Electron Spin Resonance and optical absorption spectroscopic studies of manganese centers in aluminium lead borate glasses

Electron Spin Resonance and optical absorption spectroscopic studies of manganese centers in aluminium lead borate glasses

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 105–109 Contents lists available at SciVerse ScienceDirect Spectrochim...

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 105–109

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Electron Spin Resonance and optical absorption spectroscopic studies of manganese centers in aluminium lead borate glasses G. SivaRamaiah a,⇑, J. LakshmanaRao b,1 a b

Department of Physics, Government College for Men, Kadapa 516004, India Department of Physics, Sri Venkateswara University, Tirupati 517502, India

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

" We report for the first time the

" " " "

optical absorption and EPR in this glass system. The Gibbs free energy is new and interesting in this glass system. The optical results are interesting. Our optical results are supported by recent literature on oxide glasses. The relaxation times are useful for knowing the environment of dopants in glasses.

a r t i c l e

i n f o

Article history: Received 29 March 2012 Received in revised form 7 July 2012 Accepted 12 August 2012 Available online 19 August 2012 Keywords: Glasses EPR Optical absorption Magnetic properties Band gap and Urbach energy

a b s t r a c t Electron Spin Resonance (ESR) and optical absorption studies of 5Al2O3 + 75H3BO3 + (20x)PbO + xMnSO4 (where x = 0.5, 1,1.5 and 2 mol% of MnSO4) glasses at room temperature have been studied. The ESR spectrum of all the glasses exhibits resonance signals with effective isotropic g values at 2.0, 3.3 and 4.3. The ESR resonance signal at isotropic g  2.0 has been attributed to Mn2+ centers in an octahedral symmetry. The ESR resonance signals at isotropic g  3.3 and 4.3 have been attributed to the rhombic symmetry of the Mn2+ ions. The zero-field splitting parameter (zfs) has been calculated from the intensities of the allowed hyperfine lines. The optical absorption spectrum exhibits an intense band in the visible region and it has been attributed to 5Eg ? 5T2g transition of Mn3+centers in an octahedral environment. The optical band gap and the Urbach energies have been calculated from the ultraviolet absorption edges. Ó 2012 Elsevier B.V. All rights reserved.

Introduction Electron Spin Resonance (ESR) spectroscopy is one of the most efficient tools for the characterization of non-crystalline solids like glasses. Manganese ions have been used as paramagnetic probes for exploring the structure and properties of glassy materials. The coordination, bonding and covalency of paramagnetic metal ions in glasses are very helpful in understanding the structure of the

⇑ Corresponding author. Tel./fax: +91 8562 244422. E-mail addresses: [email protected] (G. SivaRamaiah), [email protected] (J. LakshmanaRao). 1 Tel.: +91 877 2249666x272. 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.08.023

glassy materials. The variation in the chemical composition of glass may change the local environment of transition metal ion embedded into the glass, leading to ligand field variations, which may be transmitted in the optical absorption and EPR spectra [1]. The ESR spectra of transition metal ions in glasses give the ground state properties of the species. The optical absorption spectra give the excited state properties of the species. In order to understand the complete spectral properties, both ESR and optical absorption methods are necessary. The knowledge obtained from optical absorption can be used to interpret the details of the EPR spectra. The glasses with Al2O3 and PbO have been little studied [2]. The high PbO contents, which have been reported earlier [2], indicate that Pb is not acting purely as a modifier in the glasses.

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By understanding the structure of the glass and the effect of composition on the extension of the lone pair cations such as Al3+ and Pb2+, the paramagnetic and optical properties can be better explained [2]. The transition metal ions doped glasses can be used as magnetic refrigerants over wide range of temperatures. The purpose of this detailed research is described as follows. (1) To study the ESR and optical absorption studies of Mn2+ centers in aluminium lead borate glasses at room temperature. (2) It is proposed to study the population difference between the Zeeman levels and to evaluate the magnetic susceptibility. Further the optical band gap energy and Urbach energy of manganese ions in glasses have been studied. To the best of our knowledge, there are no previous reports on the above parameters in this glass system. Therefore the authors have undertaken the detailed studies using EPR and optical absorption as analytical methods. Experimental procedure Glass synthesis The glasses were prepared using melt quenching technique. The starting materials used for the preparation of glasses were analar Al2O3, H3BO4, PbO and MnSO4. The chemicals were weighed accurately according to their molecular weights (mol%) in an electrical balance. These were mixed thoroughly and ground to fine powder with an agate mortar for nearly 30 min. The batches were taken in porcelain crucibles and melted in an electric furnace at 1174 K for 3 h. The melts were then poured on a brass plate and pressed quickly with another brass plate. The glasses thus obtained were transparent and pink in color. The pink color is due to the presence of both Mn2+ and Mn3+ ions in the glasses. Care should be taken to obtain glasses of uniform thickness (approximately 1 mm) for recording optical absorption spectra. The glasses were annealed at 330 K for several hours to remove the thermal strains. Good quality glasses obtained after polishing were used for optical measurements. ESR measurements The ESR spectra were recorded at room temperature on a JEOL FE1X EPR spectrometer. The ESR spectrometer was operated at the X-band frequency with a modulation frequency of 100 kHz. The microwave frequency was kept at 9.205 GHz. The magnetic field was scanned from 0 to 500 mT with a scan speed of 25 mT. Powdered glass specimen of 100 mg was taken in a quartz tube for EPR measurements. The EPR spectrum of the CuSO45H2O powdered material was also recorded at 9.205 GHz as a reference to calculate the absolute number of spins. Optical absorption measurements The optical absorption spectra of all the glass samples doped with manganese centers were recorded at room temperature on a JASCO UV–Vis–NIR spectrophotometer (model V-570) in the wavelength region from 400 to 600 nm. The band position is measured digitally and the accuracy is ±1 nm. Results ESR studies ESR studies at room temperature No EPR resonance signal was detected in the spectra of undoped glasses. This indicates that the starting chemicals were free from paramagnetic impurities. When paramagnetic impurities such as

manganese ions were inserted into APB (aluminium lead borate) glasses, the EPR spectra of all the investigated samples exhibit resonance signals due to Mn2+ ions entering the glass as paramagnetic probes. The ESR spectrum of Mn2+ ions exhibits resonance signal with effective isotropic g value at g  2.0 and is shown in Fig. 1. The hyperfine sextet is observed for APB 0.5 Mn glass at isotropic g  2.0 and is absent for the remaining glasses. In addition to this, two broad peaks were also observed at isotropic g  3.3 and 4.3 for all the glasses. The resonance signal centered at isotropic g  4.3 is less intense and broadened due to the unresolved hyperfine structure (hfs). Overlapping this signal, a small resonance signal was observed for APB 0.5 Mn glass. As the starting chemicals used in the present work contain trace amounts (10–30 ppm) of transition metal (TM) impurities (mostly Fe), we observed a weak signal of Fe3+ ions in undoped glasses also. Nevertheless, the resonance signal at g  4.3 is often observed for Fe3+ centers for trace amounts, and we assigned the resonance signal at g  4.3 to impurities of Fe3+ (3d5, 6S) centers. Generally EPR studies of Mn2+ ions were studied at lower concentrations of manganese ions [3]. The hyperfine lines disappeared for larger than 0.5 mol% of Mn2+ ions. Afterwards the intensity of isotropic g  2.0 resonance signal increases and no significant changes are observed. The EPR spectra of these glasses are similar to those reported for manganese ions [4–15] in several glass systems. Calculation of absolute number of spins The population difference between the Zeeman levels (N) can be calculated using the expression given by Weil et al. [16]. The variation of absolute number of spins with concentration is shown in Fig. 2. Gibbs energy The Gibbs energy DG [17,18] can be calculated using the expression developed by SivaRamaiah et al. [19]. The Gibbs energy of the Mn2+ center at g  2.0 for APB 0.5 Mn glass has been calculated to be 30 (kJ/mol). This calculated value is consistent with the values reported for glasses and minerals in the literature [19,20]. The Gibbs energy increases with the concentration of manganese ions and is shown in Fig. 2. The DG is a kind of chemical potential which indicates the stability of magnetic system. If DG is larger, stability of magnetic system is smaller. Magnetic susceptibility The magnetic susceptibility v for the isotropic g  2.0 resonance signal has been calculated using the expression given by Sreekanth Chakradhar et al. [3]. The magnetic susceptibilities were calculated for all glasses and are presented in Table 1. The magnetic susceptibility for APB 0.5 Mn glass at 295 K is calculated as 26  104 m3/kg. The magnetic susceptibilities increase from 26 to 94  104 m3/kg when the concentration increases from 0.5 to 2 mol% of manganese ions. This is due to increase in number of spins with the composition. The calculated magnetic susceptibilities are consistent with the values reported in literature [3]. Table 1 shows the number of spins (N), Gibbs energy (DG), magnetic susceptibility (v), band gap energy (Eg) and Urbach energy (DE) for manganese ions doped in aluminium lead borate glasses. Zero-field splitting parameter The zero-field splitting (zfs) arises due to a combined action of the spin-orbit (and electronic spin–spin) coupling taken as perturbation on the crystal field (CF) states [21,22]. The zfs parameter is usually defined as D. The zfs is the removal of spin microstate degeneracy for systems with S > 1/2 in the absence of an applied field. In the present case, D is axial form of zfs. The zero-field splitting parameter is calculated using the formula [21]. The zfs param-

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(kJ/mol)

30

N x 10

17

40

(Spin/100mg), ΔG

Fig. 1. Electron Spin Resonance spectra of 0.5, 1, 1.5 and 2 mol% of manganese ions doped in aluminium lead borate glasses observed at room temperature.

Table 1 Number of spins (N), Gibbs energy (DG), magnetic susceptibility (v), band gap energy (Eg) and Urbach energy (DE) for manganese ions doped in aluminium lead borate glasses.

35 ΔG

25 20 N

15

1.0

1.5

2.0

Fig. 2. Gibbs energy and number of spins of manganese ions in aluminium lead borate glasses against concentration of manganese ions (mol%).

eter D for APB 0.5 Mn glass is found to be 25 mT and is of the same order as reported by Sreekanth Chakradhar et al. [8]. Effective magnetic moment (leff) The theoretical magnetic moment for high spin Mn2+ is 5.92 lB (lB is Bohr magneton) where as the experimental effective magnetic moment [23]

N

DG (kJ/ mol)

v (m3/kg)

Eg (eV)

DE (eV)

APB 0.5 Mn APB 1.0 Mn APB 1.5 Mn APB 2 Mn

10.5  1017

30

26  104

3.60

0.41

16.0  1017

31

39.5  104

3.50

0.34

26.2  1017

32

65  104

3.45

0.29

33

4





38.2  10

17

94  10

where C is Curie constant (0.76 emu/mol) and N is number of spins per m3 (10.5  1021) is found to be 5.91lB.

Concentration (mol%)

pffiffiffiffiffiffiffiffiffiffiffi 3k C

N (spin/ 100 mg)

10 0.5

leff ¼ pffiffiffiffiB

Glass

ð1Þ

Hyperfine splitting constant The isotropic hyperfine splitting constant A can be calculated using the expression given by Sreekanth Chakradhar et al. [3]. The hyperfine-splitting constant for APB 0.5 Mn glass is found to be 9 mT and is of the same order as reported in literature [8]. Spin-lattice relaxation time The spin-lattice relaxation time T1 can be calculated using the expression [20]. The spin-lattice relaxation time for Mn2+ ions at g  2.0 at room temperature is found to be 120 ps, which is of the same order of magnitude as reported in the glasses [20]. It is

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observed that the obtained T1 values are independent of concentration. Spin–spin relaxation time The spin–spin relaxation time T2 can be calculated using the expression [20]. The T2 value is determined as 116 ns at 295 K for APB 0.5 Mn glass and is in good agreement with the reported value [20]. The T2 value decreases from 116 to 32 ns with increasing concentration from 0.5 to 2 mol% manganese ions in APB glasses. Optical studies The optical absorption spectrum of manganese centers in APB glasses (Fig. 3) shows one broad band at 471 nm for APB 0.5 Mn glass, and this band has been attributed to 5Eg ? 5T2g transition of Mn3+ ions [3]. From the ultraviolet absorption edges, the band gaps and the Urbach energies have been determined and are presented in Table 1. The band gap and Urbach energy for APB 0 Mn glass is found to be 3.65 and 0.52 eV, respectively. The study of optical absorption spectra in glasses provide crucial information about the band structure and the band gap in non-crystalline materials. An expression for absorption coefficient a as a function of phonon energy hm for direct and indirect optical transitions can be written as [24].



a0 ðhm  Eg Þn hm

ð2Þ

where the exponent n is an index that can take any value between 1/2 and 2 [24]. a0 is a constant related to the extent of the band tailing, and Eg is optical band gap energy. By plotting (ahm)1/2 versus hm, the values of Eg are obtained by extrapolating (ahm)1/2 to 0 for indirect transitions. The reciprocal of slope of ln a and hm graph gives the Urbach energy [8]. Refractive index from band gap energy The refractive index from band gap energy can be calculated using the expression [25].

ðn2  1Þ ¼ 1  ðEg =20Þ1=2 ðn2 þ 2Þ

ð3Þ

where n is refractive index and Eg is indirect band gap energy. The value of Eg is different for indirect and direct transitions. The refractive index for APB 0 Mn glass is found to be 1.17. It increases slightly

absorbance (arb.units)

5E g

1.0

5T

2g

471 nm

0.8

0.6

0.4 400

450 500 550 Wavelength (nm)

600

Fig. 3. Optical absorption spectrum of 1 mol% of manganese ions in aluminium lead borate glass observed at room temperature.

when the concentration increases from 0.5 to 1.5 mol% of manganese ions.

Discussion ESR studies In case of 3d5 transition metal ions, it is known that axial distortion of octahedral symmetry gives rise to three Kramers’ doublets, l ± 5/2>, l ± 3/2> and l ± 1/2>. The external magnetic field (B) lifts the spin degeneracy of the Kramers’ doublets. As the zero field splitting is normally much greater than the Zeeman field, the resonance signals observed are due to transitions within Kramers’ doublets split by B. The resonance signal at isotropic g  2.0 arises from the energy levels of the lower Kramers’ doublet and can be attributed to 3d5 ion in octahedral symmetry. The resonance signals at isotropic g  3.3 and 4.3 are due to the rhombic environment of Mn2+ ions [3] and arises from the middle Kramers’ doublets. The resonance signal at isotropic g  2.0 shows six hyperfine lines for APB 0.5 Mn glass due to the interaction of electron spin of manganese ions with its nuclear spin I = 5/2. The EPR spectra of Mn2+ centers in APB glasses have been studied systematically for different manganese ions concentration. The structure of the spectra strongly depends on Mn2+ centers concentration. At lower concentrations of Mn2+ ions (60.5 mol%) (Fig. 1), the spectrum shows a sextet hyperfine structure. When the concentration of Mn2+ ions is increased beyond 0.5 mol%, the sextet hyperfine structure disappears, leaving behind single broad line due to ligand field fluctuations in the Mn2+ ion vicinity and also due to the dipolar interactions. It is observed that the isotropic g  2.0 resonance signal is more intense than the resonance signals at g  3.3 and 4.3. This indicates that more number of Mn2+ centers is present in an octahedral environment than in the rhombic environment [3]. The absence of the resolved hfs at g  2.0 resonance in all the glasses except for APB 0.5 Mn glass strongly indicates that Mn2+ centers are in asymmetric sites (octahedral). The calculated population difference between the Zeeman levels (N) for the Mn2+ center at isotropic g  2.0 in APB glass at room temperature is of the order of 1017 spin/g and agrees with that for Mn2+ center in alkali lead tetraborate glasses [3]. The number of spins calculated for APB 0.5 Mn, APB 1 Mn, APB 1.5 Mn, APB 2 Mn glasses are presented in Table 1. It is observed from Table 1 that the number of spins increases with increasing concentration of Mn2+ ions. The increase in number of spins with the concentration of Mn2+ ions is due to the increase of paramagnetic centers. The Gibbs energy is useful for coupling the thermodynamic parameters with the phase equilibrium calculations [19]. The Gibbs energy is useful for knowing the stability of the thermochemical processes such as the isobaric and isothermal processes. The Gibbs energy increases with increasing concentration of Mn2+ ions (Fig. 2 and Table 1). This is due to the increase in population difference between the Zeeman levels with the increase in concentration of manganese ions. The calculated magnetic susceptibilities confirm the paramagnetic nature of glasses and offer useful information concerning the distribution of manganese ions in the glass and the nature of the magnetic interactions between manganese ions in various compositions ranges [26]. The theoretical and experimental magnetic moments are equal, thus indicating high spin Mn2+ ions. The equality of the ‘theoretical and experimental magnetic moments indicates that the orbital moment (L) is quenched [21,22]. The value of the ‘magnetic moment’ is indicative of the value of the effective spin quantum num-

G. SivaRamaiah, J. LakshmanaRao / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 105–109

ber (S) [21,22]. The experimental effective magnetic moments are equal for all the glasses and close to those for the free Mn2+ ions. This indicates that the paramagnetic interactions are predominant in the glasses [26]. This is due to effective spin magnetic moment. EPR is active for Mn2+ ions and optical absorption is active for Mn3+ ions in oxide glasses. The change of oxidation state for manganese ions occurs during sample preparation such as melting. Therefore single and an intense band is observed in the optical absorption spectrum. The calculated hyperfine splitting constant value (9 mT) indicates that ionic bonding is present between the TM ion and nearby ions in the glass. The calculated T1 value is supporting for the pairs or the surface Mn2+ ions in clusters with strong spin-lattice interactions [20]. This indicates that the structural environment is invariant between the TM ion and the lattice with the concentration. The short T2 values reveal that the clusters of Mn2+ ions are found in the glasses [20]. The short T2 also indicates that the spins are localized near the surface region with strong spin–spin interactions between ions [20]. It is observed that the T2 values are decreasing with decreasing concentration of Mn2+ ions. The decrease of T2 values have been attributed to an increase in the number of spins with the concentration of Mn2+ ions. The ionic radii of Al3+, Pb2+ and Mn2+ ions are 0.067, 0.133, 0.026–0.04 nm, respectively. Therefore Mn2+ ions will enter at the sites of Al3+ ions because of nearly equal ionic radii to both the ions. Optical absorption studies The manganese ions usually found in oxide glasses are Mn2+ and Mn3+. As Mn2+ ions has 3d5 configuration, all transitions are spin-forbidden and therefore of low intensity [27,28]. The Mn3+ ions has 3d4 configuration and in an octahedral symmetry sites exhibits single spin-allowed transition 5Eg?5T2g. Although Mn2+ ions are in high concentrations in these glasses before melt quenching of Mn2+ ions into Mn3+ ions, the Mn3+ species exhibit pronounced absorption, which masks the Mn2+ spin-forbidden absorption bands. These types of interpretations have been reported previously in literature [26,29]. Our interpretations are supported by Rada et al. [30] and Siva Prasad et al. [31] in a recent discussion on optical absorption spectroscopy of manganese ions in oxide glasses. Siva Prasad et al. [31] obtained the optical absorption bands due to Mn2+ and Mn3+ ions in oxide glasses. The low intense bands are due to Mn2+ ions and single band in one glass is due to Mn3+ ions [31]. From Table 1, it is found that the optical band gap energy and the Urbach energy decrease with the increase of concentration of manganese ions. The decrease in indirect optical band gap can be attributed to the increase in non-bridging oxygen with increase in manganese content. Generally an increase in Urbach energy can be considered as due to increased defects [3]. Hence, the decrease in the Urbach energy with manganese content confirms that the number of defects decreases with manganese content. The indirect optical band gap and Urbach energies obtained in the present work are of the same order as reported earlier for oxide glasses [3]. Conclusions The EPR spectra of Mn2+ ions in aluminium lead borate glasses exhibit three resonance signals with effective isotropic g values at g  2.0, 3.3, 4.3. The resonance signal at isotropic g  2.0 has been attributed to an octahedral symmetry. The resonance signals at isotropic g  3.3 and 4.3 have been attributed to the rhombic surroundings of Mn2+ ions. The theoretical and experimental

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effective magnetic moment values support high spin Mn2+ ions. From these values, it is observed that the paramagnetic interactions are dominant in theses glasses. It is observed that the Gibbs energy increases with the concentration of manganese ions. The manganese ions enter at the sites of aluminium ions because of nearly equal ionic radii to both the ions. The optical absorption spectrum exhibits an intense and broad band in the visible region and this has been attributed to Mn3+ ions in an octahedral symmetry. It is observed that the optical band gap energy and the Urbach energy decrease with increasing concentration of manganese ions, which is assigned to increase in the non-bridging oxygen content. Acknowledgements The authors thank the Editor Dr. J. M. Bowman and two anonymous reviewers for incisive criticism and valuable suggestions. References [1] R.P. Sreekanth Chakradhar, K.P. Ramesh, J.L. Rao, J. Ramakrishna, J. Phys. Chem. Solids 64 (2003) 641–650. [2] E.R. Barney, A.C. Hannon, D. Holland, D. Winslow, B. Rijal, M. Affatigato, S.A. Feller, J. Non-Cryst. Solids 353 (2007) 1741–1747. [3] R.P. Sreekanth Chakradhar, G. SivaRamaiah, J. Lakshmana Rao, N.O. Gopal, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 62 (2005) 761–768. [4] J.O. Warne, J.R. Pilbrow, D.R. MacFarlane, J. Non-Cryst. Solids 140 (1992) 314– 318. [5] P. Rumori, B. Deroide, N. Abidi, H. El Mkami, J.V. Zanchetta, J. Phys. Chem. Solids 59 (1998) 959–967. [6] B. Vaidhyanathana, C. Prem Kumar, J.L. Rao, K.J. Rao, J. Phys. Chem. Solids 59 (1998) 121–128. [7] I. Ardelean, S. Cora, R. Ciceo Lucacel, O. Hulpus, Solid State Sci. 7 (2005) 1438– 1442. [8] R.P. Sreekanth Chakradhar, B. Yasoda, J.L. Rao, N.O. Gopal, J. Non-Cryst. Solids 353 (2007) 2355–2362. [9] Y.B. Saddeek, J. Alloys Compd. 467 (2009) 14. [10] R.S. Kaundel, S. Kaur, N. Singh, K.J. Singh, J. Phys. Chem. Solids (2010) 1191– 1195. [11] S. Rada, P. Pascuta, E. Culea, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 77 (2010) 832–837. [12] S.P. Singh, R.P.S. Chakradhar, J.L. Rao, B. Karmakar, Physica B: Condens. Matter 405 (2010) 2157–2161. [13] D. Möncke, E.I. Kamitsos, A. Herrmann, D. Ehrt, M. Friedrich, J. Non-Cryst. Solids 357 (2011) 2542–2551. [14] A. Terczyñska-Madej, K. Cholewa-Kowalska, M. £a˛czka, Opt. Mater. 33 (2011) 1984–1988. [15] G. SivaRamaiah, J. Lakshmana Rao, Proc. Indian Natl. Sci. Acad 78 (2012) 51– 57. [16] J.A. Weil, J.R. Bolton, J.E. Wertz, Electron Paramagnetic Resonance. Elementary Theory and Practical Applications, Wiley, New York, 1994. p. 498. [17] Greiner, Walter, Neise, Ludwig, Stöcker, Horst, Thermodynamics and Statistical Mechanics, Springer-Verlag, 1995. p. 101. [18] J.W. Gibbs, A method of geometrical representation of the thermodynamic properties of substances by means of surfaces, Trans. Conn. Acad. Arts Sci. 2 (Dec 1873) 382–404. [19] G. SivaRamaiah, Jinru Lin, Yuanming Pan, Phys. Chem. Miner. 38 (2011) 159– 167. [20] G. SivaRamaiah, Proc. Indian Natl. Sci. Acad. 77 (2011) 241–248. [21] C. Rudowicz, Magn. Res. Rev. 13 (1987) 1–89; C. Rudowicz, Magn. Res. Rev. 13 (1988) 335. [22] C. Rudowicz, S.K. Misra, Appl. Spectrosc. Rev. 36 (1) (2001) 11–63. [23] A.M. Hashem, H.M. Abuzeid, D. Mikhailova, H. Ehrenberg, A. Mauger, C.M. Julien, J. Mater. Sci. (2011), http://dx.doi.org/10.1007/s10853-011-6071-x. [24] E.I. Moustafa, Y.B. Saddeek, E.R. Shaaban, J. Phys. Chem. Solids 69 (2008) 2281– 2287. [25] S. Rada, M. Rada, E. Culea, J. Non-Cryst. Solids 357 (2011) 62–66. [26] Y.B. Saddeek, I.S. Yahia, K.A. Aly, W. Dobrowolski, Solid State Sci. 12 (2010) 1426–1434. [27] D.S. McClure, F. Seitz, P. Turnbull (Eds.), Solid State Physics, vol. 9, Academic Press, New York, 1959. p. 502. [28] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1968. p. 292. [29] S.P. Singh, A. Kumar, Phys. Chem. Glasses 33 (1992) 61. [30] S. Rada, A. Dehelean, M. Culea, E. Culea, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 79 (2) (2011) 320–324. [31] Y.D. Siva Prasad, A. Veerabhadra Rao, K. Srikanth, K.A. Emmanue, Rasayan, J. Chem. 4 (2011) 358–370.