Available online at Journal of Non-Crystalline Solids 354 (2008) 3343–3347 Optical properties of Cr3+ ion in lithium metasilicate Li2O Á SiO2 Shigeki Morimoto a,*, Sasithorn Khonthon a, Yasutake Ohishi b a School of Ceramic Engineering, Institute of Engineering, Suranaree University of Technology, 111 University Avenue, Muang District, Nakhon Ratchasima 30000, Thailand b Research Center for Advanced Photon Technology, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan Received 15 August 2007; received in revised form 14 January 2008 The optical properties of Cr3+ ions in lithium metasilicate (Li2O Á SiO2) transparent glass–ceramics were investigated. The main crys- talline phase precipitated was the lithium metasilicate (Li2O Á SiO2) crystal. The percent crystallinity and crystalline size were ranging 65–75% and 20–35 nm, respectively. The color changes drastically to deep pink from emerald green upon crystallization. New and strongabsorption bands appeared and the absorption intensity increases by about 10 times that in glass. These new absorption bands are foundto be derived from Cr3+ ions in octahedral sites in the lithium metasilicate crystal lattice. Cr3+ ions substitute for three Li+ ions andoccupy the distorted octahedral site between single [SiO4]n chains of lithium metasilicate crystal. The ligand field parameters can be esti-mated: 10Dq = 13 088 cmÀ1, B = 453 cmÀ1, Dq/B = 2.89 and C = 2036 cmÀ1. The near-infrared luminescence centered at 1250 nm wasnot detected in the deep pink glass–ceramics unlike emerald green glass.
Ó 2008 Elsevier B.V. All rights reserved.
PACS: 42.70.C; 78.40; 78.55; 78.60; 78.66.J Keywords: Glass–ceramics; Optical spectroscopy; Nanocrystals; Nanoparticles; Absorption; Luminescence; Alkali silicates; Electron spin resonance ($700 nm) under excitation by shorter wavelengths oflight. In this case, Cr3+ is located in a strong ligand field.
Much research on color generation and luminescent On the other hand, recently, Cr3+ ions located in weak behavior of Cr3+ ions in glasses and single crystals have ligand field were reported in a Zn(Mg)-doped LiNbO3:Cr3+ been examined in terms of the transition metal coordina- single crystal . Usually, crystal hosts enhance the tion number . These results have been interpreted ligand field strength, but this crystal showed the opposite based on ligand field theory. It is well known that the Cr3+ ions preferentially occupy octahedral sites in glasses Glass–ceramics are two-phase systems consisting of a and crystals as indicated by their large site stabilizing base glass within which crystals are grown by heat treat- energy . The color and the positions of absorption bands ment. Transparent glass–ceramics, in which nano-scale change depending on the ligand field parameters.
crystals are precipitated, can be obtained by controlling In the case of ruby crystals (Cr3+:Al2O3) and spinel base glass composition and heat treatment condition.
crystals (Cr3+:MgAlO4) the absorption bands shift to Transparent glass–ceramics are currently being considered shorter wavelengths and they emit a characteristic red line as hosts for luminescent ions because of the possibilitiesafforded by crystal site control.
In this paper the optical properties of the Cr ion in Corresponding author. Tel.: +66 44 22 4475; fax: +66 44 22 4612.
E-mail address: (S. Morimoto).
Li2O Á SiO2 transparent glass–ceramics were investigated.
0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jnoncrysol.2008.01.025 S. Morimoto et al. / Journal of Non-Crystalline Solids 354 (2008) 3343–3347 It was found that the Cr3+ ion is present in the weak octa- where k is the wavelength of X-ray radiation (1.54 A hedral ligand field of lithium metasilicate nano-crystal.
true half width (radian) and h the diffraction angle (degree).
True half width was determined by Jones method and In order to confirm the presence of Cr3+ ion, electron spin resonance (ESR) spectra were measured using JEOLJES RE-2X at room temperature.
The base glass composition was 65SiO2 Á 5Al2O3 Á 2.4. 4. Optical absorption and emission spectra 2O Á 4K2O (wt%), and 0.2 wt% of Cr2O3 and 1.0 wt% of Sb2O3 were added to 100 g of glass by excess. In thisglass the addition of Sb The absorption spectra were measured with Varian Cary and eliminated the charge transfer absorption of Cr6+ 5E UV–VIS–NIR Spectrophotometer in the range of 300– High purity silica sand, alumina and reagent grade of The emission spectra in near-infrared (NIR) region as raw materials. Batch corresponding to 25 g of glass was (1000–1700 nm) were also measured under the excitation mixed thoroughly, and then melted in a 50 ml Pt/Rh10 cru- of 974 nm laser diode at room temperature. Emission from cible at 1300 °C for 1 h in an electric furnace in air. The samples was dispersed by a single monochromator (blaze, molten glass was poured onto iron plate and pressed 1.0 lm: grating, 600 grooves/mm: resolution, 3 nm) and quickly by another iron plate to avoid crystallization.
The glass was annealed at 450 °C for 30 min to removestresses and cooled slowly to room temperature in the furnace, and then they were heat treated under conditionsof 650 °C – 5 h, 700 °C – 1.5 h and 700 °C – 5 h for The glass and glass–ceramics were cut and polished The glass–ceramics appeared to be transparent after the optically into about 1 mm in thickness for optical heat treatment below 800 °C. In these conditions, only lith- ium metasilicate (Li2O Á SiO2, hereafter LS) crystal (JCPDS00-029-0829) was detected. shows appearance, crystalline phases, percent crystallinity and crystal size of transparent glass–ceramics. The percent crystallinity and the crystallite size were ranging 65–75 wt% and 25– solid state reaction to investigate whether Cr3+ ion exists 35 nm, respectively. Both the percent crystallinity and the or not in this crystal. A mixture of high purity silica sand, crystallite size increased with increase in heating tempera- 2CO3 and Cr2O3 (1 wt% by excess) corresponding to the crystallization of LS crystal, the amount of LS crystal 2O Á SiO2 composition was heated at 1100 °C for 40 h would be about 78 wt%, and hence the measured valuesmay be reasonable.
Crystalline phases, amount of crystal and crystalline size were examined by powder X-ray diffraction (XRD) (Bru- The color changes drastically from pale emerald green to ker, AXS Model D5005) under the condition of Cu-Ka deep pink upon crystallization and the pink color becomes radiation, 40 kV–40 mA, 0.01° step, 1 s/step. Amount of deeper with increasing heating temperature and time. crystal was determined using Ohlberg and Strickler’smethod and was calculated by 100x½Ig À IxŠ=½Ig À IcŠ ¼ percent crystallinityð%CÞ; Crystalline phase, percent crystallinity and crystallite size of transparentglass–ceramics where Ig is the background intensity of glass, Ix that of specimens and Ic that of a-Quartz at 2h = 23°. The calibra- tion curve was obtained using parent glass and a-Quartz at various ratios, and showed good linearity. The percent crystallinity can be obtained by wt%.
S. Morimoto et al. / Journal of Non-Crystalline Solids 354 (2008) 3343–3347 700˚C-5h
Optical density
Synthesized LS :Cr crystal
Fig. 2. Diffuse reflectance spectra of glass–ceramics and synthesized polycrystalline Li2O Á SiO2:Cr at room temperature. Glass–ceramics:700 °C – 5 h.
360 nm, 530 nm and 750 nm, and the broad bands at Optical density
around 950 nm and 1150 nm are also observed in NIR 700˚C-1.5h
region. Furthermore, the weak absorption band at around 650˚C-5h
420 nm appears. This indicates that the Cr ion was incorporated into Li2O Á SiO2 crystal lattice in glass– Wavelength/nm
Fig. 1. Absorption spectra of glass and glass–ceramics at room temper-ature. Inset denotes heat treatment condition for crystallization (°C – h).
shows the emission spectra of glass and glass– ceramics in NIR region under the excitation of 974 nm,and no emission was observed in glass–ceramics. On the shows the absorption spectra of glass and glass–ceramics.
contrary, the broad emission centered at around 1250 nm In the glass the weak absorption bands appear at around was observed in glass. This emission might be due to4 450 and 650 nm and are due to d–d transition of Cr3+ T2 ? 4A2 transition of Cr3+ in octahedral coordination.
ion in octahedral site, and they can be assigned to4A2 ? 4T1 (%450 nm) and 4A2 ? 4T2 (%650 nm) transi-tions No absorption band was observed in NIR region.
On the contrary, in glass–ceramics, the new and strong absorption bands appear at around $370, $530 and $750 nm in VIS region, and the broad bands at around Ex=974 nm
950 nm and 1150 nm are observed in NIR region.
Furthermore, the weak absorption band at around420 nm is also found. It should be noted that the absorp- tion intensity of new absorption bands in glass–ceramicsincreases with increase in heating temperature and time,and is about 10 times greater than that in glass (700 °C Intensity/a.u.
5 h glass–ceramics). This suggests that Cr3+ ion enters intoLS nano-crystals in glass–ceramics.
The polycrystalline Li2O Á SiO2:Cr appeared to be pink Glass-ceramics(700˚C-5h)
00-029-0289). shows the diffuse reflectance spectra Wavelength/nm
polycrystalline Li2O Á SiO2:Cr is almost the same as Fig. 3. NIR emission spectra of glass and glass–ceramics under the that of glass–ceramics. The absorption bands appear at excitation of 974 nm laser diode at room temperature.
S. Morimoto et al. / Journal of Non-Crystalline Solids 354 (2008) 3343–3347 As shown in the absorption spectra in visible region of glass–ceramics are quite similar to that of pink 4.1. Assignment and coordination number of Cr3+ ion in LS Zn co-doped LiNbO3:Cr3+ single crystal in which Cr3+ ion occupies octahedral Nb5+ site, and its ligand fieldparameters were given, 10Dq = 13 420 cmÀ1, B = 646 cmÀ1 It was found that Cr3+ ion in octahedral site is present in and Dq/B = 2.1 . By assuming that three absorption glass by optical absorption spectra. In order to confirm the bands, $370 nm, %520 and %750 nm in glass–ceramics valence state and ligand field of Cr ion (Cr3+) in glass– observed here, would be assigned to 4A2g ? 4T1g(P), ceramics, ESR measurement was carried out. shows 4A2g ? 4T1g and 4A2g ? 4T2g transitions, the ligand field ESR spectra of glass and glass–ceramics. The ESR signals parameters can be estimated, 10Dq = 13 088 cmÀ1, B = were detected at g $ 4.3 and g $ 1.9 in both glass and 453 cmÀ1, C = 2036 cmÀ1, Dq/B = 2.89 and C/B = 4.50.
glass–ceramics, and these signals become more clear and The agreement between observed spectra and predicted the signal intensity increases upon crystallization, espe- taking above values is reasonable ). These values cially g $ 1.9 signal. Generally it was reported that ESR are also quite similar to those of Cr3+doped LiNbO3 single signal at g $ 4.3 is due to isolated Cr3+ ion in octahedral crystal and CrCl3 crystal In Cr3+doped lithium metasi- site and the signal at g $ 1.9 is due to exchange coupled licate (LS) glass–ceramics discussed here, the absorption pairs of Cr3+ ion in octahedral site . Furthermore, the bands shift to longer wavelength (red shift) unlike Cr3+ ESR signal at g $ 1.9 was found in Zn co-doped LiN- ion-doped ruby and spinel crystals . Usually crystal bO3:Cr3+ single crystal , and this signal is interpreted hosts enhance the ligand field strength, but this result to be due to Cr3+ ion occupying Nb5+ site (octahedral site) shows opposite feature. In this glass–ceramics, the absorp- in that crystal . Thus, it was confirmed that Cr3+ ion in tion due to the spin-forbidden transition 4A2g ? 2A2g octahedral site is present in glass and glass–ceramics by ($420 nm) also appeared, and this indicates the large dis- It is well known that Cr3+ ion occupies preferentially octahedral site in crystals and glasses. The color and absorption bands change with the ligand field strengthHere, the assignment of absorption bands will be dis- Lithium metasilicate (LS) crystal belongs to base cen- cussed bellow based on octahedral symmetry.
tered orthorhombic symmetry with Cmc21 space group, The energy diagram (Tanabe–Sugano diagram) in octa- and the LS crystal has a single chain structure in which hedral symmetry predicts that the spectrum of Cr3+ will each [SiO4] tetrahedron shares two vertices In the crys- consist essentially of three bands corresponding to the tal the parallel chains pack so as to provide suitable spin-allowed transitions 4A2g ? 4T1g, 4A2g ? 4T2g and environment for the cations, e.g. 6-coordination of Li+ 4A2g ? 4T1g(P) together with several weak lines and bands (0.68 A) and Mg2+(0.66A). Cr3+ ion (0.63 A) is slightly corresponding to spin-forbidden transition. The ligand smaller than Li+ and Mg2+ ions and hence Li+ ion might field parameters were analyzed by multi-peak fitting be substituted by Cr3+ ion, in this case however, three method with Gaussian distribution, and results are summa-rized in The examples of another crystal are also listed in this table for comparison.
Ligand field parameters of Cr3+ ion in glass and glass–ceramics usingGaussian distribution g=4.26305
10Dq = 13 088 cmÀ1, B = 453 cmÀ1,C = 2036 cmÀ1, Dq/B = 2.89, C/B = 4.50 Magnetic field/mT
Fig. 4. ESR spectra of Cr3+ ion in glass and glass–ceramics at room 10Dq = 13 600 cmÀ1, B = 510 cmÀ1,Dq/B = 2.66 S. Morimoto et al. / Journal of Non-Crystalline Solids 354 (2008) 3343–3347 Li+ ions should be replaced by one Cr3+ ion in order to cence was not detected in deep pink glass–ceramics unlike neutralize the electrical charge. NIR luminescence disap- peared and few spin-forbidden transitions appeared upon Thus, while crystal hosts usually enhance ligand field crystallization. Usually these phenomena can be observed strength, the lithium metasilicate crystal provides the oppo- when the symmetry of ligand polyhedra is lacking or decreases. As mentioned above, three Li+ ions should bereplaced by one Cr3+ ion to neutralize the electrical charge, and hence it seems that the ligand polyhedra of Cr3+ ionmight be distorted.
This study was supported by Special Coordination Fund Generally, the polarizable ligand induces the dramatic of Suranaree University of Technology, to which the decrease in Dq and B, and further the absorption intensity author indebted. The author also would like to thank Dr tends to increase according to spectrochemical series.
Sathorn Suwan, STREC, Chulalongkorn University, Thai- Therefore, this indicates that mixing of electron between land, for helpful discussion on ESR measurement.
central cation and ligands increases. The absorption inten-sity in glass–ceramics increases drastically.
[1] T. Bates, in: J.D. Mackenzie (Ed.), Modern Aspects of Vitreous State, vol. 2, Butterworth, London, 1962, p. 195.
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[4] C. Koepke et al., J. Lumin. 78 (1998) 135.
lithium metasilicate (Li2O Á SiO2) crystal. The percent crys- [5] I. Nikilov et al., Opt. Mater. 25 (2004) 53.
tallinity and crystalline size ranged from 60–70% and 20– [6] B.N. Figgis, Introduction to Ligand Fields, Interscience Publishers, 35 nm, respectively. Polycrystalline Li2O Á SiO2:Cr3+ was also prepared and its optical absorption was compared [7] G.A. Torchia, J.A. Sanz-Garcia, J. Diaz-Caro, F. Jaque, T.P.J. Han, with that of the transparent glass–ceramics.
[8] F. Jaque, T.P.J. Han, V. Bermuudez, E. Dieguez, J. Lumin. 102–103 The color changes drastically to deep pink from emerald green upon crystallization. New and strong absorption [9] T.P.J. Han, F. Jaque, V. Vermudez, E. Dieguez, Chem. Phys. Lett.
bands appeared and the absorption intensity increases by about 10 times that in the glass. The same absorption [10] G.H. Sigel, M. Tomozawa, R.H. Doremus (Eds.), Treatise on spectra can be obtained in both polycrystalline Li Materials Science and Technology, vol. 12, Academic Press, New SiO2:Cr3+ and in the glass–ceramics. These new absorption [11] S.M. Ohlberg, D.W. Strickler, J. Amer, Ceram. Soc. 45 (1962) 170.
bands are found to be derived from Cr3+ ions in octahedral [12] I. Nitta, X-ray Crystallography, vol. 1, Maruzen, Tokyo, 1975 (in sites in the lithium metasilicate crystal lattice. Cr3+ ions substitute for three Li+ ions and occupy the distorted octa- [13] D.K. Durga, N. Veeriah, Physica B 324 (2002) 127.
[14] G.A. Torchia, O. Martinez Santos, P. Vaveliuk, J.O. Tocho, Solid metasilicate crystal. The ligand field parameters can be esti- [15] A.F. Wells, Structural Inorganic Chemistry, 4th Ed., Clarendon mated: 10Dq = 13 088 cmÀ1, B = 453 cmÀ1, Dq/B = 2.89 and C = 2036 cmÀ1, respectively. Near-infrared lumines-


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