Coordination and valence state of transition metal ions in alkali-borate glasses

Optical Materials 33 (2011) 1984–1988

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Coordination and valence state of transition metal ions in alkali-borate glasses A. Terczyn´ska-Madej ⇑, K. Cholewa-Kowalska, M. Ła˛czka AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Glass and Amorphous Coatings, Ave. Mickiewicza 30, Krakow, Poland

a r t i c l e

i n f o

Article history: Received 22 November 2010 Received in revised form 29 March 2011 Accepted 30 March 2011 Available online 23 April 2011 Keywords: Alkali-borate glasses Transition metal ions Coordination environment Oxidation state UV–VIS–NIR spectroscopy

a b s t r a c t Borate glasses of the 20R2O80B2O3 type, where R = Li, Na and K, were colored by doping with transition metal ions (Co, Ni, Cr and Mn). The glasses were obtained by melting at the temperature of 1150 °C. For these glasses optical absorption in UV–VIS–NIR range were recorded. Analysis of the spectra allows to be determined the coordination and oxidation states of the doping transition metal ions. Changes of their coordination or oxidation are presented as a function of the optical basicity K after Duffy. Cobalt and nickel are present in examined borate glasses as divalent ions (Co2+, Ni2+) in octahedral coordination mainly, but the tetrahedral coordination state of cobalt is also possible. Chromium and manganese are present in the borate glasses in various oxidation state, though Cr3+ and Mn3+ ions in the octahedral coordination are probably dominant. A decrease of the electronegativity of the modifiers (Li ? Na ? K) and an increase of the glass matrix basicity cause a shift of the oxidation/reduction equilibrium towards higher valences of the transition metals (Cr6+, Mn3+). Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Borate glasses are very interesting amorphous materials considering their specific structure and physical properties. In these glasses two group of bands; one due to trigonal BO3 and the second due to the tetrahedral BO4 units are present. Borate glasses are also interesting as inorganic hosts for transition metal ions [2,3]. However, literature interpretations of optical and EPR spectra of the transition metal ions are ambiguous. Lakshminarayana, Thulasiramudu and Buddhudu [4,5] examined the spectral properties of B2O3–ZnO–PbO glasses doped with Mn2+, Co2+ and Ni2+ ions [4,5]. It has been found that a little shifting of the characteristic bands of the BO3 and BO4 units to higher energy regions occurs with the increase in the concentration of the transition metal ions. Moreover, absorption and emission spectra of the transition metal ions indicate that in B2O3–ZnO–PbO glasses both Mn2+ ions and Co2+ ions are present in tetrahedral coordination while Ni2+ ions occur in octahedral coordination. Alkali-fluoroborate glass systems containing manganese cations have been examined in order to obtain information about the structural role of manganese in such glass hosts [2]. As manganese is introduced replacing lithium or sodium, it acts as a network modifier and the infrared intensity of the bands due to the presence of BO4 units is increased at the expense of the BO3 units. The EPR spectra of these glasses show a six-line hyper-fine structure due to the presence of Mn2+ ions (g = 2.01) and Mn3+ ions (g = 4.3). The intensity of optical absorption bands and the EPR signal due to the presence of Mn2+ ions ⇑ Corresponding author. Fax: +48 12 617 25 09. E-mail address: [email protected] (A. Terczyn´ska-Madej). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.03.046

decreases with increasing MnO concentration indicating the conversion of Mn2+ ions into Mn3+ ions taking place at the networkforming positions. This conclusion is incompatible with the interpretation of Venkat Reddy et al. [2] indicating that the resonance signal at g = 2.0 is due to the presence of Mn2+ ions in octahedral environment, whereas the resonance at g = 4.3 and g = 3.3 are attributed to rhombic surroundings of the Mn2+ ions. Our paper is a continuation of research concerning the behavior of transition metal ions in glasses of various chemical compositions based on their optical spectra. Earlier, we tried to determine the relationship between the optical basicity of the host glasses of the R2O–SiO2 system and valence and coordination states of the transition metal ions, where R = Li, Na and K [1]. It was found that with the increasing of the optical basicity the tetrahedral coordination of transition metal ions becomes preferential in the host glasses. The purpose of the present paper is to determine influence the optical basicity of oxygen ions in the borate glasses of the R2O–B2O3 system on the local state (coordination and valence) of transition metal ions in the glass matrix and finally the glass color itself. 2. Experimental Colored alkali-borate glasses of the type (20-x)R2O80B2O3: xMmOn (where R = Li, Na and K) studied in the present work were obtained by introducing transition metal ions (M = Co, Ni, Cr and Mn) into the glass matrix, using the classical melt quenching technique. The mentioned glasses were prepared by mixing the starting materials of p.a. quality, Li2CO3, Na2CO3, K2CO3 and H3BO3 (by POCh SA, Poland and Merck KGaA, Germany) with one of the following CoO, NiO, K2Cr2O7 and MnO2 (by POCh SA, Poland) as

´ ska-Madej et al. / Optical Materials 33 (2011) 1984–1988 A. Terczyn

dyes in an agate mortar. The mixtures were heated in an electric furnace (Superkhantal Nabertherm TL6) using a Pt/Au crucibles, at a temperature of 1150 °C for 1.5 h utill a homogeneous, bubble free liquid was formed. Finally, all glass samples were subsequently annealed for 1 h at 500 °C to relieve residual internal stress and slowly cooled to room temperature. The obtained, colored alkali-borate glasses were then polished and prepared in form of 10  20  1 mm plates for the measurement of the optical spectra. Chemical composition of the final base glass samples was performed using an Energy dispersive X-ray microanalyser Link ISIS – EDX. Table 1 shows the compositions and optical basicity of the glasses considered. The content of dyes in all kinds of the borate glasses is equal (Table 2). The theoretical optical basicity Kcal of the glass matrix was determined by the Eq (1) according to Duffy and Ingram as follows [6,7]:

Kcal ¼ 1  fðzA r A =2Þð1  1=cA Þ þ ðzB r B =2Þð1  1=cB Þ þ . . . ; g

ð1Þ

where zA, zB, . . . are the oxidation numbers of the cations A, B, . . .; whereas rA, rB, . . . are their ionic ratios with respect to the total number of oxides and cA, cB, . . . are basicity moderating parameters of these cations (c = 1.36 (x - 0.26), where x is the Pauling electronegativity [6]). The theoretical optical basicity Kcal of the investigated alkali-borate glasses increases with decrease of the Pauling electronegativity of the modifiers changing in the order: Li2O ? Na2O ? K2O (Table 1). The matrix glasses were also characterized with regards to the glass structure by measurements of FTIR absorption spectra in the range 400–4000 cm1 using a Digilab FTS-60 V (Bio-Rad) Fourier transform spectrometer. The spectra were collected after 256 scans at 4 cm1 resolution. The samples were prepared by the standard KBr pellets method. The Win-IRTM software has been applied for the decomposition of the IR spectra. The optical absorption spectra of all colored glass samples doped with chosen transition metal ions were recorded at room temperature using a JASCO V-630 UV–VIS–NIR spectrophotometer in the wavelength range 380–980 nm Spectral parameters such as positions, shapes and intensities of bands, providing the information about valence and coordination states of transition metal ions were determined (Tables 3–6). The results were expressed in terms of the absorbance for the studied glasses as a function of wavelength. The optical absorption spectra of the glasses with doping by transition metal ions were determined by the molecular extinction coefficient for the transition metal. 3. Results and discussion 3.1. FTIR spectra FTIR spectra of the examined alkali-borate glasses of the type (20-x)R2O80B2O3:xMmOn (where R = Li, Na and K) glasses (Fig. 1) Table 1 Chemical compositions and optical basicity of the base borate glasses. Oxide component

Chemical compositions, % R2OaB2O3 From synthesis

Li2O B2O3 Na2O B2O3 K2O B2O3 a

Optical basicity, Kcal

From analysis

Mass

Molar

Mass

Molar

9.69 90.31 18.21 81.79 25.27 74.73

20 80 20 80 20 80

–a

–a

0.426

17.50 82.50 23.84 76.16

19.24 80.76 18.79 81.21

0.427

not determined by used EDX method.

0.432

1985

Table 2 Colored compounds content of the borate glasses. Borate glasses compositions (mol%)

MmOn content, x values CoO

NiO

Cr2O3

MnO2

(20-x)Li2OxMmOn80B2O3 (20-x)Na2OxMmOn80B2O3 (20-x)K2OxMmOn80B2O3

0.058

0.463

0.173

0.777

contain two groups of bands; one at 1200–1650 cm1, deriving from the stretching relaxation of the B–O bond of the trigonal BO3 units and the second one in the region 800–1200 cm1 due to the tetrahedral BO4 units; in the FTIR spectra another band at about 695–725 cm1 corresponding to the bending relaxation of the B–O linkages in the borate network also appears. From the infrared spectra it follows that in the examined alkali-borate glasses both trigonal BO3 units and tetrahedral BO4 units are present and the type of the modifier of the host glasses does not affect significantly the boron coordination equilibrium [2].

3.2. Optical absorption spectra Figs. 2–5 present the optical absorption spectra of the examined alkali-borate glasses containing transition metal ions. These visible spectra are the result of d–d electron transfers in the transition metal ions when linked with oxygen.

3.2.1. Cobalt The color of the glasses with cobalt addition was pale and changed from mauve through pink to blue with the change of the modifiers: Li2O ? Na2O ? K2O. In the optical spectra of the investigated Co-doped alkali-borate glasses the broad unsymmetrical bands are observed in the visible part of the spectrum. Optical spectra of glasses modified with Li2O and Na2O consist of one duplex band centered at 557 nm with a small shoulder at 501 nm. These spectra are similar to each other in respect of shape and band position and differ only in the absorption intensity; the intensity of the bands of the Li2O-modified glass is higher than that of the Na2O-modified one. In the spectrum of the K2O-modified glass one triple split band (503, 570 and 635 nm) in the visible spectral range is observed (Fig. 2, Table 3). Cobalt (d7 configuration) may be present in glasses as divalent ion in octahedral or tetrahedral coordinations [4,5,8–10,12]. The Co2+ ion in octahedral field has a 4F ground state which splits into the three states 4T1, 4T2 and 4A2 and the spin-allowed transitions 4 T1(F) ? 4T2(F), 4T1(F) ? 4A2(F) and 4T1(F) ? 4T1(P) may be expected [4,5] (absorption band position at approx. 540 nm). For the tetrahedral symmetry of Co2+, with the ground state of 4 A2(4F) two spin-forbidden transitions 4A2(4F) ? 4T1(P) and 4 A2(4F) ? 4T1(4F) are mainly present (absorption band position 605–667 nm) [4,5,11]. Based on the visible absorption spectra of cobalt in alkali-borate glasses it can be concluded that in Li2O- and Na2O-modified borate glasses cobalt occurs in form of divalent Co2+ ions in octahedral coordination mainly. This is confirmed by the presence of absorption bands in the range of 500–560 nm attributed to the 4T1g(F) ? 4 T1g(P) transition in octahedral coordinated Co2+ ions, responsible also for the bright pink color of the glasses. A triple split band of the K2O-modified borate glass with an additional third maximum at 635 nm, assigned to the 4A2(F) ? 4T1(P) transition in tetrahedral coordinated Co2+, indicates that in this glass Co2+ ions occur in both octahedral and tetrahedral coordination with oxygen. This phenomenon is confirmed by the obtained pale blue color of the glass. Therefore, higher basicity of the K2O-modified borate glasses

´ ska-Madej et al. / Optical Materials 33 (2011) 1984–1988 A. Terczyn

1986

Fig. 1. FTIR spectra of base alkali-borate glasses modified with Li, Na and K oxides. Recorded at room temperature.

favors the forming of tetrahedral network position of cobalt ions in these glasses.

Fig. 2. Absorption spectra of the borate glasses modified with Li, Na or K oxides and doped with Co ions.

Table 3 Band positions (mmax) in the absorption spectra and optical basicity of the alkaliborate glasses doped with cobalt ions in octahedral and tetrahedral coordination spheres. Oxide modifier Optical basicity, Band position (Co2+), m (cm1) R2O Kcal Octahedral Tetrahedral

m1 = m2 = 4T1g(F) m3 = 4A2(F) Molar ? T1g(P)

? 4T1(P)

extinction coefficient, e

19 960 17 889 (501 nm) (559 nm) 19 960 17 986 (501 nm) (556 nm) 19 881 17 575 (503 nm) (569 nm)

–

28,421a

–

20,016a

15 748 (635 nm)

24,456a

4

a

Li2O

0426

Na2O

0427

K2O

0432

e = emax.

3.2.2. Nickel All alkali-borate glasses with Ni addition are characterized by a pale yellow shade and their visible absorption spectra are very similar to each other in respect of spectral shape as well as band position and intensities (Fig. 3, Table 4). Nickel can be present in the glasses as divalent Ni2+ ions in both octahedral Oh and tetrahedral Th symmetry but the presence of Ni ions in different formations is also possible e.g. square planar [4,5,13–16]. Divalent nickel in octahedral coordination gives a spin-allowed absorption band at about 415 nm corresponding to the 3A2g(F) ? 3T1g(P) transition and a very weak spin-forbidden transition 3A2g(F) ? 1Eg(D) at ca. 755 nm, accompanied by a very weak yellow color of the glass [4,5,13,17]. Absorption spectra of the Ni-doped alkali-borate glass consist of both a sharp absorption band with a maximum at about 415 nm and a broad absorption band located at about 700–800 nm. Both, color and optical spectra of examined alkali-borate glasses indicate that Ni occurs in these glasses in octahedral coordination with oxygen ions and the type of the modifier does not affect the Ni position in the glass structure. 3.2.3. Chromium All alkali-borate glasses with Cr addition were green with various shades, from rich green (Li2O-modified glass) to yellow-green (K2O-modified glass). The absorption spectra consist of a very strong absorption in the UV spectral range as well as of a weaker absorption band located in the visible part of the spectrum at 600–700 nm. The intensity of the visible absorption decreases with the increase of the host glasses basicity ((Li ? Na ? K) (Fig. 4, Table 5). This relationship has been earlier observed in the case of alkali-silicate glasses containing chromium ions [1,12,18]. Chromium may be present in the glasses in various oxidation states, but Cr3+ and Cr6+ ions are the most common. Cr3+ ions (3d3 configuration) occurring in octahedral forms show two broad absorption bands due to the transitions from the 4A2g(F) ground state to the 4T2g (F) (660 nm) and 4T1g(F) (ca. 450 nm) excited state [19–23]. The very strong absorption in the UV region is probably

´ ska-Madej et al. / Optical Materials 33 (2011) 1984–1988 A. Terczyn

Fig. 3. Absorption spectra of the borate glasses modified with Li, Na or K oxides and doped with Ni ions.

Table 4 Band positions (mmax) in absorption spectra and optical basicity of the alkali-borate glasses doped with nickel ions. Ni2+ ions in octahedral sphere. Oxide modifier R2O

Optical basicity, Kcal

a

Na2O

0.427

K2O

0.432

Table 5 Band positions (mmax) in the absorption spectra and optical basicity of the alkaliborate glasses doped with chromium ions. Cr3+ in an octahedral coordination sphere. Oxide modifier R2O

Optical basicity, Kcal

Band position (Cr3+, octahedral), mmax (cm1)

m1 = 4A2g(F) ? m2 = 3A2g(F) ?

? T1g(P)

1

Eg(D) ca. 755 nm

Molar extinction coefficient, e

24,039 (416 nm) 24,213 (413 nm) 24,156 (414 nm)

Shoulder

3

0.426

Fig. 4. Absorption spectra of the borate glasses modified with Li, Na or K oxides and doped with Cr ions.

Band position (Ni2+, octahedral), mmax (cm1)

m1 = 3A2g(F)

Li2O

1987

4

T2g(F)

Li2O

0.426

11,821a

Na2O

0.427

Shoulder

12,183a

K2O

0.432

Shoulder

12,013a

a

16,234 (616 nm) 16,234 (616 nm) 16,313 (613 nm)

Molar extinction coefficient, e 8329a 6166a 4351a

e = emax.

e = emax.

connected with the presence of Cr ions in a higher valence state (Cr6+ ions) and it is not a d–d absorption [12,18,21]. In the case of our Cr-doped alkali-borate glasses the absorption in the visible part of the spectrum derives most probably from Cr3+ in an octahedral ligand field Oh of oxygen ions. Similarly to the alkali-silicate glasses [1], with the increase of the host glass basicity (Li ? Na ? K) a decrease of the 4T2g(F) band intensity occurs indicating a decrease of the number of Cr3+ octahedral forms. Probably, together with the basicity increase, part of chromium oxidizes to higher valence states responsible for strong UV absorption and clears yellow-green color shade of the K2O-modified borate glasses. 3.2.4. Manganese Glasses with manganese addition were brown–red with various depths of color. The color intensity was the highest in the case of the K2O-modified borate glasses. The UV–VIS spectrum consists of a strong absorption band in the visible range at about 470– 476 nm; intensity of this band increases with the increase of the host glass basicity (Li ? Na ? K); a similar correlation was been observed in the case of alkali-silicate glasses [1] (Fig. 5). Manganese ions occur in the glasses usually as divalent (Mn2+) or trivalent (Mn3+) ions but the presence of Mn4+ ions is also possible. The absorption spectra of the zinc lead borate glasses containing Mn2+ ions in Oh symmetry (d5 configuration) possess a quite broad absorption bands at about 420 nm and 510– 520 nm corresponding to the spin-forbidden 6A1g(S) ? 4T1g(G) transition [4,5]. The visible spectrum of trivalent manganese (d4

Fig. 5. Absorption spectra of the borate glasses modified with Li, Na or K oxides and doped with Mn ions.

configuration) consists of a single asymmetric band at 450– 490 nm, which is identified as the 5Eg ? 5T2g transition in octahedral coordinated Mn3+ ions [2,4,5,9]. In the case of Mn4+ (3d3 configuration) the band at about 450 nm appears corresponding to the 4 A2 ? 4T2 transition [24]. Moreover, from optical and ESR studies of other authors [1,2,5,12] it follows that in the case of manganese ions tetrahedral symmetry and network-forming positions are also possible.

´ ska-Madej et al. / Optical Materials 33 (2011) 1984–1988 A. Terczyn

1988

Table 6 Band positions (mmax) in absorption spectra and optical basicity of the alkali-borate glasses doped with manganese ions. Oxide modifier R2O

Optical basicity, Kcal

Band position of Mn ions, mmax (cm1) Molar extinction coefficient, e

a

Li2O

0.426

Na2O

0.427

K2O

0.432

This paper has been presented on the Conference ‘‘XXII International Congress on Glas. ICG 2010’’, September 20–25, 2010, Bahia, Brazil, as a poster.

21,322 (469 nm) 21,186 (472 nm) 21,008 (476 nm)

8752

a

12,883a 29,452a

e = emax.

Because of the similar range of the visible absorption deriving from manganese ions in various oxidation states, the interpretation of the spectra of our Mn-doped alkali-borate glasses is difficult. The observed 470–476 nm absorption band derives probably from Mn3+ ions, but the presence of Mn ions in other oxidation states cannot be excluded. The most important information resulting from the analysis of optical spectra is the increase of absorption band intensity with the increase of the host glass basicity (Table 6). This indicates that this structural factor plays an important role in the formation of valence and coordination state of Mn ions in both alkali-borate and alkali-silicate glasses. However, further research is necessary to define the character of the changes, which the Mn ions undergo. 4. Conclusions 1. Transition metals Co, Ni, Cr and Mn are present in alkali-borate glasses in various oxidation and valence states, which is the cause for the different glass color. 2. Optical basicity of the host glasses is an important structural factor affecting the valence and coordination states of the transition metal ions in the glasses; this is especially evident in the case of Cr- and Mn-doped alkali-borate glasses. 3. Alkali-borate glasses prefer rather the octahedral coordination of Co2+ and Ni2+ ions and the type of glass structure and modifiers (Li2O, Na2O, K2O) do not affect significantly their coordination equilibrium (some minor changes were observed in the case of Co2+ only). This situation is different than in alkali-silicate glasses, where the contribution of the tetrahedral coordinated Co2+ and Ni2+ ions is considerable in glasses of high basicity (K2O-modified silicate glasses). 4. Obtained results can be helpful in the design of the chemical compositions of glasses of defined color, as well as in the research concerning mechanism of color in amorphous materials.

Acknowledgements The research presented is supported financially by the Polish State Committee for Scientific Research Project No.: 3 T08D 41 29.

References [1] A. Terczynska-Madej, K. Cholewa-Kowalska, M. Laczka, The effect of silicate network modifiers on color and electron spectra of transition metal ions, Opt. Mater. 32 (11) (2010) 1456–1462. [2] P. Venkat Reddy, C. Laxami Kanth, V. Prashanth Kumar, N. Veeraiah, P. Kistaiah, Optical and thermoluminescence properties of R2O–RF–B2O3 glass systems doped with MnO, J. Non-Cryst. Solids 351 (2005) 3752–3759. [3] R.P. Sreekanth Chakradhar, B. Yasoda, J.L. Rao, N.O. Gopal, EPR and optical studies of Mn2+ ions in Li2O–Na2O–B2O3 glasses – An evidence of mixed alkali effect, J. Non-Cryst. Solids 353 (2007) 2355–2362. [4] G. Lakshminarayana, S. Buddhudu, Spectral analysis of Mn2+, Co2+ and Ni2+: B2O3–ZnO–PbO glasses, Spectrochim. Acta A 63 (2006) 295–304. [5] A. Thulasiramude, S. Buddhudu, Optical characterization of Mn2+, Ni2+, and Co2+ ions doped zinc lead borate glasses, J. Quant. Spectrosc. Radiat. Transfer 102 (2006) 212–227. [6] J.A. Duffy, M.D. Ingram, An interpretation of glass chemistry in terms of the optical basicity concept, J. Non-Cryst. Solids 21 (1976) 373–410. [7] J.K. West, G. La Torre, L.L. Hench, The UV–visible spectrum in porous type VI silica: application and theory, J. Non-Cryst. Solids 195 (1996) 45–53. [8] N.V. Suzdal, O.A. Prokhorenko, V.D. Khalilev, Absorption spectra of cobalttinted alkali-borate glasses, Glass Ceram. 60 (3–4) (2003) 71–74. [9] S. Zhang, H. Xia, J. Wang, Y. Zhang, Growth of Mn2+, Co2+ and Ni2+ -doped nearstoichiometric LiNbO3 by Bridgman method using K2O flux, J. Alloys Compd. 463 (2008) 446–452. [10] I. Ardelean, Gh. Ilonca, S. Simon, S. Filip, T. Jurcut, Magnetic properties of cobalt-strontium-borate oxide glasses, Mater. Res. Bull. 32 (2) (1997) 191– 196. [11] J. Sponer, J. Cejka, J. Dedecek, B. Wichterlova, Coordination and properties of cobalt in the molecular sieves CoAPO-5 and -11, Microporous Mesoporous Mater. 37 (2000) 117–127. [12] C.R. Bamford, The application of the ligand field theory to colored glasses, Phys. Chem. Glasses 3 (6) (1962) 189–202. [13] H. Keppler, N. Bagdassarov, The speciation of Ni and Co in silicate melts from optical absorption spectra to 1500 °C, Chem. Geol. 158 (1999) 105–115. [14] A. Paul, R.W. Douglas, Co-ordination equilibria of nickel (II) in alkali borate glasses, Phys. Chem. Glasses 8 (6) (1967) 233–237. [15] L. Galoisy, L. Cormier, S. Rossano, A. Ramos, G. Calas, P. Gaskell, M. Le Grand, Cationic ordering in oxide glasses: the example of transition elements, Mineralog. Mag. 64 (3) (2000) 409–424. [16] A. Paul, A.N. Tiwari, Optical absorption of nickel (II) in Na2O–NaCl–B2O3 and Na2O NaBr–B2O3 glasses, J. Mater. Sci. 9 (1974) 1057–1063. [17] S. Roy, D. Ganguli, Optical properties of Ni2+s-doped silica and silicate gel monoliths, J. Non-Cryst. Solids 151 (1992) 203–208. [18] S.R. Ramanan, D. Ganguli, Spectroscopic studies of Cr-doped silica gels, J. NonCryst. Solids 212 (1997) 299–302. [19] L. Galoisy, G. Calas, L. Cormier, Chemical stability of Ni-enriched nanodomains in alkali borate glasses, J. Non-Cryst. Solids 321 (2003) 197–203. [20] K. Morinaga, T. Murata, M. Torisaka, H. Takebe, Compositional dependence of the valency state of Cr ions in oxide glasses, J. Non-Cryst. Solids 220 (2–3) (1997) 139–146. [21] G. Calas, O. Majerus, L. Galoisy, L. Cormier, Crystal field spectroscopy of Cr3+ in glasses: Compositional dependence and thermal site expansion, Chem. Geol. 229 (2006) 218–226. [22] Y. Tanabe, S. Sugano, On the absorption spectra of complex ions, J. Phys. Soc. Jpn. 9 (1954) 753–766. [23] V. Felice, B. Dussardier, J.K. Jones, G. Monnom, D.B. Ostrowsky, Chromiumdoped silica optical fibres: influence of the core composition on the Cr oxidation states and crystal field, Opt. Mater. 16 (2001) 269–277. [24] N.W. Barnett, B.J. Hindson, P. Jones, A.S. Trevor, Chemically induced phosphorescence from manganese (II) during the oxidation of various compounds by manganese (III), (IV) break and (VII) in acidic aqueous solutions, Anal. Chim. Acta 451 (2002) 181–188.