Optical spectroscopy of La3Ga5SiO14 disordered crystals doped with Fe3+ ions

Optical Materials 43 (2015) 55–58

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Optical spectroscopy of La3Ga5SiO14 disordered crystals doped with Fe3+ ions L. Alyabyeva a,⇑, V. Burkov a, B. Mill b a b

Moscow Institute of Physics and Technology, 9 Institutskii per., Dolgoprudny 141700, Moscow Oblast, Russia Moscow State University, 1 Leninskie Gory, Moscow 119991, Russia

a r t i c l e

i n f o

Article history: Received 23 December 2014 Received in revised form 17 February 2015 Accepted 22 February 2015 Available online 7 March 2015

a b s t r a c t The circular dichroism, absorption and luminescence spectra of langasite (La3Ga5SiO14) doped with iron ions (Fe) are investigated at temperatures T of 300 and 8 K. Analysis of the data obtained reveals the Fe ions are trivalent. The Fe3+ impurity ions substitute Ga3+ ions and occupy 3f tetrahedral sites. Luminescence from the 4T1 excited state of the dopant was detected at 8 K. Ó 2015 Elsevier B.V. All rights reserved.

Keywords: Circular dichroism Langasite Optical spectroscopy Fe3+ Disordered structure La3Ga5SiO14

1. Introduction La3Ga5SiO14 (LGS) is a member of the well-known langasite family, which are a series of crystals with the same structure as calcium gallium germanate (Ca3Ga2Ge4O14) in the space group P321. Mill first grew Ca3Ga2Ge4O14 in the 1980s [1]. One interesting property of the structures of these materials is the presence of ordered and disordered crystal phases [1]. At present, over 200 disordered and 50 ordered isostructural compounds have been characterized. In LGS structure, 2d tetrahedral position has an equal probability of being occupied by Ga3+ and Si4+ ions. This structure causes marked broadening of the lines in optical spectra of the electronic transitions of impurities in LGS because of local distortions of the crystal field. Their unique piezoelectric properties [2] mean that langasites are an attractive substitute for quartz in various piezoelectric and acousto-optic devices. To date, devices based on langasites include acoustic wave sensors, resonators, high-frequency filters and many other related products. The crystal structure of LGS has a bilayer composition [1,3]. One layer contains Thomson cubes with La3+ ions in 3e site positions with local symmetry 2 and distorted octahedra occupied by Ga3+ ions at 1a sites with local symmetry of 32. The octahedral triad axis is parallel to the threefold axis of the crystal. In accordance with the threefold axis rule, it is surrounded by 3f tetrahedra with ⇑ Corresponding author. Tel.: +7 9268351330. E-mail address: [email protected] (L. Alyabyeva). http://dx.doi.org/10.1016/j.optmat.2015.02.023 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

Ga3+ ions with a local symmetry of 2 positioned in upper and lower neighboring tetrahedral layers. Small distorted 2d tetrahedra inhabited by Ga3+ and Si4+ ions in a ratio of 1:1 and local symmetry of 3 with threefold axes coaxial to the C3 crystal axis are found in the same layers. Kaldybaev et al. [4] described the gyrotropy of a set of langasitetype crystals including LGS. After a stimulated emission in LGS doped with Nd3+ was discovered [5] an interest in crystal family of langasites has been raised. Circular dichroism (CD) has proved to be a useful technique to study the nature of optical defect centers [6]. Obtaining of the stimulated emission on chromium doped LGS [7] gave a rise for growing a whole series of crystals with langasite structure doped with transitional metal ions. Further investigation of CD spectra of different langasites with various contents of chromium impurities allowed spectral lines broadened by structural disorder to be ascribed to corresponding electronic transitions of Cr3+ or Cr4+ [8]. Chromium was also shown to enter different langasite analogues with valences of either 3+ or both 3+ and 4+. The situation is more complex for manganese impurities. Investigation of La3Ga5SiO14:Mn [9] revealed that manganese ions were tetravalent only and substituted Ga3+ in octahedral positions. Meanwhile, in La3Ga5SiO14:Co crystals, Co2+ ions occupy tetrahedral 3f positions [10]. Because of the large number of defects, the electroneutrality of the lattice was maintained in both materials, even though crystals were grown without adding any charge compensators. In langasites doped with transition metal ions, the dopant ions are able to enter various

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sites, and adopt one or several valence states simultaneously. Iron in crystals is usually di- or trivalent [11–27]. As a result, Fe2+ and Fe3+ ions doped in various crystalline materials customarily occupy tetrahedral or octahedral sites. It is also possible for impurity ions to form both 2+ and 3+ states or enter both four- and sixfold coordinations. For example, Fe3+ in LiAl5O8 inhabits octahedral positions [11], while in an isostructural analog of this spinel, LiGa5O8, Fe3+ ions occupy tetrahedral positions as well as octahedral [12]. In KTaO3, octahedral positions are shared between Fe2+ and Fe3+ [13]. Thus, the valence state and site position of impurity ions doped in disordered structures with numerous defects including oxygen defects, such as LGS, are not obvious and need to be determined by careful analysis. In present paper we report for the first time an investigation of optical properties of LGS:Fe crystal. 2. Method and materials LGS:Fe crystals were grown by the Czochralski technique in 3%O2–97%N2 atmosphere using an Fe concentration of 0.1 at.% (6  1019 cm3). The resulting sample was pale yellow-green and had a thickness of 3.12 mm. A z-cut sample was used for measurements. Optical absorption spectra in the visible region were recorded on a spectrophotometer (Hitachi-330). Infrared (IR) absorption measurements to 2 lm were performed using a Fourier transform IR spectrometer (Varian 3100). CD measurements were performed on a dichrograph (Mark 3S, Jobin Yvon) at 300 and 8 K. Luminescence of the sample was obtained by excitation using a continuous-wave laser (LCM-S-111, k = 532 nm, P = 55 mW). Luminescence spectra were recorded on a monochromator (MDR-23) using phase-lock detection. Samples were cooled with a closed-cycle cryostat (CCS-150, Janis Research Company). 3. Results and discussion The axial absorption, CD and luminescence spectra of the Fe-doped LGS samples measured at 8 K are shown in Fig. 1. A CD spectrum of undoped LGS is also shown for comparison. In the luminescence spectrum, a broad band is observed with a peak at

676 nm (14,900 cm1) with a shoulder on the long-wavelength side at 710 nm (14,100 cm1). The CD spectrum of LGS:Fe exhibits the first transition at 600–700 nm with a peak at 640 nm (15,600 cm1). A zero-phonon line at 676 nm is not seen in the CD spectrum because of broadening of the signal caused by the structural disorder of LGS. The CD spectrum contains five weak positive bands with maxima at 520 nm (19,100 cm1), 480 nm (20,800 cm1), 460 nm (21,700 cm1), 394 nm (25,400 cm1) and 382 nm (26,200 cm1), an intense positive band at 349 nm (28,700 cm1), and an intense negative band on the edge of the fundamental absorption at 324 nm (30,900 cm1). The absorption spectrum is not informative; nonetheless, several weak bands are detected at 460, 480 and 520 nm at 8 K. A wing of absorption edge is intensive, so no weak kinks were observed in the region of the short waves. No bands were detected in the IR region for LGS:Fe at 8 K. It is well known that Fe ions can enter different compounds in various valence states, generally in Fe2+ or Fe3+. To correctly analyze the spectra of LGS:Fe, it is important to know the degree of oxidation of dopant ions. So let us consider the electron configurations of iron in detail. In iron garnets containing trivalent iron, 6 A1g ð6 SÞ ! 4 T1g ð4 GÞ and 6 A1g ð6 SÞ ! 4 T2g ð4 GÞ transitions of d5 configuration are manifested as two absorption bands centered at 11,000 and 16,000 cm1, respectively [14]. An additional six bands are observed between 20,000 and 30,000 cm1 in the short wavelength part of the visible region for Y3(Ga0,93Fe0,07)5O12 and Y3(Ga0,97Fe0,03)5O12. A doublet at 24000 cm1 is associated with 6 A1g ð6 SÞ ! 4 Eg ð4 GÞ and 6 A1g ð6 SÞ ! 4 A1g ð4 GÞ transitions, and a band at 27,000 cm1 is attributed to the 6 A1g ð6 SÞ ! 4 T2g ð4 DÞ transition. a-Fe2O3 contains iron with sixfold coordination, and its spectra contain bands at 11,000, 12,000 and 16,000 cm1, consistent with Fe3+ ions located in an octahedral environment. Meanwhile, Y3Fe5O12 exhibits strong bands at 10,900 and 14,289 cm1 corresponding to the transitions of octahedral Fe3+ and a band at 16,400 cm1 that is typical of tetrahedral Fe3+ [15]. Luminescence spectra of forsterite (Mg2SiO4) contain a band at 13,300 cm1 that is a manifestation of transitions of tetrahedral Fe3+ [16]. In addition, complex bands at 405 nm (24,690 cm1)

Fig. 1. Absorption (1), CD (2), and luminescence (3) spectra of LGS:Fe at T = 8 K, and CD spectrum (4) of pure LGS at T = 300 K. The inset shows a magnified view of the absorption spectrum from 450 to 540 nm.

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and 510 nm (19,610 cm1) are attributed to transitions to excited states of fourfold coordinated Fe3+, 4E(4D) and 4A1, 4E(4G), respectively. In spectra of Fe-doped LiNbO3 [17], a very broad band centered at approximately 20,400 cm1 is attributed to charge-transfer transitions of Fe2+, a broad band in the IR region at 9100 cm1 is consistent with the 5 A ! 5 E transition of Fe2+, and bands at 20,600 and 23,500 cm1 are assigned to spin-forbidden d–d transitions of Fe3+. An investigation of tetrachloroferrate, [FeCl4], in different compounds revealed bands at 14,200, 16,200 and 18,600 cm1 corresponding to transitions to 4T1, 4T2 and 4E states of trivalent iron with tetrahedral coordination [18]. Meanwhile, the first two transitions of octahedral Fe3+ in perovskite BiFeO3 [19] are observed at approximately 10,500 and 15,400 cm1. A brief summary of literature data with proposed assignments of Fe3+ bands is given in Table 1. The electron configuration of divalent iron is d6. Spectra of this configuration should contain one allowed transition 5 Eg ! 5 T2g and a series of spin-forbidden transitions. According to literature data, all the transitions are situated in the near-IR region. However, no transitions were detected in the IR spectrum of LGS:Fe. Furthermore, Table 2 reveals that the ionic radius of Fe2+ is substantially larger than those of Si4+ and Ga3+. Therefore, it is improbable that LGS:Fe contains Fe2+. It is also unlikely the Fe ions would substitute La3+ in the crystal lattice of LGS because of their considerable difference in size. In addition, the ionic radius of Fe3+ is much larger than that of Si4+, so it is also improbable that Fe ions occupy Si positions. Comparison of ionic radii (see Table 2) reveals that the substitution of Ga3+ with Fe3+ ions in LGS:Fe seems the most realistic possibility. In the crystal lattice of LGS, Ga3+ ions occupy both tetrahedral and octahedral positions. Comparison of the ionic radii of Fe3+ and Ga3+ suggests that Fe3+ could occupy both of these positions with equal probability. Fe3+ ions have d5 electronic configuration. The ground state 6A1 for tetrahedral (and 6A1g for octahedral) environment originates from the free ion term 6S, so its multiplicity is six. All excited states of Fe3+ are either doublets or quartets; that is, all transitions of high-spin Fe3+ are spin-forbidden. The energy levels of the tetrahedral field are higher than those of their octahedral equivalents [28]. Several energy levels of d5 electron configuration do not depend on crystal field. According to the literature data, the transition from 4 A1,4E(4G) (4A1g,4Eg (4G)) levels usually appears as a peak in 440– 480 nm region, and that of 4E(4D) (4Eg(4D)) at 380–395 nm. Thus, the absorption and CD bands observed at 460–470 and 385– 395 nm for LGS:Fe can be reasonably assigned to these transitions.

Table 2 Ionic radii of ferric ions possible substituted ions in LGS:Fe. Ionic radii (high spin) (Å)

Si4+

Ga3+

Fe3+

Fe2+

CN = 4 CN = 6

0.26 0.40

0.47 0.62

0.49 0.64

0.64 0.78

Note: CN is coordination number.

However, this analysis still does not clearly reveal the environment of the impurity Fe ions in LGS:Fe. The spectra of compounds containing [FeO6]9 ions typically contain a band in a range 9000–11,000 cm1 corresponding to the 4T1g(4G) transition. Meanwhile, the luminescence spectra of compounds containing Fe3+ in an environment with four oxygen ions usually exhibit a band in the region 14,000–16,000 cm1 originating from the transition from the 4T1(4G) excited state. For example, LiAl5O8 exhibits luminescence at 15,220 cm1. Despite the fact that in Ref. [24] the transition from 4T1(4G) is observed at 19,123 cm1, the peak at 15,220 cm1 is assigned to a split component of the 4T1(4G) term. In several other reports concerning tetrahedral coordination of Fe3+ [11,25,26], the luminescence band in the region of 15,000 cm1 is unambiguously assigned to the 4 T1(4G) state, while that in the region of 18,000–19,000 cm1 is ascribed to the transition to the 4T2(4G) state. It follows from Tanabe–Sugano diagrams (Fig. 2) that the positions of the three energy terms 4A1,4E(4G), and 4E(4D), do not depend on the crystal field strength. According to the literature, the transition 6 A1 ! 4 A1 ; 4 Eð4 GÞ is usually observed at 440– 480 nm (20,800–22,700 cm1), while the 6 A1 ! 4 Eð4 DÞ transition appears at 380–395 nm (25,300–26,300 cm1). Therefore, it is reasonable to assign the absorption and CD bands to these transitions. The band observed at 15,600 cm1 for LGS:Fe is a manifestation of the 4 T1 ð4 GÞ ! 6 A1 transition of tetrahedral Fe3+.

Table 1 Energies (cm1) of d–d transitions of Fe3+ in various crystals. Tetrahedral

4

T1(4G)

LiAl5O8 [8] Y3Fe5O12 [12] Mg2SiO4 [13] [FeCl4] [15] KAlSi3O8 [22] Ca3Fe2Si3O12 [24] Fe3Al2Si3O12 [24]

15,255 16,400 13,300 14,200 16,200 21,400 20,300

18695

21,300

17,200 16,200 19,800

19,610 18,920 22,800 23,900 24,200

Octahedral

4

4

Y3Fe5O12 [12] CsCl [18] Ca3Fe2Si3O12 [24] Fe3Al2Si3O12 [24] Al2O3 [25]

10,900 12,755 12,000 14,300 9450

14,280 20,408 16,700 17,500 14,350

T1g(4G)

4

T2(4G)

4

A1,4E (4G)

4

4

22,550

25,720

19,570 23,800

24,690 22,200 26,700

4

4

4

20,833, 22,727 22,700 23,000 22,270

23,800

25,575 26,000 26,000 26,800

21,800 T2g(4G)

A1g,4Eg (4G)

T2(4D)

T2g(4D)

25,510

E(4D)

Eg(4D)

Fig. 2. Tanabe–Sugano diagram of d5 electronic configuration with C/B = 4.75 [29].

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Table 3 d–d transitions of tetrahedral Fe3+ in LGS. Transition 6

A1 A1 6 A1 6 A1 6 A1 6 A1 6

4

4

$ T1 ð GÞ ! 4 T2 ð4 GÞ ! 2 T2 ð2 IÞ ! 4 A1 ; 4 Eð4 GÞ ! 4 T2 ð4 DÞ ! 4 Eð4 DÞ

References E (cm1)

k (nm)

15,600 19,100 20,800 21,700 25,400 26,200

640 520 480 460 394 382

Sugano et al. [30] showed that the energies of crystal fieldindependent terms could be expressed in terms of Racah parameters; namely, E(4A1,4E(4G)) = 10B + 5C, E(4E(4D)) = 17B + 5C. Thus, with respect to the experimental values of the energies of these transitions, we find that B = 643 cm1, C = 3060 cm1, and C/B = 4.75. The best agreement with the Tanabe–Sugano diagram in Fig. 2 was obtained for the value of the crystal field parameter Dq = 900 cm1. The origin of the two bands at 19,100 and 20,800 cm1 is still questionable. In view of the Tanabe–Sugano diagram, the second transition caused by the strength of the crystal field could be a manifestation of either the 6 A1 ! 2 T2 ð2 IÞ transition (if 10Dq/B > 15) or 6 A1 ! 4 T2 ð4 GÞ (if 10Dq/B < 15). For the values obtained, 10Dq/B = 12.4. This means that the band at 19,100 cm1 is a manifestation of 6 A1 ! 4 T2 ð4 GÞ, and at the same time, the weak peak at 20,800 cm1 could be assigned to the 6 A1 ! 2 T2 ð2 IÞ transition only. The low intensity of the peak at 20,800 cm1 could be caused by the large difference in the multiplicity of the ground state (6) and excited state (2), which is also further evidence that our interpretation of the spectra for LGS:Fe is correct. All of the bands observed in the spectra of LGS:Fe are ascribed to d–d transitions of the d5 configuration. The transitions ascribed to each band are listed in Table 3. All of the transitions of the d5 configuration are spin-forbidden, which explains why the optical spectra are not demonstrative. The intense bands in the ultraviolet region of the CD spectrum are caused by charge-transfer transitions.

4. Conclusion Careful analysis of the low-temperature absorption, CD and luminescence spectra of LGS:Fe crystals allowed us to determine the site position and valence state of the iron impurity doped in LGS. Because of the structural disorder in the langasite crystal, all of the bands in the spectra are broadened, and vibronic structure is not observed even at liquid-helium temperature. In the luminescence spectrum, only the lowest excited state 4 T1 ð4 GÞ is active. The calculated crystal field parameters for LGS:Fe are B = 643 cm1, C = 3060 cm1, C/B = 4.75, and Dq = 900 cm1. These results indicate that Fe ions doped into LGS occupy 3f tetrahedral site positions in the lattice with a 3+ valence state, substituting Ga3+ ions.

Acknowledgements Authors are grateful to Shared Facilities Center of Moscow Institute of Physics and Technology for providing equipment for low-temperature measurements. This research was supported by the Russian Foundation for Basic Research, Grant No. 13-07-00242a.

[1] A.A. Kaminskii, B.V. Mill, C.E. Sarkisov, Physics and Spectroscopy of Laser Crystals, Nauka, Moscow, 1986. [2] I.A. Andreev, M.F. Dubovik, A new piezoelectric material, langasite (La3Ga5SiO14), with a zero temperature coefficient of the elastic vibration frequency, Sov. Tech. Phys. Lett 10 (1984) 205–207. [3] E.L. Belokoneva, M.A. Simonov, B.V. Mill, A.V. Butashin, N.V. Belov, Crystal structure of Cagallogermanate, Ca3Ge[(Ga2Ge)Ge2O14] and its analog, Ba3Fe[(FeGe2)Ge2O14], Dokl. Akad. Nauk SSSR 255 (5) (1980) 1099–1104. [4] K.A. Kaldybaev, A.F. Konstantionva, Z.B. Perekalina, Gyrotropy of Uniaxial Absorbing Crystals, Institute of Investment Press, Moscow, 2000. [5] A.A. Kaminskii, S.E. Sarkisov, B.V. Mill, G.G. Khodzhabagyan, Generation of stimulated emission of Nd3+ ions in a trigonal acentric La3Ga5SiO14 crystal, Sov. Phys. Doklady 27 (1982) 403. [6] V.I. Burkov, E.P. Peredereı˘, E.V. Fedotov, B.V. Mill’, Yu.V. Pisarevskii, Circular dichroism spectra of langasite family crystals in the range of electronic transitions of structure defects, Crystallogr. Rep. 53 (5) (2008) 843–846. [7] A.A. Kaminskii, A.V. Butashin, A.A. Demidovich, et al., Broad-band tunable stimulated emission from octahedral Cr3+ ions in new acentric crystals with Ca-Gallogermanate structure, Phys. Status Solidi (a) 112 (1) (1989) 197–206. [8] V.I. Burkov, A.F. Konstantinova, B.V. Mill, T.F. Veremeichik, Yu.N. Pyrkov, V.P. Orekhova, E.V. Fedotov, The absorption and circular dichroism spectra of langasite family crystals doped with chromium ions, Crystallogr. Rep. 54 (4) (2009) 613–618. [9] V.I. Burkov, S.V. Gudenko, L.N. Alyabyeva, Optical and EPR spectroscopy of a La3Ga5SiO14:Mn crystal, JETP 119 (4) (2014) 723–736. [10] V.I. Burkov, L.N. Alyabyeva, Yu.V. Denisov, B.V. Mill, Optical spectroscopy of a La3Ga5SiO14:Co2+ crystal, Inorg. Mater. 50 (11) (2014) 1119–1124. [11] G.T. Pott, B.D. McNicol, ZeroPhonon transition and fine structure in the phosphorescence of Fe3+ ions in ordered and disordered LiAl5O8, Chem. Phys. 56 (1972) 5246–5254. [12] N.T. Melamed, J.M. Neto, T. Abritta, F. de Souza Barros, A comparison of the luminescence of LiAl5O8:Fe and LiGa5O8:Fe–II. Fe3+ in octahedral sites, J. Lumin. 24–25 (1) (1981) 249–252. [13] H.J. Reyher, N. Hausfeld, M. Pape, A magnetic circular dichroism and optically detected magnetic resonance investigation of Fe2+ and Fe3+ centres in KTaO3, J. Phys. Condens. Matter 12 (2000) 10599–10610. [14] K.A. Wickersheim, R.A. Lefever, Absorption spectra of ferric ironcontaining oxides, Chem. Phys. 36 (1962) 844–850. [15] D.L. Wood, J.P. Remeika, Effect of impurities on the optical properties of yttrium iron garnet, J. Appl. Phys. 38 (1967) 1038–1045. [16] G. Walker, T.J. Glynn, Infra-red luminescence of iron-doped synthetic forsterite, J. Lumin. 54 (1992) 131–137. [17] S.A. Basun, D.R. Evans, T.J. Bunning, S. Guha, J.O. Barnes, et al., Optical absorption spectroscopy of Fe2+ and Fe3+ ions in LiNbO3, J. Appl. Phys. 92 (2002) 7051–7055. [18] M.T. Vala Jr., P.J. McCarthy, Tetrahedral transition metal complex spectra: the tetrachloroferrate (III) anion, Spectrochim. Acta 26A (1970) 2183–2195. [19] X.S. Xu, T.V. Brinzari, S. Lee, Y.H. Chu, L.W. Martin, A. Kumar, S. McGill, R.C. Rai, R. Ramesh, V. Gopalan, S.W. Cheong, J.L. Musfeldt, Optical properties and magnetochromism in multiferroic BiFeO3, Phys. Rev. B 79 (2009) 134425-4. [20] T. Sanamyan, M. Dubinskii, S. Trivedi, Fluorescence Properties of Fe2+ and Co2+ Doped Hosts of CdMnTe Compositions as Potential Mid-infrared Laser Materials, Army research laboratory, Adelphi, MD 20783-1197, 2011, ARLTR-5770. [21] M.G. Misra, R. Kripal, EPR, optical absorption and superposition model studies of Fe3+-doped cesium chloride single crystals: a case of substitutional as well as interstitial sites, Mol. Phys. 110 (24) (2012) 3001–3013. [22] R. Kripal, Sh.D. Pandey, M.G. Misra, EPR, optical absorption and superposition model studies of Fe3+-doped diammonium hexaaqua magnesium sulfate: a case of hyperfine structure, Appl. Magn. Reson. 44 (2013) 1295–1310. [23] N. Pathak, S.K. Gupta, K. Sanyal, M. Kumar, R.M. Kadam, V. Natarajan, Photoluminescence and EPR studies on Fe3+ doped ZnAl2O4: an evidence for local site swapping of Fe3+ and formation of Inverse and Normal phase, Dalton Trans. 43 (2014) 9313–9323. [24] N.T. Melamed, F. de S. Barros, P.J. Viccaro, J.O. Artman, Optical properties of Fe3+ in ordered and disordered LiAl5O8, Phys. Rev. B 5 (9) (1972) 3377–3387. [25] W.B. White, M. Matsumura, D.G. Linnehan, T. Furukawa, B.K. Chandrasekhar, Absorption and luminescence of Fe3+ in single-crystal orthoclase, Am. Mineral. 71 (1986) 1415–1419. [26] R. Heitz, A. Hoffmann, I. Broser, Fe3+ center in ZnO, Phys. Rev. B 4 (16) (1992) 8977–8988. [27] P.G. Manning, Optical absorption spectra of Fe3+ in tetrahedral and octahedral sites in natural garnets, Can. Mineral. 11 (1972) 826–839. [28] A.B.P. Lever, Inorganic Electronic Spectroscopy, second ed., Elsevier, Amsterdam, 1984. [29] J.J. Krebs, W.G. Maisch, Exchange effects in the optical-absorption spectrum of Fe3+ in Al2O3, Phys. Rev. B 4 (3) (1971) 757–769. [30] S. Sugano, Y. Tanabe, H. Kamimura, Multiplets of Transition Metal Ions in Crystals, Academic Press, New York, 1970.