- Email: [email protected]
Structure study of bromocadmate and iodocadmate glasses by 113Cd MAS NMR spectroscopy
Journal of Non-Crystalline Solids 290 (2001) 15±19
www.elsevier.com/locate/jnoncrysol
Structure study of bromocadmate and iodocadmate glasses by 113 Cd MAS NMR spectroscopy Shinichi Sakida a,*, Yoji Kawamoto b b
a Venture Business Laboratory, Kobe University, Nada, Kobe 657-8501, Japan Division of Molecular Science, Graduate School of Science and Technology, Kobe University, Nada, Kobe 657-8501, Japan
Received 19 January 2001; received in revised form 16 May 2001
Abstract 113 Cd magic-angle spinning (MAS) NMR spectra have been measured for glasses of 50:0CdBr2 xKBr xCsBr 50:0±2xBaBr2 (x 17:5 and 20.0), 40CdI2 40KI 20CsI, 50CdI2 yKI 50±yCsI (y 10; 20, and 30), 60CdI2 10KI 30CsI, and 60±zCdI2 40CsI zBaI2 (z 0; 5 and 10) in mol% to reveal the anion co-ordination environments around Cd2 in the glasses. The bromocadmate and iodocadmate glasses gave isotropic chemical shift values of )27 ppm and 78121 ppm, respectively, indicating that the former and the latter glasses consist of CdBr6 octahedra and CdI4 tetrahedra, respectively. Moreover, the octahedral anion co-ordination around Cd2 in the bromocadmate glasses has higher symmetry than that in the chlorocadmate glasses, and the iodine co-ordination environment around Cd2 in the iodocadmate glasses is dependent on the CdI2 content, but is independent of the kind of glass-modifying cation. Ó 2001 Elsevier Science B.V. All rights reserved.
PACS: 61.18.F; 61.43.F
1. Introduction Bromide and iodide materials are more desirable halide hosts for photo-active rare earth ions than ¯uoride materials because the transparency in the far infrared region is superior to that in ¯uorides and the maximum phonon energies are extremely low. Thus far, several studies have been already carried out on the upconversion properties of bromide and iodide single-crystals [1,2] from scienti®c and also practical interests. On the other hand, the near-infrared to visible upconversion characteristics of Er3 in 50CdBr2 20KBr * Corresponding author. Tel.: +81-78 803 5703; fax: +81-78 803 5679. E-mail address: [email protected] (S. Sakida).
20CsBr 10BaBr2 3ErBr3 and 50CdI2 40CsI 10BaI2 3ErI3 glasses in mol% have been clari®ed by Shojiya et al. [3]. CdI2 -based glasses have relatively large glassforming regions whereas CdX2 -based glasses (X Cl and Br) have very small ones [4,5]. This is probably due to the dierence in the anion co-ordination environments around Cd2 between the CdI2 -based glasses and the CdX2 -based glasses (X Cl and Br). Therefore, the study on the structure of CdX2 -based glasses (X Cl, Br, and I) is very interesting from the viewpoint of glassforming ability. Since most of the bromide and iodide systems cannot be vitri®ed in one or two components, however, the structural investigations of these glasses are considerably dicult. Nevertheless, the structures of CdX2 -based glasses (X
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 7 2 9 - 3
16
S. Sakida, Y. Kawamoto / Journal of Non-Crystalline Solids 290 (2001) 15±19
Cl, Br, and I) have been investigated by means of Raman [4] and EXAFS [6] spectroscopies by Kadono et al. However, all the results obtained by EXAFS spectroscopy are not consistent with the structural models predicted from Raman spectroscopy. On the bromine co-ordination number of Cd2 in CdBr2 -based glasses, for example, Raman spectroscopy gives ®ve or six, but EXAFS spectroscopy gives four. In order to solve the contradictions, Kadono et al. have proposed the following structure model: By taking into account the Cd±X (X Br or I) bond distances acquired from EXAFS spectra, the anion co-ordination numbers of Cd2 in the bromocadmate glasses are ®ve or six, and those in the iodocadmate glasses are four. Thus, the structure model is not still wellde®ned. 113 Cd magic angle spinning (MAS) NMR spectroscopy is a powerful technique that directly gives de®nitive informations about the anion coordination environments around Cd2 . By using this technique Sakida et al. [7] have revealed that, in 50CdF2 30NaF 20BaF2 and 50CdCl2 15NaCl 35BaCl2 glasses, the CdF6 , CdF7 , and CdF8 polyhedra and CdCl6 octahedra are present as the structural units, respectively, and that, in 50CdF2 30NaCl 20 xBaF2 xBaCl2 glasses, the CdF6 p Clp and CdF7 q Clq polyhedra (p 1±5 and q 1±5) which have not been found in crystalline compounds are present. Therefore, this technique is highly useful for the structural analysis of multicomponent bromocadmate and iodocadmate glasses. In the present study, the anion co-ordination environments around Cd2 in two bromocadmate glasses in the CdBr2 KBr CsBr BaBr2 system and eight iodocadmate glasses in the CdI2 KI CsI and CdI2 CsI BaI2 systems are investigated by means of 113 Cd MAS NMR spectroscopy. 2. Experimental 2.1. Glass preparation Bromocadmate glasses prepared have the 50:0CdBr2 xKBr xCsBr 50:0±2xBaBr2 com-
position (x 17:5 and 20.0) in mol%, and iodocadmate glasses prepared have the 40CdI2 40KI 20CsI, 50CdI2 yKI 50±yCsI (y 10; 20, and 30), 60CdI2 10KI 30CsI, and 60±zCdI2 40CsI zBaI2 (z 0; 5, and 10) compositions in mol%. In the preparation of bromocadmate and iodocadmate glasses, 0.8 mol% ErBr3 and 0.8 mol% ErI3 , respectively, were added to shorten the relaxation time of a Cd nucleus with I 1=2. High purity reagents of CdBr2 , KBr, CsBr, BaBr2 , ErBr3 , CdI2 , KI, CsI, BaI2 , and ErI3 were used as starting materials. These reagents were dried by heating at appropriate temperatures under vacuum. Five gram batches of well mixed reagents were melted at 600±650°C for 15 min in silica crucibles in a glove box ®lled with a dry Ar gas. In the preparation of the bromocadmate and iodocadmate glasses, small amounts of NH4 Br and NH4 I were added to the batches and/or the melts in order to remove residual water and also to convert very slight inclusions such as oxides into bromides and iodides, respectively. Then the melts were quickly quenched by pressing them between a pair of brass plates in the glove box. No ErBr3 -containing 50:0CdBr2 17:5KBr 17:5CsBr 15:0BaBr2 and no ErI3 -containing 50CdI2 40CsI 10BaI2 glasses were also prepared by the same procedures to examine eects on the 113 Cd isotropic chemical shift diso due to the addition of a small amount of ErBr3 and ErI3 . 2.2. NMR measurements 113
Cd MAS NMR spectra were measured on powdered glasses at 88.738 MHz (9.4 T) with a Varian Unity Inova 400 MAS FT±NMR spectrometer. A single pulse sequence was used for all the measurements. The acquisition parameters were as follows; a 4:0 ls pulse length, 480±2400 scans per spectrum, and 1.0±60.0 s pulse delays. Samples in cylindrical zirconia rotors were spun at spinning rates of about 5±6 kHz. Chemical shifts were referenced to a 1 M Cd ClO4 2 aqueous solution at 0 ppm. The experimental errors in both diso and full width at half maximum (FWHM) were within 1 ppm.
S. Sakida, Y. Kawamoto / Journal of Non-Crystalline Solids 290 (2001) 15±19
3. Results Fig. 1 shows the 113 Cd MAS NMR spectra of glasses of 50:0CdBr2 17:5KBr 17:5CsBr 15:0BaBr2 with no and 0.8 mol% ErBr3 additions and 50CdI2 40CsI 10BaI2 with no and 0.8 mol% ErI3 additions. The diso values of the 50:0CdBr2 17:5KBr 17:5CsBr 15:0BaBr2 and 50CdI2 40CsI 10BaI2 glasses are )27 and 81 ppm, respectively. It was found that the additions of small amounts of ErBr3 and ErI3 give no eects on diso and line pro®le, in other words, cause no changes in the anion co-ordination environments around Cd2 . The glasses with no and 0.8 mol% ErX3 (X Br or I) additions were able to be measured by using 60.0 and 1.0 s pulse delay, respectively. These facts indicate that ErX3 (X Br or I) can remarkably shorten the relax-
Fig. 1. 113 Cd MAS NMR spectra of glasses of 50:0CdBr2 17:5KBr 17:5CsBr 15:0BaBr2 with no and 0.8 mol% ErBr3 additions and 50:0CdI2 40:0CsI 10:0BaI2 with no and 0.8 mol% ErI3 additions.
17
ation time of a Cd nucleus, being useful as a relaxation reagent. In the present study, therefore, 113 Cd MAS NMR measurements were made of glasses containing 0.8 mol% ErX3 (X Br or I). The 113 Cd MAS NMR spectra of 50:0CdBr2 xKBr xCsBr 50:0±2xBaBr2 glasses (x 17.5 and 20.0) are shown in Fig. 2. As given in the respective spectra, the diso values are )27 ppm. The spectra of these bromocadmate glasses consist of only one peak without splitting. The peaks spread out over the range of )150 to 100 ppm and have the FWHMs of 78 to 88 ppm. The diso and FWHM values of these bromocadmate glasses are listed in Table 1. The 113 Cd MAS NMR spectra of glasses of 40CdI2 40KI 20CsI, 50CdI2 yKI 50±yCsI (y 10; 20, and 30), 60CdI2 10KI 30CsI, and 60±zCdI2 40CsI zBaI2 (z 0; 5, and 10) are shown in Fig. 3. The numerals in the respective spectra mean the diso values. The spectra of these glasses gave only one peak over the range of )150 to 300 ppm without splitting. The diso values of these glasses are in the range of 78 to 121 ppm. The peaks have the FWHM values of 89 to 185 ppm. The diso and FWHM values of these iodocadmate glasses are listed in Table 1.
113 Fig. 2. Cd MAS NMR 50:0CdBr2 xKBr xCsBr 50:0±2xBaBr2 and 20.0).
spectra of glasses (x 17:5
18
S. Sakida, Y. Kawamoto / Journal of Non-Crystalline Solids 290 (2001) 15±19
Table 1 Isotropic chemical shifts diso and FWHM determined by
113
Cd MAS NMR at 9.4 T
Glass composition/mol%
diso =ppm
FWHM/ppm
50:0CdBr2 17:5KBr 17:5CsBr 15:0BaBr2 50CdBr2 20KBr 20CsBr 10BaBr2 40CdI2 40KI 20CsI 50CdI2 30KI 20CsI 50CdI2 20KI 30CsI 50CdI2 10KI 40CsI 60CdI2 10KI 30CsI 50CdI2 40CsI 10BaI2 55CdI2 40CsI 5BaI2 60CdI2 40CsI
)27 ()27) )27 121 82 86 85 78 81 (81) 86 84
78 (83) 88 136 135 149 171 93 167 (174) 185 89
Experimental errors in diso and FWHM are within 1 ppm. The number in parentheses indicates the diso or FWHM value of glass containing neither ErBr3 nor ErI3 .
4. Discussion 4.1. Bromine co-ordination environments around Cd 2 in bromocadmate glasses
for the CdI6 octahedra range from )715 to )713 ppm and those for the CdI4 tetrahedra range from 65 to 156 ppm [8]. Therefore, it is concluded that
Based on the 113 Cd MAS NMR measurements of several bromocadmate crystals with known structures, Sakida et al. have revealed that the diso values for the CdBr6 octahedra range from )64 to )24 ppm and that for the CdBr4 tetrahedron is 421 ppm [8]. Hence, it may be concluded that the present bromocadmate glasses are composed of CdBr6 octahedra alone. The FWHM values of these bromocadmate glasses are much smaller than those of 50CdCl2 15NaCl 35BaCl2 glasses with no and 0.5 mol% NiCl2 additions (133 and 131 ppm, respectively) [7]. This fact indicates that the octahedral anion-co-ordination around Cd2 in these bromocadmate glasses have higher symmetry than those in the chlorocadmate glasses. A diculty in obtaining glasses in the CdBr2 -based system, which has been described in [4], is probably due to the higher symmetry of the CdBr6 octahedra caused by the larger ionic radius of Br compared with Cl . 4.2. Iodine co-ordination environments around Cd 2 in iodocadmate glasses The diso values of several iodocadmate crystals with known structures have been already determined by means of 113 Cd MAS NMR spectroscopy and then it has been found that the diso values
Fig. 3. 113 Cd MAS NMR spectra of glasses of 40CdI2 40KI 20CsI, 50CdI2 yKI 50±yCsI (y 10; 20, and 30), 60CdI2 10KI 30CsI, and 60±zCdI2 40CsI zBaI2 (z 0; 5, and 10).
S. Sakida, Y. Kawamoto / Journal of Non-Crystalline Solids 290 (2001) 15±19
the present iodocadmate glasses consist of CdI4 tetrahedra alone. This conclusion is consistent with that obtained from Refs. [4] and [6]. The diso and FWHM values of the 50CdI2 10KI 40CsI glass are very similar to those of the 50CdI2 40CsI 10BaI2 glass, indicating that the iodine coordination environment around Cd2 is independent of the kind of glass-modifying cation. Two kinds of iodocadmate glasses containing 60 mol% CdI2 have the FWHM values of 89 and 93 ppm. On the other hand, the other iodocadmate glasses containing 40±55 mol% CdI2 have the FWHM values of 135±185 ppm. This suggests that CdI4 tetrahedra in the glasses with 60 mol% CdI2 have higher symmetry than those in the other glasses and are closer to those in iodocadmate crystals. Thus, an increase in CdI2 content probably leads to a decrease in the degree of freedom of glassnetwork. When the CdI2 content exceeds 60 mol%, therefore, the degree of freedom of glass-network composed of CdI4 tetrahedra may be too low to give glasses. The diso value of the 40CdI2 40KI 20CsI glass is 121 ppm whereas those of the other glasses are 78±86 ppm, suggesting that CdI4 tetrahedra in the glass have iodine co-ordination environments which are dierent from those in the other glasses. The dierence in diso values between the 40CdI2 40KI 20CsI glass and the other glasses may be explained based on the relationship between the diso values and the structures in iodocadmate crystals with known structures. RbCdI3 H2 O consists of chains of CdI4 tetrahedra [9] whereas a±Cs2 CdI4 is composed of isolated CdI4 tetrahedra [10]. The diso values of RbCdI3 H2 O and a±Cs2 CdI4 are 115 and 156 ppm, respectively [8], suggesting that the number of non-bridging iodide anions in the CdI4 tetrahedra increases with an increase of diso . Therefore, a decrease in CdI2 content in these iodocadmate glasses is considered to cause an increase in the number of non-bridging iodide anions in the CdI4 tetrahedra, that is, a decrease
19
of the number and length of chains consisted of CdI4 tetrahedra. In the case of the CdI2 contents less than 40 mol%, the networks consisted of CdI4 tetrahedra are probably insucient to generate glass-formation. 5. Conclusion The summaries of the present study are as follows: (1) The 50:0CdBr2 xKBr xCsBr 50:0±2x BaBr2 glasses (x 17:5 and 20.0) in mol% are composed of CdBr6 octahedra alone; (2) The octahedral anion co-ordination around Cd2 in the bromocadmate glasses has higher symmetry than that in chlorocadmate glasses; (3) In the glasses of 40CdI2 40KI 20CsI, 50CdI2 yKI 50±yCsI (y 10; 20, and 30), 60CdI2 10KI 30CsI, and 60±zCdI2 40CsI zBaI2 (z 0; 5, and 10) in mol%, the CdI4 tetrahedra alone are present as the structural units; (4) The iodine co-ordination environment around Cd2 in the iodocadmate glasses is dependent on the CdI2 content, but it is independent of the kind of glass-modifying cation. References [1] N.J. Cockroft, G.D. Jones, D.C. Nguyen, Phys. Rev. B 45 (1992) 5187. [2] M.P. Hehlen, K. Kramer, H.U. G udel, Phys. Rev. B 49 (1994) 12475. [3] M. Shojiya, M. Takahashi, R. Kanno, Y. Kawamoto, Appl. Phys. Lett. 67 (1995) 2453. [4] K. Kadono, T. Shimomura, H. Tanaka, Phys. Chem. Glasses 32 (1991) 29. [5] E.I. Cooper, C.A. Angell, J. Non-Cryst. Solids 56 (1983) 75. [6] K. Kadono, N. Kamijo, H. Tanaka, H. Kageyama, Phys. Chem. Glasses 38 (1997) 232. [7] S. Sakida, M. Shojiya, Y. Kawamoto, Solid State Commun. 115 (2000) 553. [8] S. Sakida, Y. Kawamoto, Chem. Lett. (2000) 1046. [9] M.N. Iyer, R. Faggiani, I.D. Brown, Acta Crystallogr. B 33 (1977) 129. [10] R. Sj ovall, Acta Crystallogr. C 45 (1989) 667.
www.elsevier.com/locate/jnoncrysol
Structure study of bromocadmate and iodocadmate glasses by 113 Cd MAS NMR spectroscopy Shinichi Sakida a,*, Yoji Kawamoto b b
a Venture Business Laboratory, Kobe University, Nada, Kobe 657-8501, Japan Division of Molecular Science, Graduate School of Science and Technology, Kobe University, Nada, Kobe 657-8501, Japan
Received 19 January 2001; received in revised form 16 May 2001
Abstract 113 Cd magic-angle spinning (MAS) NMR spectra have been measured for glasses of 50:0CdBr2 xKBr xCsBr 50:0±2xBaBr2 (x 17:5 and 20.0), 40CdI2 40KI 20CsI, 50CdI2 yKI 50±yCsI (y 10; 20, and 30), 60CdI2 10KI 30CsI, and 60±zCdI2 40CsI zBaI2 (z 0; 5 and 10) in mol% to reveal the anion co-ordination environments around Cd2 in the glasses. The bromocadmate and iodocadmate glasses gave isotropic chemical shift values of )27 ppm and 78121 ppm, respectively, indicating that the former and the latter glasses consist of CdBr6 octahedra and CdI4 tetrahedra, respectively. Moreover, the octahedral anion co-ordination around Cd2 in the bromocadmate glasses has higher symmetry than that in the chlorocadmate glasses, and the iodine co-ordination environment around Cd2 in the iodocadmate glasses is dependent on the CdI2 content, but is independent of the kind of glass-modifying cation. Ó 2001 Elsevier Science B.V. All rights reserved.
PACS: 61.18.F; 61.43.F
1. Introduction Bromide and iodide materials are more desirable halide hosts for photo-active rare earth ions than ¯uoride materials because the transparency in the far infrared region is superior to that in ¯uorides and the maximum phonon energies are extremely low. Thus far, several studies have been already carried out on the upconversion properties of bromide and iodide single-crystals [1,2] from scienti®c and also practical interests. On the other hand, the near-infrared to visible upconversion characteristics of Er3 in 50CdBr2 20KBr * Corresponding author. Tel.: +81-78 803 5703; fax: +81-78 803 5679. E-mail address: [email protected] (S. Sakida).
20CsBr 10BaBr2 3ErBr3 and 50CdI2 40CsI 10BaI2 3ErI3 glasses in mol% have been clari®ed by Shojiya et al. [3]. CdI2 -based glasses have relatively large glassforming regions whereas CdX2 -based glasses (X Cl and Br) have very small ones [4,5]. This is probably due to the dierence in the anion co-ordination environments around Cd2 between the CdI2 -based glasses and the CdX2 -based glasses (X Cl and Br). Therefore, the study on the structure of CdX2 -based glasses (X Cl, Br, and I) is very interesting from the viewpoint of glassforming ability. Since most of the bromide and iodide systems cannot be vitri®ed in one or two components, however, the structural investigations of these glasses are considerably dicult. Nevertheless, the structures of CdX2 -based glasses (X
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 7 2 9 - 3
16
S. Sakida, Y. Kawamoto / Journal of Non-Crystalline Solids 290 (2001) 15±19
Cl, Br, and I) have been investigated by means of Raman [4] and EXAFS [6] spectroscopies by Kadono et al. However, all the results obtained by EXAFS spectroscopy are not consistent with the structural models predicted from Raman spectroscopy. On the bromine co-ordination number of Cd2 in CdBr2 -based glasses, for example, Raman spectroscopy gives ®ve or six, but EXAFS spectroscopy gives four. In order to solve the contradictions, Kadono et al. have proposed the following structure model: By taking into account the Cd±X (X Br or I) bond distances acquired from EXAFS spectra, the anion co-ordination numbers of Cd2 in the bromocadmate glasses are ®ve or six, and those in the iodocadmate glasses are four. Thus, the structure model is not still wellde®ned. 113 Cd magic angle spinning (MAS) NMR spectroscopy is a powerful technique that directly gives de®nitive informations about the anion coordination environments around Cd2 . By using this technique Sakida et al. [7] have revealed that, in 50CdF2 30NaF 20BaF2 and 50CdCl2 15NaCl 35BaCl2 glasses, the CdF6 , CdF7 , and CdF8 polyhedra and CdCl6 octahedra are present as the structural units, respectively, and that, in 50CdF2 30NaCl 20 xBaF2 xBaCl2 glasses, the CdF6 p Clp and CdF7 q Clq polyhedra (p 1±5 and q 1±5) which have not been found in crystalline compounds are present. Therefore, this technique is highly useful for the structural analysis of multicomponent bromocadmate and iodocadmate glasses. In the present study, the anion co-ordination environments around Cd2 in two bromocadmate glasses in the CdBr2 KBr CsBr BaBr2 system and eight iodocadmate glasses in the CdI2 KI CsI and CdI2 CsI BaI2 systems are investigated by means of 113 Cd MAS NMR spectroscopy. 2. Experimental 2.1. Glass preparation Bromocadmate glasses prepared have the 50:0CdBr2 xKBr xCsBr 50:0±2xBaBr2 com-
position (x 17:5 and 20.0) in mol%, and iodocadmate glasses prepared have the 40CdI2 40KI 20CsI, 50CdI2 yKI 50±yCsI (y 10; 20, and 30), 60CdI2 10KI 30CsI, and 60±zCdI2 40CsI zBaI2 (z 0; 5, and 10) compositions in mol%. In the preparation of bromocadmate and iodocadmate glasses, 0.8 mol% ErBr3 and 0.8 mol% ErI3 , respectively, were added to shorten the relaxation time of a Cd nucleus with I 1=2. High purity reagents of CdBr2 , KBr, CsBr, BaBr2 , ErBr3 , CdI2 , KI, CsI, BaI2 , and ErI3 were used as starting materials. These reagents were dried by heating at appropriate temperatures under vacuum. Five gram batches of well mixed reagents were melted at 600±650°C for 15 min in silica crucibles in a glove box ®lled with a dry Ar gas. In the preparation of the bromocadmate and iodocadmate glasses, small amounts of NH4 Br and NH4 I were added to the batches and/or the melts in order to remove residual water and also to convert very slight inclusions such as oxides into bromides and iodides, respectively. Then the melts were quickly quenched by pressing them between a pair of brass plates in the glove box. No ErBr3 -containing 50:0CdBr2 17:5KBr 17:5CsBr 15:0BaBr2 and no ErI3 -containing 50CdI2 40CsI 10BaI2 glasses were also prepared by the same procedures to examine eects on the 113 Cd isotropic chemical shift diso due to the addition of a small amount of ErBr3 and ErI3 . 2.2. NMR measurements 113
Cd MAS NMR spectra were measured on powdered glasses at 88.738 MHz (9.4 T) with a Varian Unity Inova 400 MAS FT±NMR spectrometer. A single pulse sequence was used for all the measurements. The acquisition parameters were as follows; a 4:0 ls pulse length, 480±2400 scans per spectrum, and 1.0±60.0 s pulse delays. Samples in cylindrical zirconia rotors were spun at spinning rates of about 5±6 kHz. Chemical shifts were referenced to a 1 M Cd ClO4 2 aqueous solution at 0 ppm. The experimental errors in both diso and full width at half maximum (FWHM) were within 1 ppm.
S. Sakida, Y. Kawamoto / Journal of Non-Crystalline Solids 290 (2001) 15±19
3. Results Fig. 1 shows the 113 Cd MAS NMR spectra of glasses of 50:0CdBr2 17:5KBr 17:5CsBr 15:0BaBr2 with no and 0.8 mol% ErBr3 additions and 50CdI2 40CsI 10BaI2 with no and 0.8 mol% ErI3 additions. The diso values of the 50:0CdBr2 17:5KBr 17:5CsBr 15:0BaBr2 and 50CdI2 40CsI 10BaI2 glasses are )27 and 81 ppm, respectively. It was found that the additions of small amounts of ErBr3 and ErI3 give no eects on diso and line pro®le, in other words, cause no changes in the anion co-ordination environments around Cd2 . The glasses with no and 0.8 mol% ErX3 (X Br or I) additions were able to be measured by using 60.0 and 1.0 s pulse delay, respectively. These facts indicate that ErX3 (X Br or I) can remarkably shorten the relax-
Fig. 1. 113 Cd MAS NMR spectra of glasses of 50:0CdBr2 17:5KBr 17:5CsBr 15:0BaBr2 with no and 0.8 mol% ErBr3 additions and 50:0CdI2 40:0CsI 10:0BaI2 with no and 0.8 mol% ErI3 additions.
17
ation time of a Cd nucleus, being useful as a relaxation reagent. In the present study, therefore, 113 Cd MAS NMR measurements were made of glasses containing 0.8 mol% ErX3 (X Br or I). The 113 Cd MAS NMR spectra of 50:0CdBr2 xKBr xCsBr 50:0±2xBaBr2 glasses (x 17.5 and 20.0) are shown in Fig. 2. As given in the respective spectra, the diso values are )27 ppm. The spectra of these bromocadmate glasses consist of only one peak without splitting. The peaks spread out over the range of )150 to 100 ppm and have the FWHMs of 78 to 88 ppm. The diso and FWHM values of these bromocadmate glasses are listed in Table 1. The 113 Cd MAS NMR spectra of glasses of 40CdI2 40KI 20CsI, 50CdI2 yKI 50±yCsI (y 10; 20, and 30), 60CdI2 10KI 30CsI, and 60±zCdI2 40CsI zBaI2 (z 0; 5, and 10) are shown in Fig. 3. The numerals in the respective spectra mean the diso values. The spectra of these glasses gave only one peak over the range of )150 to 300 ppm without splitting. The diso values of these glasses are in the range of 78 to 121 ppm. The peaks have the FWHM values of 89 to 185 ppm. The diso and FWHM values of these iodocadmate glasses are listed in Table 1.
113 Fig. 2. Cd MAS NMR 50:0CdBr2 xKBr xCsBr 50:0±2xBaBr2 and 20.0).
spectra of glasses (x 17:5
18
S. Sakida, Y. Kawamoto / Journal of Non-Crystalline Solids 290 (2001) 15±19
Table 1 Isotropic chemical shifts diso and FWHM determined by
113
Cd MAS NMR at 9.4 T
Glass composition/mol%
diso =ppm
FWHM/ppm
50:0CdBr2 17:5KBr 17:5CsBr 15:0BaBr2 50CdBr2 20KBr 20CsBr 10BaBr2 40CdI2 40KI 20CsI 50CdI2 30KI 20CsI 50CdI2 20KI 30CsI 50CdI2 10KI 40CsI 60CdI2 10KI 30CsI 50CdI2 40CsI 10BaI2 55CdI2 40CsI 5BaI2 60CdI2 40CsI
)27 ()27) )27 121 82 86 85 78 81 (81) 86 84
78 (83) 88 136 135 149 171 93 167 (174) 185 89
Experimental errors in diso and FWHM are within 1 ppm. The number in parentheses indicates the diso or FWHM value of glass containing neither ErBr3 nor ErI3 .
4. Discussion 4.1. Bromine co-ordination environments around Cd 2 in bromocadmate glasses
for the CdI6 octahedra range from )715 to )713 ppm and those for the CdI4 tetrahedra range from 65 to 156 ppm [8]. Therefore, it is concluded that
Based on the 113 Cd MAS NMR measurements of several bromocadmate crystals with known structures, Sakida et al. have revealed that the diso values for the CdBr6 octahedra range from )64 to )24 ppm and that for the CdBr4 tetrahedron is 421 ppm [8]. Hence, it may be concluded that the present bromocadmate glasses are composed of CdBr6 octahedra alone. The FWHM values of these bromocadmate glasses are much smaller than those of 50CdCl2 15NaCl 35BaCl2 glasses with no and 0.5 mol% NiCl2 additions (133 and 131 ppm, respectively) [7]. This fact indicates that the octahedral anion-co-ordination around Cd2 in these bromocadmate glasses have higher symmetry than those in the chlorocadmate glasses. A diculty in obtaining glasses in the CdBr2 -based system, which has been described in [4], is probably due to the higher symmetry of the CdBr6 octahedra caused by the larger ionic radius of Br compared with Cl . 4.2. Iodine co-ordination environments around Cd 2 in iodocadmate glasses The diso values of several iodocadmate crystals with known structures have been already determined by means of 113 Cd MAS NMR spectroscopy and then it has been found that the diso values
Fig. 3. 113 Cd MAS NMR spectra of glasses of 40CdI2 40KI 20CsI, 50CdI2 yKI 50±yCsI (y 10; 20, and 30), 60CdI2 10KI 30CsI, and 60±zCdI2 40CsI zBaI2 (z 0; 5, and 10).
S. Sakida, Y. Kawamoto / Journal of Non-Crystalline Solids 290 (2001) 15±19
the present iodocadmate glasses consist of CdI4 tetrahedra alone. This conclusion is consistent with that obtained from Refs. [4] and [6]. The diso and FWHM values of the 50CdI2 10KI 40CsI glass are very similar to those of the 50CdI2 40CsI 10BaI2 glass, indicating that the iodine coordination environment around Cd2 is independent of the kind of glass-modifying cation. Two kinds of iodocadmate glasses containing 60 mol% CdI2 have the FWHM values of 89 and 93 ppm. On the other hand, the other iodocadmate glasses containing 40±55 mol% CdI2 have the FWHM values of 135±185 ppm. This suggests that CdI4 tetrahedra in the glasses with 60 mol% CdI2 have higher symmetry than those in the other glasses and are closer to those in iodocadmate crystals. Thus, an increase in CdI2 content probably leads to a decrease in the degree of freedom of glassnetwork. When the CdI2 content exceeds 60 mol%, therefore, the degree of freedom of glass-network composed of CdI4 tetrahedra may be too low to give glasses. The diso value of the 40CdI2 40KI 20CsI glass is 121 ppm whereas those of the other glasses are 78±86 ppm, suggesting that CdI4 tetrahedra in the glass have iodine co-ordination environments which are dierent from those in the other glasses. The dierence in diso values between the 40CdI2 40KI 20CsI glass and the other glasses may be explained based on the relationship between the diso values and the structures in iodocadmate crystals with known structures. RbCdI3 H2 O consists of chains of CdI4 tetrahedra [9] whereas a±Cs2 CdI4 is composed of isolated CdI4 tetrahedra [10]. The diso values of RbCdI3 H2 O and a±Cs2 CdI4 are 115 and 156 ppm, respectively [8], suggesting that the number of non-bridging iodide anions in the CdI4 tetrahedra increases with an increase of diso . Therefore, a decrease in CdI2 content in these iodocadmate glasses is considered to cause an increase in the number of non-bridging iodide anions in the CdI4 tetrahedra, that is, a decrease
19
of the number and length of chains consisted of CdI4 tetrahedra. In the case of the CdI2 contents less than 40 mol%, the networks consisted of CdI4 tetrahedra are probably insucient to generate glass-formation. 5. Conclusion The summaries of the present study are as follows: (1) The 50:0CdBr2 xKBr xCsBr 50:0±2x BaBr2 glasses (x 17:5 and 20.0) in mol% are composed of CdBr6 octahedra alone; (2) The octahedral anion co-ordination around Cd2 in the bromocadmate glasses has higher symmetry than that in chlorocadmate glasses; (3) In the glasses of 40CdI2 40KI 20CsI, 50CdI2 yKI 50±yCsI (y 10; 20, and 30), 60CdI2 10KI 30CsI, and 60±zCdI2 40CsI zBaI2 (z 0; 5, and 10) in mol%, the CdI4 tetrahedra alone are present as the structural units; (4) The iodine co-ordination environment around Cd2 in the iodocadmate glasses is dependent on the CdI2 content, but it is independent of the kind of glass-modifying cation. References [1] N.J. Cockroft, G.D. Jones, D.C. Nguyen, Phys. Rev. B 45 (1992) 5187. [2] M.P. Hehlen, K. Kramer, H.U. G udel, Phys. Rev. B 49 (1994) 12475. [3] M. Shojiya, M. Takahashi, R. Kanno, Y. Kawamoto, Appl. Phys. Lett. 67 (1995) 2453. [4] K. Kadono, T. Shimomura, H. Tanaka, Phys. Chem. Glasses 32 (1991) 29. [5] E.I. Cooper, C.A. Angell, J. Non-Cryst. Solids 56 (1983) 75. [6] K. Kadono, N. Kamijo, H. Tanaka, H. Kageyama, Phys. Chem. Glasses 38 (1997) 232. [7] S. Sakida, M. Shojiya, Y. Kawamoto, Solid State Commun. 115 (2000) 553. [8] S. Sakida, Y. Kawamoto, Chem. Lett. (2000) 1046. [9] M.N. Iyer, R. Faggiani, I.D. Brown, Acta Crystallogr. B 33 (1977) 129. [10] R. Sj ovall, Acta Crystallogr. C 45 (1989) 667.