands are second-order peaks. The result indicates that, after the carriers from the traps corresponding to the two bands were released, the probability of retrapping carriers is increased in comparison with that of the first-order case (19).
60 60000
CaMgSi2O6:Eu
40
5 min UV Radiation (Whole curve) (a) st (1 peak) (b) nd (c) (2 peak)
50000
0
500
1000
1500
2000
2500
3000
3500
Time (sec.) Figure 6. Decay curves of CaMgSi2O6:Eu
2+
phosphors.
TL Intensity (a.u.)
20
0
2+
40000
30000
20000
10000
Table 1. Decay curves for exponential components of CaMgSi2O6:Eu2+ phosphors Phosphors 2
CaMgSi2O6:Eu +
τ1 (s)
τ2 (s)
35.153
267.655
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0 50
100
150
200
250
300
Temperature (degree celcius) Figure 8. TL curves of CaMgSi2O6:Eu 80.04 °C (b) and 151.14 °C (c).
Copyright © 2015 John Wiley & Sons, Ltd.
2+
phosphor: whole curve (a), separated at
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Characterization and luminescence properties We used the following equation to estimate the depth of the traps, E: (3) E α ¼ cα kTm 2 =α – bα ð2kTm Þ
The trap parameters of depth E, frequency factor s and the lifetime τ at the temperature of 300 K (room temperature) are given in the Table 2, calculated according to eqns 2–5.
where k is the Boltzmann constant. The relationship between the frequency factor s and the depth E of the trap is given by
ML analysis
βE=kTm ¼ s expf–E=kTm g½1 þ ðb – 1Þ2kTm =E
Figure 9 shows the time dependence of ML induced by the impact of a piston of mass 400 g dropped from different heights on to the phosphors. Initially, the ML intensity increases with time, reaches a peak value and then decreases. The peak ML intensity increases with increasing impact velocity υ0 (= √2gh, where g is the acceleration due to gravity and h is the height through which the load was dropped) of the load used to deform the phosphor. Figure 10 shows the dependence of the ML intensity on the impact velocity of the load used to deform the phosphor. It is
(4)
where β is the heating rate, b is the order of the kinetics, which is 2 in this case. The lifetime τ of a trap with depth E at temperature T is given by τ ¼ s–1 expðE=kTÞ
(5)
Table 2. Trapping parameters of CaMgSi2O6:Eu2+ phosphors CaMgSi2O6:Eu2 + phosphors
Peak temperature Tm/K
Trap depth E (eV)
Frequency factor S (Hz)
Lifetime τ (s)
353.04 424.14
0.48 0.61
1.50 × 109 4.38 × 108
5.4 × 103 3.8 × 105
1st peak 2nd peak
8
14
2+
CaMgSi2O6:Eu
ML Intensity (a. u.)
ML Intensity (a. u.)
6
4
2
0
2+
30 cm
12
20 cm
10 8 6 4 2 0
5
10
15
20
25
5
30
20
(a)
(b) CaMgSi2O6:Eu
2+
40 cm
10
15
Time (ms)
22 20 18 16 14 12 10 8 6 4 2 0 5
10
Time (ms)
ML Intensity (a. u.)
ML Intensity (a. u.)
CaMgSi2O6:Eu
15
20
25
30
30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0
30
2+
CaMgSi2O6:Eu
50 cm
5
Time (ms)
25
10
15
20
25
30
Time (ms)
(c)
(d)
Figure 9. Mechanoluminescence behavior of CaMgSi2O6:Eu
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2+
when a load of 400 g was dropped from different heights.
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P. Chandrakar et al. 30
newly created (22). Thus, the ML intensity will increase with increasing υ0. It should be noted that the stress near the tip of a moving crack is of the order of Y/100 ~ 1010 dynes/cm2 = 109 Newton/m2 (where Y is the Young’s modulus of the materials). Thus, a fixed charge density will be produced on the newly created surfaces and the increase in the ML intensity with impact velocity will primarily be caused by the increase in the rate of newly created surface area with increasing impact velocity. Moreover, the total ML intensity will also increase with υ0 because more compression of the sample will take place with increasing impressing impact velocity.
2+
CaMgSi2O6:Eu
ML intensity (a.u.)
25
20
15 (i)
10 (ii)
5
Conclusions
0 1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
Impact velocity (m/s) Figure 10. Plot of ML intensity versus different impact velocity.
clear from the figure that the ML intensity increases linearly with the increasing impact velocity. For the ML excitation of CaMgSi2O6:Eu2+ phosphor, the following three possible models may be considered: (1) charged dislocation model, (2) piezoelectrification-induced electroluminescence model, and (3) local piezoelectricity-induced electron bombardment model. The charged dislocation model is not applicable for the following experimental reasons: (1) in elastic region, the ML intensity of aluminate and silicate phosphors is repetitive; and (2) intense ML appears when the phosphor is placed inside the transparent insulating liquid and the ML was excited by the application of hydrostatic pressure. Because the dislocations cannot move under hydrostatic pressure, the dislocation origin of ML may not be the dominating process. In the case of nanophosphors, the mean free path of detrapped electrons is of the order of the crystal size, which is in the nm range. For such a value of the mean free path, the de-trapped electrons may not obtain sufficient energy for the impact excitation of luminescence centers. In the case of EL, the intensity increases nonlinearly with the electric field (B = B0exp(b/F), where B is the EL brightness, B0 and b constants, and F is the electric field. However, practically, ML intensity increases linearly with pressure or the piezoelectric field. In fact, for the ML of CaMgSi2O6:Eu2+ phosphor, a local piezoelectricity-induced electron bombardment model must be applicable. The space group of CaMgSi2O6:Eu2+ phosphor is C2/c, which indicates that the crystal belongs to nonpiezoelectric structure. Therefore, in such crystallites the ML excitation may be caused by the local piezoelectric field near the impurities and defects in the crystals (20). During the impact of a load on the sample, new surfaces are created in which one surface will be positively charged and the other will be negatively charged. It has been shown that the piezoelectric field between the two walls of a crack is in the order of 108 v/m (21). Under such a piezoelectric field, the ejected electrons from the negatively charged surface may be accelerated and their impact on the positively charged surfaces of the cracks may excite luminescence centers such as Eu2+. Subsequently, the de-excitation of excited Eu2+ ions may give rise to the light emission due to 4f7 → 4f 65d1 transition, emitting at 450 nm. With increasing impact velocity, υ0, more compression of the sample will take place, and therefore, more surface area will be
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Eu2+-doped CaMgSi2O6 phosphors were prepared using the solid-state reaction method in a reducing atmosphere. From the sample powder, the CaMgSi2O6:Eu2+ crystal phase was identified by XRD and the optical properties were investigated. The particle size of the Eu2+-doped CaMgSi2O6 phosphors varies between 35 and 52 nm with an average value of 45 nm. XRD results also show that the synthesized CaMgSi2O6:Eu2+ phosphors are crystalline with a single phase. The excitation spectra show a broad absorption band within the 200–400 nm UV range and this is due to transition from the 4f7 (ground state) of Eu2+ to the 4f65d1 excited state. Under 356 nm radiation, the CaMgSi2O6:Eu2+ phosphor shows an intense and broad blue emission at 450 nm. In the TL glow curve, two peaks were observed at around 82.03 and 151.14 °C. The value of the trapdepths was found to be 0.48 and 0.61 eV. The ML intensity strongly depends on the impact velocity of the piston used to deform the phosphor and a linear relationship was found between the ML intensity and the impact velocity. The local piezoelectricity-induced bombardment model was shown to be responsible for the ML emission of CaMgSi2O6:Eu2+ phosphor. Acknowledgements We thank Prof. K. V. R. Murthy, M. S. University of Baroda, India, for providing the facility for recording the photoluminescence spectra.
References 1. Takasaki H, Tanabe S, Hanada T. Long-lasting afterglow characteristics of Eu,Dy co-doped SrO-Al2O3 phosphor. J Ceram Soc Jpn 1996;104:322. 2. Yamaoto H, Matsuzawa T. Mechanism of long phosphorescence of 2+ 3+ 2+ 3+ and CaAl2O4:Eu , Nd . J Luminescence SrAl2O4:Eu , Dy 1997;73:287. 3. International Commission on Illumination (CIE) (Ed.), Guide on the emergency lighting of building interiors. CIE: Wien, 1981. 4. International Commission on Illumination (CIE) (Ed.), Lighting of outdoor work places – lighting requirements for safety and security. CIE: Wien, 2005. 5. Fu ZL, Zhou SH, Zhang SY. Study on optical properties of rare-earth 3+ 3+ 3+ ions in nanocrystalline monoclinic SrAl2O4:Ln (Ln= Ce , Pr , Tb ). J Phys Chem 2005;B109:14396. 6. Clabau F, Rocquefelte X, Jobic S. Mechanism of phosphorescence 2+ appropriate for the long-lasting phosphors Eu doped SrAl2O4 with 3+ 3+ co-dopants Dy and B . Chem Mater 2005;17:3904. 7. Yu XB, Zhou CL, He XH. The influence of some processing conditions 2+ on luminescence of SrAl2O4:Eu nanoparticles produced by combustion method. Mater Lett 2004;58:1087. 8. Peng TY, Yang HP, Pu XL. Combustion synthesis and photoluminescence of SrAl2O4:Eu, Dy phosphor nanoparticles. Mater Lett 2004;58:352.
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Characterization and luminescence properties 9. Iwata NY, Lee GH, Tsunakawa S, Tokuoka Y, Kawashima N. Preparation of diopside with apatite-forming ability by sol-gel process using metal alkoxide and metal salts. Colloid Surf 2004;B33:1. 10. Jiang L, Chang C, Mao D. Luminescent properties of CaMgSi2O6 and 2+ 3+ 3+ Ca2MgSi2O7 phosphors activated by Eu , Dy and Nd . J Alloys Compd 2003;360:193. 11. Cameron M, Papike J. Structural and chemical variations in pyroxenes. J Am Miner 1981;66:1. 12. Kim JS, Kwon AK, Park YH, Chol JC, Park HL, Kim GC. Luminescent 2+ 2+ and thermal properties of full-color emitting X3MgSi2O8:Eu , Mn (X= Ba, Sr, Ca) phosphors for white LED. J Lumin 2007;122–123:583–7. 13. Jung KY, Hana KH, Kang YC, Jung HK. Preparation of CaMgSi2O6:Eu blue phosphor particles by spray pyrolysis and its VUV characteristics. Mater Chem Phys 2006;98:330. 14. Chandra BP, Tiwari S, Ramrakhiani M, Ansari MH. Mechanoluminescence in centrosymmetric crystals. Cryst Res Tech 1991;26:769.
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15. Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst 1976;A32:751. 16. Furusho H, Holsa J, Laamanen T, Lastusaari M, Nittykoski J, Okajima 2+ 3+ Y, Yamamoto A. Probing lattice defects in Sr2MgSi2O7:Eu , Dy . J Lumin 2008;128:882. 17. Chang C, Mao D. Luminescent properties of Sr2MgSi2O7 and 2+ 3+ Ca2MgSi2O7 long lasting phosphors activated by Eu , Dy . J Alloys Compd 2005;390:134. 18. Vij DR (ed.). Thermoluminescent materials. PTR prentice Hall: NJ, 1993. 19. SWS Mc Keever. Thermoluminescence of solids. London: Cambridge University Press, 1985. 20. Chandra BP, Rathore AK. Classification of mechanoluminescence. Cryst Res Tech 1995;30:885. 21. Chandra BP, Vij DR (ed.). Luminescence of Solids. (New York: Plenum), 1998; chapter 10:361–89. 22. Chandra BP, Chandra VK, Jha P, Patel R, Shende SK, Thaker S, Baghel RN. Fracto-mechanoluminescence and mechanics of fracture of solids. J Lumin 2012;132:2012–22.
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Accepted: 28 December 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2855
Characterization and luminescence properties of CaMgSi2O6:Eu2+ blue phosphor P. Chandrakar, R. N. Baghel,* D. P. Bisen and B. P. Chandra ABSTRACT: A blue CaMgSi2O6:Eu2+ phosphor was prepared by the solid-state reaction method and the phosphor characterized in terms of crystal structure, particle size, photoluminescence (PL), thermoluminescence (TL) and mechanoluminescence (ML) properties using X-ray diffraction (XRD), transmission electron microscopy (TEM), PL spectroscopy, TLD reader and ML impact technique. The XRD result shows that phosphor is formed in a single phase and has a monoclinic structure with the space group C2/c. Furthermore, the PL excitation spectra of Eu2+-doped CaMgSi2O6 phosphor showed a strong band peak at 356 nm and the PL emission spectrum has a peak at 450 nm. The depths and frequency factors of trap centers were calculated using the TL glow curve by deconvolution method in which the trap depths were found to be 0.48 and 0.61 eV. The formation of CaMgSi2O6:Eu2+ phosphor was confirmed by Fourier transform infrared spectroscopy. The ML intensity increased linearly with the impact velocity of the piston used to deform the phosphor. It was shown that the local piezoelectricityinduced electron bombardment model is responsible for the ML emission. Finally, the optical properties of CaMgSi2O6:Eu2+ phosphors are discussed. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: CaMgSi2O6:Eu2+ phosphor; TEM; photoluminescence; thermoluminescence; mechanoluminescence; solid-state reaction synthesis
Introduction Over the last 10 years, aluminate-based rare earth (RE)-ion-doped phosphors have attracted a great deal of interest because of their excellent properties, e.g., high brightness, long duration, excellent photoresistance and environmental capability. However, the properties of these phosphors are decreased greatly on soaking in water for several hours, which limits their application in pigments, paints and other fields (1,2). Recently, silicate-based phosphors have attracted much attention because of their many advantages over sulfide and aluminate phosphors (3-8). CaMgSi2O6 is one of the pyroxene minerals (diopside) and is known to be a good bioactive material (9). Jiang et al. have prepared a long-lasting alkaline earth silicate, CaMgSi2O6 activated by Eu2+, Dy3+ and Nd3+, and have shown that the phosphor has good blue emission under UV illumination (10). A characteristic feature of the CaMgSi2O6:Eu2+ (CMS:Eu2+) blue light-emitting phosphor is that it is stable and its emission efficiency decreases less than that of BaMgSi2O6: Eu2+ (BAM:Eu2+), which is widely utilized as a blue light-emitting material of Plasma display panel (PDP) or as a rare gas lamp. However, it is known that the emission intensity of CMS:Eu2+ is lower than that of the BAM:Eu2+. Accordingly, there has been a demand for the development of a CMS:Eu2+ blue light-emitting phosphor with a high emission intensity. The pyroxene structure consists of single chains of SiO4 tetrahedra extending along the c-axis of the unit cell. Cameron and Papike (11) described two types of cation positions, designated as M1 and M2. Site M1 is smaller than M2 and lies between the apices of opposing tetrahedra forming an almost regular octahedron. Site M2 lies between the bases of the tetrahedral forming distorted six- or eight-fold sites. In the diopside, position M1 is occupied by Mg and position M2 is occupied by Ca.
Luminescence 2015
Optical transitions of divalent europium (4f 6–5d1) have been investigated in many phosphors. The spectra of Eu2+-doped compounds are caused by electric dipole transitions with parity allowed so that they occur with high transition probabilities. The emission colors vary from ultraviolet to red depending on the host lattice (12). Recently, the luminescent properties of solidstate reaction-derived CaMgSi2O6:Eu2+ on UV excitation have been reported. As an important family of luminescent materials, phosphate compounds have attracted more attention because of their excellent thermal and chemical stability (13). In this work, the blue-emitting CaMgSi2O6:Eu2+ phosphor was prepared by the solid-state reaction method. The excitation spectra, emission spectra, thermoluminescence (TL), Fourier transform infrared spectroscopy (FTIR) and mechanoluminescence (ML) of the crystalline properties were investigated. The morphology of the phosphor was been characterized.
Experimental Synthesis A sample with the general formula CaMgSi2O6:Eu2+ was prepared by the solid-state reaction method. The starting reagents were CaCO3, MgO, SiO2 and Eu2O3 with small quantities of H3BO3 added as flux. The raw materials were weighed stoichiometrically and mixed homogeneously in a mortar for 2 h. The mixture contained 2.5 mol% Eu2O3. The mixture was sintered * Correspondence to: R. N. Baghel, School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur- 492010 (C.G) India. E-mail: [email protected] School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur, India
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P. Chandrakar et al. at 1000 °C for 3 h under a reducing atmosphere, which was created using activated charcoal. Characterization For structure determination, powder X-ray diffraction (XRD) patterns of the samples were recorded using Panalytical Empyrean XRD with CuKα radiation (λ = 1.5418 Å). A transmission electron microscope (TECNAI G2 20 S-TWIN (IIT Roorkee)) was used to take the TEM images. The surface morphology of the sample was analyzed using a scanning electron microscope (Hitachi SU6600 (SEM centre, NIT Calicut Kerela)). Room temperature PL emissions of the powder samples were measured using a Shimadzu RF 5301 PC (M. S. University of Baroda, India) spectrofluorophotometer. The PL decay was measured using a home-made setup comprising a RCA 931 photomultiplier tube (PMT); after 2 min the UV-irradiated sample was placed in front of the PMT. TL glow curves were plotted between emitted TL intensity and the temperature of the sample with the help of TLD Reader (TL 1009I). ML was monitored using a home-made setup comprising a RCA 931 PMT positioned below the transparent lucite plate and connected to a storage oscilloscope (Scientific 300 MHz, SM 340 (Pt. R. S. University Raipur, India)) (14). The ML activity of
CaMgSi2O6:Eu2+ was determined for different impact velocities, ranging from 3.1305 to 1.9798 m/s. In each measurement, 4 mg of the phosphor was used. FTIR spectra were also recorded (IR-Prestige 21, Shimadzu) to evaluate the vibrational features of the sample.
Results and Discussion XRD analysis Figure 1 shows the crystal phase of the phosphor determined from the X-ray powder diffraction pattern with CuKα (λ = 1.5405 Å). Comparison of the recorded XRD patterns with the standard JCPDS card number 01-070-3482 showed good agreement. The XRD data were measured over a scattering angle range of 10° to 80°. The crystal has a monoclinic structure, with cell parameters of a = 9.7397 Å, b = 8.9174 Å, c = 5.2503 Å, α = 90°, β = 105.866 and γ = 90°, belonging to space group C2/c. The XRD pattern shows that the sample is single phased, which is consistent with JCPDS file no. 01-070-3482. The average particle size obtained from the XRD measurement is 45 nm.
TEM analysis of CaMgSi2O6:Eu2+ phosphors TEM image and SAED patterns of Eu2+-doped CaMgSi2O6 phosphors are shown in Fig. 2(a,b). The average particle size determined from the TEM images is ~ 45 nm. Particle sizes from TEM and XRD analysis are similar. The TEM analysis result confirms via the SAED pattern that the phosphor is polycrystalline, smooth and has uniform layers.
SEM analysis of CaMgSi2O6:Eu2+ phosphors
Figure 1. XRD of CaMgSi2O6:Eu
2+
phosphors.
The surface morphology of the synthesized Eu2+-doped CaMgSi2O6 phosphor was examined by SEM and it is shown in Fig. 3. From the SEM image, it can be observed that the prepared sample consists of particles 30–60 nm or more in size. In addition, there are some large aggregates caused by the high temperature heat treatment. Fig. 3(b) presents the EDX spectra of
Figure 2. TEM image and SAED patterns of CaMgSi2O6:Eu
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2+
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phosphors.
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Characterization and luminescence properties
Figure 3. SEM image (a) and EDX analysis (b) of CaMgSi2O6:Eu
the as-prepared phosphors and confirms doping of Eu2+ in the CaMgSi2O6 host. FTIR analysis The IR spectra of Eu2+-doped CaMgSi2O6 phosphor in the range 4000 to 400 cm1 are shown in Fig. 4. Vibration modes for the deformation of SiO2 at 489.92 and 632.35 cm1 and tetrahedral Si4+ lie at 682.80, 875.68, 975.98 and 1080.14 cm1. A typical feature of the IR spectrum is the observed influence of the vibration from the doping material Eu2+ in the CaMgSi2O6 host structure. The spectrum of CaMgSi2O6: Eu2+ shows a vibration band at 1654.92 and 1446.61 cm1, which indicate the influence of Eu2+. Eu2+ ions are expected to replace Ca2+ in the CaMgSi2O6 host because the ionic radii of Eu2+ and Ca2+ are 1.25 and 1.12 Å, respectively, and match closely (15). Eu2+ does not replace Mg2+ because the ionic radius of Mg2+ (0.72 Å) is far smaller than that of Eu2+ (16). The vibration modes at 1654.92 and 1446.61 cm1 represent the vibration modes of Mg2+ and Ca2+, respectively, in the CaMgSi2O6 host. When Eu2+ enters the lattice, it replaces Ca2+ in the CaMgSi2O6 host and occupies Ca2+ lattice sites due to distortion of the CaMgSi2O6 host crystal lattice (17). The original position of Ca2+ was replaced by Eu2+ and the original Ca2+ is located somewhere else. Therefore,
2+
phosphors.
the vibration mode of Ca2+ at 1446.61 cm1 is clearly observed with CaMgSi2O6:Eu2+. The vibration mode of Mg2+ was observed at 1654.92 cm1 because of the same of distortion in the CaMgSi2O6 host crystal lattice.
PL analysis Eu2+-doped phosphors showed strong absorption in the UV band and emitted a blue light when excited by UV light. The excitation and emission spectra of CaMgSi2O6:Eu2+ phosphor at room temperature are shown in Fig. 5. The excitation spectrum shows a broad absorption band at 200–400 nm in the UV range which is attributed to transition from the 4f 7 ground state of Eu2+ to the 4f 65d1 excited state. Fig. 5 shows the emission spectra of CaMgSi2O6:Eu2+ phosphor under 356 nm excitation. Under 356 nm excitation, the CaMgSi2O6:Eu2+ phosphor shows an intense and broad blue emission at 451 nm. The broadband emission centered at 451 nm can be assigned to the typical 4f 65d1 → 4f 7 transition of Eu2+, which is an allowed electrostatic dipole transition. No emission peaks of Eu3+ were observed in the spectra. This implies that the Eu3+ ions in the matrix had been completely reduced to Eu2+ in the reducing atmosphere.
400
Excitation spectra
356
Emission spectra 450
PL Intensity (a.u.)
2+
CaMgSi2O6:Eu
350 300 250 200 266
150 100 50 0 200
300
400
500
600
700
Wavelength (nm) Figure 4. FTIR spectra of CaMgSi2O6:Eu
Luminescence 2015
2+
phosphors.
Figure 5. Excitation and emission spectra of CaMgSi2O6:Eu
Copyright © 2015 John Wiley & Sons, Ltd.
2+
phosphor.
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P. Chandrakar et al. 60000
PL decay
CaMgSi2O6:Eu
2+
I ¼ A1 expð–t=τ 1 Þ þ A2 expð–t=τ 2 Þ
(1)
UV Radiation 1 (1 min) 2 (3 min) 3 (5 min) 4 (8 min)
4 2
40000
1
30000
20000
10000
where I is phosphorescence intensity, A1 and A2 are constants, t is time, and τ1 and τ2 are the decay times for the fast and slow decay exponential components, respectively. The fitting results are given in Table 1.
0 50
100
150
200
Figure 7. TL glow curve of CaMgSi2O6:Eu
TL is the emission of light from a solid, either inorganic, semiconductor or insulator in form, when it is heated after exposure to some radiation (18). Fig. 7 shows the TL glow curve of CaMgSi2O6:Eu2+ phosphors for different UV radiation times. In the present case, the TL intensity increases with increasing irradiation time up to 5 min and then decreases over time because, at a particular time, the population of trapped electrons in a metastable state reaches a maximum values. From the TL glow curve of CaMgSi2O6:Eu2+ it was observed that one broad peak exists at 151.14 °C, with another at a low temperature 80.04 °C. The corresponding depth of the trap at 80.04 °C should be shallow (low temperature). Shallow traps contribute to the initial intensity of the afterglow, and deep traps contribute to the longer duration. Therefore, TL data 120 2+
CaMgSi206:Eu
100
0.025
80
250
300
Temperature (Time in sec.)
TL analysis
Intensity (a.u.)
3
50000
TL Intensity (a.u.)
Figure 6 shows the PL decay curve of CaMgSi2O6:Eu phosphors irradiated with 254 nm light for 2 min. The phosphors show rapid decay and then long-lasting phosphorescence. The initial afterglow intensity of the phosphor was high. Decay times can be calculated using a curve fitting technique and the decay curves are fitted by the sum of two exponentials, as given below:
2+
2+
phosphor
reveal the presence of two different trapping levels in CaMgSi2O6:Eu2+ phosphor. The TL glow curve of CaMgSi2O6:Eu2+ phosphor is shown in Fig. 8. It was deconvoluted into two peaks representing curve (b) and (c). The two fitted peaks are centered around 80.04 and 151.14 °C, respectively. In order to determine the kinetic parameters of TL glow curves, Chen’s empirical formula was used. The geometrical factor μg was calculated as: μg ¼ ðT2 – Tm Þ=ðT2 – T1 Þ
(2)
where Tm is temperature corresponding to the maxima, whereas T1 and T2 correspond to the full-width at half maxima (FWHM). The μg of the two peaks were calculated and found to be 0.51 and 0.52, respectively, which are close to the value for the second-order peak (0.52). This shows that the two bands are second-order peaks. The result indicates that, after the carriers from the traps corresponding to the two bands were released, the probability of retrapping carriers is increased in comparison with that of the first-order case (19).
60 60000
CaMgSi2O6:Eu
40
5 min UV Radiation (Whole curve) (a) st (1 peak) (b) nd (c) (2 peak)
50000
0
500
1000
1500
2000
2500
3000
3500
Time (sec.) Figure 6. Decay curves of CaMgSi2O6:Eu
2+
phosphors.
TL Intensity (a.u.)
20
0
2+
40000
30000
20000
10000
Table 1. Decay curves for exponential components of CaMgSi2O6:Eu2+ phosphors Phosphors 2
CaMgSi2O6:Eu +
τ1 (s)
τ2 (s)
35.153
267.655
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0 50
100
150
200
250
300
Temperature (degree celcius) Figure 8. TL curves of CaMgSi2O6:Eu 80.04 °C (b) and 151.14 °C (c).
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2+
phosphor: whole curve (a), separated at
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Characterization and luminescence properties We used the following equation to estimate the depth of the traps, E: (3) E α ¼ cα kTm 2 =α – bα ð2kTm Þ
The trap parameters of depth E, frequency factor s and the lifetime τ at the temperature of 300 K (room temperature) are given in the Table 2, calculated according to eqns 2–5.
where k is the Boltzmann constant. The relationship between the frequency factor s and the depth E of the trap is given by
ML analysis
βE=kTm ¼ s expf–E=kTm g½1 þ ðb – 1Þ2kTm =E
Figure 9 shows the time dependence of ML induced by the impact of a piston of mass 400 g dropped from different heights on to the phosphors. Initially, the ML intensity increases with time, reaches a peak value and then decreases. The peak ML intensity increases with increasing impact velocity υ0 (= √2gh, where g is the acceleration due to gravity and h is the height through which the load was dropped) of the load used to deform the phosphor. Figure 10 shows the dependence of the ML intensity on the impact velocity of the load used to deform the phosphor. It is
(4)
where β is the heating rate, b is the order of the kinetics, which is 2 in this case. The lifetime τ of a trap with depth E at temperature T is given by τ ¼ s–1 expðE=kTÞ
(5)
Table 2. Trapping parameters of CaMgSi2O6:Eu2+ phosphors CaMgSi2O6:Eu2 + phosphors
Peak temperature Tm/K
Trap depth E (eV)
Frequency factor S (Hz)
Lifetime τ (s)
353.04 424.14
0.48 0.61
1.50 × 109 4.38 × 108
5.4 × 103 3.8 × 105
1st peak 2nd peak
8
14
2+
CaMgSi2O6:Eu
ML Intensity (a. u.)
ML Intensity (a. u.)
6
4
2
0
2+
30 cm
12
20 cm
10 8 6 4 2 0
5
10
15
20
25
5
30
20
(a)
(b) CaMgSi2O6:Eu
2+
40 cm
10
15
Time (ms)
22 20 18 16 14 12 10 8 6 4 2 0 5
10
Time (ms)
ML Intensity (a. u.)
ML Intensity (a. u.)
CaMgSi2O6:Eu
15
20
25
30
30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0
30
2+
CaMgSi2O6:Eu
50 cm
5
Time (ms)
25
10
15
20
25
30
Time (ms)
(c)
(d)
Figure 9. Mechanoluminescence behavior of CaMgSi2O6:Eu
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2+
when a load of 400 g was dropped from different heights.
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P. Chandrakar et al. 30
newly created (22). Thus, the ML intensity will increase with increasing υ0. It should be noted that the stress near the tip of a moving crack is of the order of Y/100 ~ 1010 dynes/cm2 = 109 Newton/m2 (where Y is the Young’s modulus of the materials). Thus, a fixed charge density will be produced on the newly created surfaces and the increase in the ML intensity with impact velocity will primarily be caused by the increase in the rate of newly created surface area with increasing impact velocity. Moreover, the total ML intensity will also increase with υ0 because more compression of the sample will take place with increasing impressing impact velocity.
2+
CaMgSi2O6:Eu
ML intensity (a.u.)
25
20
15 (i)
10 (ii)
5
Conclusions
0 1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
Impact velocity (m/s) Figure 10. Plot of ML intensity versus different impact velocity.
clear from the figure that the ML intensity increases linearly with the increasing impact velocity. For the ML excitation of CaMgSi2O6:Eu2+ phosphor, the following three possible models may be considered: (1) charged dislocation model, (2) piezoelectrification-induced electroluminescence model, and (3) local piezoelectricity-induced electron bombardment model. The charged dislocation model is not applicable for the following experimental reasons: (1) in elastic region, the ML intensity of aluminate and silicate phosphors is repetitive; and (2) intense ML appears when the phosphor is placed inside the transparent insulating liquid and the ML was excited by the application of hydrostatic pressure. Because the dislocations cannot move under hydrostatic pressure, the dislocation origin of ML may not be the dominating process. In the case of nanophosphors, the mean free path of detrapped electrons is of the order of the crystal size, which is in the nm range. For such a value of the mean free path, the de-trapped electrons may not obtain sufficient energy for the impact excitation of luminescence centers. In the case of EL, the intensity increases nonlinearly with the electric field (B = B0exp(b/F), where B is the EL brightness, B0 and b constants, and F is the electric field. However, practically, ML intensity increases linearly with pressure or the piezoelectric field. In fact, for the ML of CaMgSi2O6:Eu2+ phosphor, a local piezoelectricity-induced electron bombardment model must be applicable. The space group of CaMgSi2O6:Eu2+ phosphor is C2/c, which indicates that the crystal belongs to nonpiezoelectric structure. Therefore, in such crystallites the ML excitation may be caused by the local piezoelectric field near the impurities and defects in the crystals (20). During the impact of a load on the sample, new surfaces are created in which one surface will be positively charged and the other will be negatively charged. It has been shown that the piezoelectric field between the two walls of a crack is in the order of 108 v/m (21). Under such a piezoelectric field, the ejected electrons from the negatively charged surface may be accelerated and their impact on the positively charged surfaces of the cracks may excite luminescence centers such as Eu2+. Subsequently, the de-excitation of excited Eu2+ ions may give rise to the light emission due to 4f7 → 4f 65d1 transition, emitting at 450 nm. With increasing impact velocity, υ0, more compression of the sample will take place, and therefore, more surface area will be
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Eu2+-doped CaMgSi2O6 phosphors were prepared using the solid-state reaction method in a reducing atmosphere. From the sample powder, the CaMgSi2O6:Eu2+ crystal phase was identified by XRD and the optical properties were investigated. The particle size of the Eu2+-doped CaMgSi2O6 phosphors varies between 35 and 52 nm with an average value of 45 nm. XRD results also show that the synthesized CaMgSi2O6:Eu2+ phosphors are crystalline with a single phase. The excitation spectra show a broad absorption band within the 200–400 nm UV range and this is due to transition from the 4f7 (ground state) of Eu2+ to the 4f65d1 excited state. Under 356 nm radiation, the CaMgSi2O6:Eu2+ phosphor shows an intense and broad blue emission at 450 nm. In the TL glow curve, two peaks were observed at around 82.03 and 151.14 °C. The value of the trapdepths was found to be 0.48 and 0.61 eV. The ML intensity strongly depends on the impact velocity of the piston used to deform the phosphor and a linear relationship was found between the ML intensity and the impact velocity. The local piezoelectricity-induced bombardment model was shown to be responsible for the ML emission of CaMgSi2O6:Eu2+ phosphor. Acknowledgements We thank Prof. K. V. R. Murthy, M. S. University of Baroda, India, for providing the facility for recording the photoluminescence spectra.
References 1. Takasaki H, Tanabe S, Hanada T. Long-lasting afterglow characteristics of Eu,Dy co-doped SrO-Al2O3 phosphor. J Ceram Soc Jpn 1996;104:322. 2. Yamaoto H, Matsuzawa T. Mechanism of long phosphorescence of 2+ 3+ 2+ 3+ and CaAl2O4:Eu , Nd . J Luminescence SrAl2O4:Eu , Dy 1997;73:287. 3. International Commission on Illumination (CIE) (Ed.), Guide on the emergency lighting of building interiors. CIE: Wien, 1981. 4. International Commission on Illumination (CIE) (Ed.), Lighting of outdoor work places – lighting requirements for safety and security. CIE: Wien, 2005. 5. Fu ZL, Zhou SH, Zhang SY. Study on optical properties of rare-earth 3+ 3+ 3+ ions in nanocrystalline monoclinic SrAl2O4:Ln (Ln= Ce , Pr , Tb ). J Phys Chem 2005;B109:14396. 6. Clabau F, Rocquefelte X, Jobic S. Mechanism of phosphorescence 2+ appropriate for the long-lasting phosphors Eu doped SrAl2O4 with 3+ 3+ co-dopants Dy and B . Chem Mater 2005;17:3904. 7. Yu XB, Zhou CL, He XH. The influence of some processing conditions 2+ on luminescence of SrAl2O4:Eu nanoparticles produced by combustion method. Mater Lett 2004;58:1087. 8. Peng TY, Yang HP, Pu XL. Combustion synthesis and photoluminescence of SrAl2O4:Eu, Dy phosphor nanoparticles. Mater Lett 2004;58:352.
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Characterization and luminescence properties 9. Iwata NY, Lee GH, Tsunakawa S, Tokuoka Y, Kawashima N. Preparation of diopside with apatite-forming ability by sol-gel process using metal alkoxide and metal salts. Colloid Surf 2004;B33:1. 10. Jiang L, Chang C, Mao D. Luminescent properties of CaMgSi2O6 and 2+ 3+ 3+ Ca2MgSi2O7 phosphors activated by Eu , Dy and Nd . J Alloys Compd 2003;360:193. 11. Cameron M, Papike J. Structural and chemical variations in pyroxenes. J Am Miner 1981;66:1. 12. Kim JS, Kwon AK, Park YH, Chol JC, Park HL, Kim GC. Luminescent 2+ 2+ and thermal properties of full-color emitting X3MgSi2O8:Eu , Mn (X= Ba, Sr, Ca) phosphors for white LED. J Lumin 2007;122–123:583–7. 13. Jung KY, Hana KH, Kang YC, Jung HK. Preparation of CaMgSi2O6:Eu blue phosphor particles by spray pyrolysis and its VUV characteristics. Mater Chem Phys 2006;98:330. 14. Chandra BP, Tiwari S, Ramrakhiani M, Ansari MH. Mechanoluminescence in centrosymmetric crystals. Cryst Res Tech 1991;26:769.
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15. Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst 1976;A32:751. 16. Furusho H, Holsa J, Laamanen T, Lastusaari M, Nittykoski J, Okajima 2+ 3+ Y, Yamamoto A. Probing lattice defects in Sr2MgSi2O7:Eu , Dy . J Lumin 2008;128:882. 17. Chang C, Mao D. Luminescent properties of Sr2MgSi2O7 and 2+ 3+ Ca2MgSi2O7 long lasting phosphors activated by Eu , Dy . J Alloys Compd 2005;390:134. 18. Vij DR (ed.). Thermoluminescent materials. PTR prentice Hall: NJ, 1993. 19. SWS Mc Keever. Thermoluminescence of solids. London: Cambridge University Press, 1985. 20. Chandra BP, Rathore AK. Classification of mechanoluminescence. Cryst Res Tech 1995;30:885. 21. Chandra BP, Vij DR (ed.). Luminescence of Solids. (New York: Plenum), 1998; chapter 10:361–89. 22. Chandra BP, Chandra VK, Jha P, Patel R, Shende SK, Thaker S, Baghel RN. Fracto-mechanoluminescence and mechanics of fracture of solids. J Lumin 2012;132:2012–22.
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