2TeO2 (M = Li, Na, K, Rb, Cs and Tl) glasses

]OURNA

Journal of Non-Crystalline Solids 144 (1992) 128-144 North-Holland

L OF

NON-CRYSTALLINE SOLIDS

Raman spectra of MO1/z-TeO 2 (M = Li, Na, K, Rb, Cs and T1) glasses T a k a o Sekiya, N o r i o M o c h i d a , A t s u s h i O h t s u k a a n d M a m o r u T o n o k a w a a Division of Materials Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama-shi 240, Japan Received 22 August 1991 Revised manuscript received 23 March 1992

The structure of MOI/2-TeO 2 (M = Li, Na, K, Rb, Cs and T1) binary glasses has been studied by means of Raman spectroscopy. The glasses having low alkali content have a continuous network constructed by sharing corners of TeO 4 trigonal bipyramids (tbp's) and TeO3+ I polyhedra having one non-bridging oxygen atom (NBO). In the glasses containing 20-30 mol% alkali oxide, TeO 3 trigonal pyramids (tp's) having NBOs are formed in the continuous network. When alkali content exceeds 30 mol%, isolated structural units, such as Te2 O2- ion, coexist in the continuous network. The fraction of TeO 4 tbp's decreases with increasing alkali content. The glasses, which contain nearly 50 mol% alkali oxide, are composed of a continuous network constituted by TeO3+ 1 polyhedra and TeO3 tp's, and of isolated structural units, such as Te20 z and TeO 2- ions. The structure of thallium tellurite glasses having less than 30 tool% T101/2 is similar to that of alkali tellurite glasses containing equal amounts of MO1/2. The fraction of TeO 3 tp's having NBOs in the thallium tellurite glasses, when TIO~/2 content is equal to or higher than 40 mol%, is larger than that in the corresponding alkali tellurite glasses. In the 66T101/2. 34TEO2 glass, most of tellurium atoms are in a form of isolated TeO~- ion. A new hypothesis is also given for a mechanism for the basic structural changes in the tellurite glasses.

1. Introduction It was indicated in a previous paper [1] that T e O 2 glass is composed of T e O 4 trigonal bipyramids (tbp's), in which one of the equatorial sites is occupied by a lone pair of electrons, and that most of the tellurium atoms are connected at vertices by Te-eqOax-Te linkage. The T e O 2 glass has an unique structure as a consequence of the the structural unit and its connecting style differs from the conventional glass formers, such as

1 Present address: Foundation for Promotion of Material Science and Technology of Japan, Kamisoshigaya 3-11-1, Setagaya-ku, Tokyo 157, Japan. Correspondence to: Dr T. Sekiya, Division of Materials Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai 156, HodogayaTku, Yokohama-shi 240, Japan. Tel: +81-45 335 1451. Telefax: + 81-45 331 6143.

B203, SiO2, GeO 2 and P205. It is expected that T e O 2 may have a structural role differing from the conventional glass formers in binary glasses which contain network modifiers. So far, several investigations have been made to determine the structure of alkali tellurite glasses [2-7]. Mochida et al. [2] showed that the primary structural unit of tellurite glasses having high T e O 2 content is a distorted T e O 4 tbp and that the fraction of T e O 3 trigonal pyramids (tp's) increases with increasing mono- or di-valent cation content. A neutron diffraction study of L i 2 0 - T e O 2 glasses performed by Noev et al. [3] showed that, in addition to the T e O 4 tbp's, there exist regrouped tetrahedra in which each of the tellurium atoms has three oxygen atoms in equatorial sites and one oxygen atom in an axial site. It was proposed by Nishida et al. [4] that an introduction of N a 2 0 into T e O 2 matrix results in a change of the glass matrix from a three- or two-dimensional network

0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

T. Sekiya et al. / Raman spectra of MO1/2-Te02 (M = Li, Na, K, Rb, Cs and Tl) glasses

structure to a lower dimensional one. An infrared study of LiC1-Li20-TeO 2 glasses performed by Yoko et al. [5] and by Tanaka et al. [6] suggested that, on addition of Li20 to TeO2, the bond strengths of Te-axO bonds become weaker and the TeO 4 tbp network breaks up accompanied by creation of non-bridging oxygen atoms (NBOs) in both Te-eqO and Te-axO bonds. Yoko et al. [7] proposed a mechanism for the change of the coordination number of Te 4+ from 4 through 3 + 1 to 3 and from 4 to 3 in L i X - L i 2 0 - T e O 2 (X = F and Br) glasses. In the present study, the structure of MO~/2TeO 2 (M = Li, Na, K, Rb, Cs and T1) binary glasses has been investigated by means of Raman spectroscopy. It is expected that the coordination state of tellurium atom in tellurite glasses may be determined by investigating several binary tellurite glasses having typical network modifiers. In this report, the symbol NBO is used for representation of the oxygen atoms forming Te=O, T e - O and their resonating bonds.

2. Experimental procedure Reagent-grade TeO2, thallium carbonate and alkali carbonates were mixed in the required compositions and then melted in Au crucible for 10-30 min in the temperature range from 700 to 800°C. The glass specimens were obtained by quenching the crucible in air or by dipping the bottom of crucible into ice-cold water. Crystalline specimens were obtained by gradually cooling the melts in the furnace. All the crystals were identified by X-ray powder diffraction. Hygroscopic specimens depending on composition of alkali oxides were sealed in glass ampules. Physical properties such as density, d, glass transition temperature, Tg, deformation temperature, Td, and thermal expansion coefficient, a, were measured on samples of RbOl/z-TeO 2 and CsO1/z-TeO 2 glasses. Raman spectra were measured in the wavenumber range from 20 to 1000 cm -a with Ar + laser (514.5 nm, 200 mW) and R-800 laser-Raman spectrophotometer with triple-monochrometer supplied by Japan Spectroscopic Co., Ltd.

129

(JASCO). The spectra were observed at an angle of 90° to the exciting light. The intensities of Raman spectra of glass specimens with differing compositions were corrected by an internal standardization technique with a standard of 35.2T101/2 • 64.8SIO 2 (mol%) glass. A small disk pellet was prepared from a mixed powder of each specimen and the standard glass. Raman spectrum was measured while the disk pellet was rotating. Further, the Raman spectrum was deconvoluted into symmetric Gaussian functions. In order to confirm the peak deconvolution, the following procedure was used. A pair of polarized spectra, lib and I ± , was deconvoluted in the wavenumber range from 420 to 880 cm-a and was simulated by the minimal number of peaks, taking the depolarization ratio, p(=l±/Iii), into consideration. Generally, the Raman peaks differ from one another in the depolarization ratio [8]. The deconvolution parameters, peak positions, intensities, half widths and depolarization ratios, were optimized by the least-squares method. Finally, the peak intensities were optimized for intensity-normalized Raman spectrum while the other parameters were fixed. The analysis of normal vibration was carried out with the Cartesian symmetry coordinates method [9,10]. A simple Urey-Bradley force field [11] was used as the internal potential field. For TeO 2- ions which have C3v symmetry, Raman peaks have already been assigned by Siebert [12]. The force constants of stretching of valence bond, K, bending of bonds, H, and repulsion between non-bonding atoms, F, were optimized by a brute force method [13]. The initial value of the force constant for the optimization of stretching was Table 1 Optimized force constants and calculated frequency values of T e O ~ - ion Force constants (mdyn/A.)

K(Te-O ) = 4.183 H ( o - _ x e _ o - ) = 0.379 F(o- Te_o- ) = 0.250

Frequencies (cm 1) AI

E

obs. a)

calc.

obs. a)

calc.

758 364

736 329

703 326

721 353

a) Ref. [12].

130

71. Sekiya et aL / Raman spectra of MO1/2-TeO 2 ( m = Li, Na, K,, Rb, Cs and Tl) glasses

given by Siebert [12], and the other initial values were assumed to be zero. The program of optimization [9] was developed for NEC PC-9801 series computer by the group of this laboratory. The results for the force constants are shown in table 1.

_M=,J

_

T

3. Results

3.1. MO1/2-TeO 2 (M = Li, Na, K, Rb and Cs) glasses

M=

Figure 1 shows the intensity-normalized Raman spectra of NaO1/e-TeO2 glasses. The Raman spectra of M O 1 / 2 - T e O 2 (M = Li, Na, K, Rb and Cs) binary glasses containing equal amounts of MO1/2 are similar to each other. For example, the spectra of 30MO1/2 • 70TeO 2 (M = Li, Na, K and Rb) glasses are shown in fig. 2. Figure 3 shows examples of peak deconvolutions of the spectra of 3 0 N a O a / e ' 7 0 T e O 2 and 50NaO1/2.

M=Rb

1000

800 800 Wavenumber

400 (cm-~)

200

Fig. 2. Intensity-normalized Raman spectra of 30MO1/2. 70TeO 2 (M = Li, Na, K and Rb) glasses.

5 0 T e n 2 glasses. Five peaks are assigned at about 780, 720, 665, 615 and 450 cm -1 in the range from 420 to 880 cm-a, as is similar case for T e n 2 glass [1]. In order to make a quantitative judgement of the agreement between observed and simulated spectra, the R-value was defined as

T

Eil//(observed) -//(calculated) [ R=

4~ CO3 C--

1000

800 600 Wavenumber

400 (cm-I)

200

Fig. l. Intensity-normalized Raman spectra of xNa01/2(100-x)TeO 2 glasses.

F.i l li(observed) I

where I i is intensity at position u i. The averaged and maximal R-values were 0.02 and 0.03, respectively. A good agreement was obtained between the observed and simulated spectra. In the case of the binary glass system, shifts of the peak positions are small. The five peaks are named A, B, C, D and E (fig. 3). The composition dependences of individual peak intensities are shown in fig. 4. All the peaks observed in the range from 420 to 880 cm -a are attributed to the vibrations of coordination polyhedra of tellurium. It is seen in fig. 4 that peak intensities are reduced with respect to tellurium content of these binary

T. Sek~,a et al. / Raman spectra of MO1/2-TeO 2 (M = Li, Na, K, Rb, Cs and Tl) glasses

glasses. The ordinate shows the relative intensities when the intensity of peak C of TeO2 glass is taken to be 1.00. The relative intensity of peak E decreases with increasing alkali content. Peak C has a maximum intensity at about 5 mol%, and the intensity decreases similar to that of peak E. The intensities of peaks A and B decrease until alkali content reaches about 10 mol%, and then increase with further increase of alkali content. Peak B has a maximum between 20 and 30 tool%. The change of intensity of peak D is similar to that of peak B. In the composition range of alkali content > 30 mol%, the intensities of peaks A, B and D increase, while the intensities of peaks C and E decrease. These results indicate that new peaks may appear at the same wavenumber positions as peaks A, B and D on the addition of modifier. The new peaks are named peaks A', B' and D'.

131

The intensities of new peaks, I A, and Iw, are calculated from the formulas IAr,gl = / A , g l -- ( / A , T e / / / E , T e ) / E , g l , IB',gl = IB,gl -- ( I B , T e / I E , T e ) I E , m g l ,

where IA,gl, IB,gl and IE,gl are the intensities of peak A, B and E of binary glasses, respectively, and IA,Te, IB,Te and IE,Te are the intensities of peak A, B and E of TeO 2 glass, respectively. It is assumed that the intensities of the vibrations due to a continuous network composed of TeO 4 tbp's contribute to intensities of peak A and B in proportion to the composition dependence of the intensity of peak E. Taking the assignment of peak E into consideration (discussed below), it is supposed that, under this treatment, the estimated intensities of peaks A' and B' are equal to or smaller than exact intensities. The composition

1.0

' c~. o.5

cLO,5

~°f

(b)

.-

,

' ' ,..

(a)

, ...;~ '

o.ot

.

'

0.0

'-....

'

~

"

;

"

~

"

" ~');~"

T

T

~

r

_.J c

i

C

1000

800

800

Wavenumber

400

(cm -I)

200

1000

800

600

Wavenumber

400

200

(cm-I)

Fig. 3. Polarized Raman spectra and deconvolution of xNaO1/2.(lOO-x)TeO 2 glass using a symmetric Gaussian function; (a) x = 30 and (b) x = 50. The observed spectra of I Hand I ± , resolved Raman peaks (solid lines) and a sum of the peaks (broken lines) are shown. The p is depolarization ratio. The observed and simulated results are indicated by points and a solid line, respectively.

132

T. Sekiya et al. / Raman spectra of M O 1 / 2 - Z e O 2 (M = L4 Na, 1~ Rb, Cs and TI) glasses

la)

2.0 ;

~0.4

1.8-~

~0.3

\

o\

1.6-

o\

>" 1.4 I/1 C ¢-

I

1.2

i

~"/9 d

0.8

/ /

0t

--'

'

'

a:peak A o:peak B v:peakC z~:peak D o:peak E

\

0.6

~

0 10 20 30 40 Content (moP/,)

vX X

¢r

u:peak A' ^ .'. o:peak B'

~ v ~ ,,,~.-u~_~o 0.4 (i ~'~P7 - ~'v

•\

// \

>" 1.4

"

l.a c¢-

\

;o.: / /

1.6

o

eu // =01 ~ rr~ ' //~o

_~ \ ,~v ~ X L i O,n -o 1.0'

,.8

/Jo\

X~0"2 X ~o \

(b)

2.0

o~

-\

1.2

~n, / ° / ~" / u z

~

o-

/o/

1'0 2'0 3'0 4'0 s'o

1.0' / V ~ N a O l / 2

~o 0.8

a:P eakA' o : peak B'

Content (moP/,)

\\ ~oVN X~ /°~a:~,~--ta~T.--•

cr

0.6 0.4

[] :peak o : peak v:peak z~:peak

A B C D

N O

0.2

\z~'--'-'~z~~z

'

0.2 0

00 1'0 2'0 3'0 4'0 LiOn/2 Content (mol*/o)

(c) 2.0

0.4

1.8

~0.3

1.6

c-

.0.2

k_

~

\

>, 1.4 c

o~

-o 1.0 / ~ ' - ' ' ~ o £)

-~ c~

o/

//

~o

[]:oeak A'

5,0, o:peakB,

c~ 0 6z'Z~I'0 2'0 3'0 4'0

v.~

KO~z2 Content (mot°l,)

X

\~ \\ \v\ 0.6 o~---...i'~X 0.4 • ~, ~_...~a-------~'~m

0.8

~

°~

(d)

2.0

C

1.2

1'0 2'0 3'0 4'0 5'0 NaOl/2 Content ( tool°l, )

0

-o

0.2

[] :peak o :peak v :peak z~:peak o :peak

A B C

0 E

c"

0.4

1.8

~0.3

1.6

0.2

1.4

gO.1

1.2

00

-o 1.0 El

-~ o.8 0.6 0.4

~~=----"o ~ ~ ~o

0.2 .

O0

I'0 2'0 3'0 4'0

KOl/2 Content (mol*/.)

10 20 30 40

RbOln Content ( mol°/o )

O0 4 10 ~20

u : peak o:peak v :peak zx: peak o:peak

A B C D E

~ 30

RbOll2 Content (mol*l.)

Fig. 4. Composition dependences of the intensities of Raman peaks A, B, C, D and E for MO1/z-TeO 2 glasses; (a) M = L: (b) M = Na, (c) M = K, (d) = Rb and (e) = Cs. The insets show the composition dependences of peaks A' and B'.

133

T. Sekiya et al. / Raman spectra of MO1/e-TeO2 (M = Li, Na, K, Rb, Cs and Tl) glasses

(e)

2.0

>,0.4 =

80.3/

1.8 1.6 1.4

\o

0.2

"O u

.~0.1 13£

¢r" 1.2

,~/O

1'0 2'0 3'0 Cs01/2 Content ( moP/° )

0

~I1J 1.0'

T >-

09 C 4-) C

~ o.8 n-

0.6 O ."------O 0.4 ~-------D ~....____~ 0.2

o

u :peak A' o :peak B'

u:peak o:peak v:peak ~:peak o:peak

A B C D

E

i

10 2'0 3'0 o CsO~12 Content ( moP/. )

1000

Fig. 4. (continued). Fig. 5.

dependences of intensities of peak A' and B' are shown in the insets in fig. 4. The intensity of peak A' increases with increasing alkali content. The change of intensity of peak B' has a maximum at about 30 mol%. The detail of the peak assignments is discussed below. 3.2. T l O ] / 2 - T e O 2 glasses

The intensity-normalized Raman spectra of T101/2-TeO2 glasses are shown in fig. 5. The shape of Raman spectra of the glasses having TIO1/2 content up to 30 mol% is similar to that of binary alkali tellurite glasses containing equal amounts of MO1/2. The peak deconvolution indicates the presence of five peaks in the range from 420 to 880 cm -1, similar to the spectra of alkali tellurite glasses; with increasing T101/2 content, the peak at about 450 cm-1 becomes weaker and the peak at about 780 cm -1 shifts toward lower wavenumber. The peaks at about 275 and 720 cm -1 become more intense, and the peak and about 275 cm-~ shifts toward higher wavenumbet. In the spectrum of 66T101/2-34TeO 2 glass

800 G00 400 Wavenumber (cm-~)

200

Intensi~-normalizedRaman spectra of xTIO1/2"(100x)TeO 2 glasses.

sample, the peaks are located at about 100, 320, 680 and 725 cm-1. The Raman peaks of T101/z-TeO 2 glasses were more intense than those of alkali tellurite glasses. The color of the glass specimens changes from pate yellow through yellow to red with increasing T1OI/2 content. It seems that T101/2TeO 2 glasses having higher T1Oa/2 content have resonance- or preresonance-Raman effect. The resonance- and preresonance-Raman effects are observed when the exciting frequency lies in and is close to the electronic absorption band of the compound, respectively [8,14]. Therefore, no detailed discussion is made with respect to the composition dependence of the peak intensities in TIOa/z-TeO 2 glass system. 3.3. Properties

The composition dependences of molar volume, Vm, glass transition temperature, Tg, and thermal expansion coefficient, a, are shown in

7". Sekiya et al. / Raman spectra of MO1/2-Te02 (m = LL Na, K, Rb, Cs and Tl) glasses

134

30

_o~O-

~ £ - - D

[]-

30.0

"O~ • . ~ ' ~ - •
-6

• : LiOl/2 o : NaOv2 :KOv2 [] : RbO1/2

O : CSOl/2

~E 25

."

v

•

Li01/2

>E

o a. [] 0 •

NaOll2 KOl/2 RbOv2 Cs01/2 AgOv2 TlOv2

2C

•

2'0

o

"O v

~'o~

~

~•

• : AGO1/2

d~

o~ --% ~!.x "

• :TIOv2

c/

/#"

/

/

/

• /

/

/

/,,, //d __4/,~" , ~

25.0

E, x

/ / • /

20.0

4'0

6'0

MOv2 content ( m o l % ) Fig. 6. Composition dependences of molar volume, Vm, of MO1/2 - T e O 2 (M = Li, Na, K, Rb, Cs, Ag and T1) glasses.

/1 15c

2'o

4'o

6'o

MOll2 content (moP/o)

figs. 6, 7 and 8, respectively. The Vm, Tg and oz of binary glasses containing Li, Na, K, Ag and T1 are due to ref. [2]. The molar volumes change linearly with the content of MO1/2. With increasing MO1/2 content, the glass transition temperatures

300

Fig. 8. Composition dependences of thermal expansion coefficient, a, of MOI/a - T e O 2 (M = Li, Na, K, Rb, Cs, Ag and T1) glasses.

decrease and thermal expansion coefficients increase linearly.

4. Discussion

4.1. Assignment of Raman peaks 250

200

• 150

:LiO~/2

o : NaO1/2 zx : KO1/2 [] : RbOvz

\

o CsOv2 • AgOv2

• TlOlt2

1000

2'0

6'0

MOv2 content (mol°/o) Fig. 7. Composition dependences of glass transition temperature, Tg, of MO~/2 - T e O 2 (M = Li, Na, K, Rb, Cs, Ag and TI) glasses.

Assignments of Raman peaks observed in the spectra of the glasses are made on the basis of Raman spectra of MO1/2-TeO2 crystals and T e O 2 glass. Few investigations for Raman spectra of M O 1 / 2 - T e O : crystals have been made. The present authors investigated Raman spectra of o~-TeO 2 crystal [1]. In this section, a relationship between Raman peaks observed in spectra of binary tellurite crystals and their structural units is discussed in order to assign Raman peaks observed in spectra of binary tellurite glasses. The coordination states of tellurium atoms are classified to T e O 4 tbp, T e O 3 tp and their intermediate states which are found in T e O 2 crystals and binary tellurite crystals. The T e - O distance in the T e O 4 tbp ranges from 0.185 to 0.195 nm for two equatorial sites and from 0.205 to 0.215 nm for two axial sites. In the T e O 3 tp, the T e - O

T. Sekiya et al. / Raman spectra of MOl/2-Te02 (M = Li, Na, K, Rb, Cs and Tl) glasses

Li2Te03

7121689

N~Te03

T

2

r

c"

IC~LTe03 TI~Te03 1000 800

600

Wavenumber

J 400

200

(cm -' )

Fig. 9. Raman spectra of M2TeO3 (M = Li, Na, K, Rb, Cs and T1) crystals. distance is in the range 0.185-0.200 nm. The T e - O distance in the intermediate states is divided into two groups: shorter three bonds of 0.185-0.200 nm and a longer bond of 0.220-0.260 rim.

4.1.1. M2TeO 3 (M = Li, Na, K, Rb, Cs and Tl) crystals Figure 9 shows the Raman spectra of MzTeO 3 (M = Li, Na, K, Rb, Cs and T1) crystals. These crystals contain isolated T e O 2- ion [15-19]. The isolated TeO 2- ion has a trigonal pyramidal structure, in which three T e - O bonds are equivalent because of a resonating effect between one Te=O and two T e - O - bonds [12]. Raman spectra of KzTeO3, RbzTeO 3 and Cs2TeO 3 crystals are very similar to one another. Each of them has a

135

strong sharp peak at about 780 cm-1 and weak peaks in the range from 250 to 350 cm -1. Thiimmel and H o p p e [17] showed that KzTeO3, RbzTeO 3 and CszTeO 3 crystals are isomorphous and can be described as hexagonal. On the other hand, Loopstra and Goubitz [18] concluded that CszTeO 3 crystal has a hexagonal unit cell and that T e O 2- ion has C3v point symmetry. Four normal vibrations, vl(A1), uz(A1), ~,3(E) and u4(E), are Raman active. According to the result of normal vibrational analysis of the T e O 2- ion, the peak at about 780 cm-1 is assigned to symmetric stretching vibration of T e O 2- ion (v 1) and the peaks at 250-350 cm-1 are assigned to symmetric bending vibration of T e O 2- ion (re). In LizTeO 3 crystal, two strong peaks are observed at 725 and 798 cm-1. These two peaks are assigned to T e - O - stretching vibrations [2]. The decrease in the symmetry of isolated TeO3z- ion causes an increase in the peak intensity of the vibration originally associated with E species [20]. The peaks observed in the range from 310 to 410 cm-1 is assigned to bending vibrations of T e O zion. In Na2TeO 3 crystal, the Raman peaks assigned to T e - O - stretching vibrations appear in a lower wavenumber region. The averaged length of T e - O - bonds of T e O 2- ion in NazTeO 3 crystal is slightly larger than that in Li2TeO3, KzTeO3, RbzTeO 3 and CszTeO 3 crystals. The distance between the NBO and the adjacent tellurium atom is distributed in the range from 0.291 to 0.310 nm in NazTeO 3 crystal [16]. It is assumed that there is a weak interaction between these atoms. As a result, the T e - O - bond is weakened by this weak interaction. In T12TeO 3 crystal, the Raman peaks assigned to v I and v 2 of TeO32- ion are observed at 730 and 293 cm -1, respectively. The peaks due to T e - O - stretching vibrations shift to a lower wavenumber region because of partial covalency of Tl + ion, compared with K +, Rb + and Cs + ions in KzTeO3, RbzTeO 3 and CszTeO 3 crystals, respectively.

4.1.2. M2Te205 (M = Li and Cs) crystals Raman spectra of M2Te205 (M = Li and Cs) crystals are shown in fig. 10. CszTezO 5 crystal

136

T. Sekiya et a L / Raman spectra of MO1/2-ZeO 2 (M = Li~ Na, 1~ Rb, Cs and TI) glasses

645

I

~-L i2Te205

t cCD .4-) c3-k i2Ye20s 725

tion of the TeO3+ 1 polyhedron. The peak at 645 cm -1 is assigned to antisymmetric vibration of the T e u i + ~ - O - - - T e m + I linkage. The Roman numeral subscript stands for the coordination state of tellurium atom. Two types of T e - O - T e linkages are formed in a-Li2Te205 crystal: one is T e n i + i - O - - - T e m + I and the other is T e l n + i - O Teiii+ I linkage which is formed by oxygen-sharing of almost equivalent T e - O bonds. The former linkage is similar to the Te-eqOax-Te linkage in paratellurite. The peak at 645 cm -1 is compared with that at 649 cm -1 and is assigned to combined effect of usiTeO 4 and antisymmetric vibration of Te-eqOa~-Te linkage in paratellurite [1]. The difference in polarizability between Te m + i O (Te-eqO) and T e n i + r - -O ( T e - ~ O ) bonds may cause a change in Raman peak intensity of Te iii + i - O - -Tein ÷ I ( T e - eqOax-Te) linkage. The peaks observed in the range from 270 to 400 cm-1 are assigned to symmetric stretching (and bending) vibrations of T e l n + i - O - - - T e l n + i and Te ni + i - O - T e iii + i linkages. In I3-Li2Te205 crystal [21], a pair of TeO3+ 1 polyhedra is connected by a common edge to form a Te206 unit. Each tellurium atom has one NBO. The Raman spectrum of I3-Li2Te205 crystal shows two strong peaks at 691 and 817 cm-1. The peaks at 691 and 817 cm -~ are assigned to antisymmetric vibration of T e m + i - O - - -Teill + i linkage and T e - O - stretching vibration, respectively. These peak positions do not correspond to those in the spectra of binary glasses and are observed in the higher wavenumber region compared with those of o~-Li2Te205 crystal. The vibrations are localized because of the edge sharing of T e O 3 + 1 polyhedra and the peaks, which have similar vibrational modes, are shifted to a higher wavenumber region. This tendency has been found for polymorphic forms of T e O 2 crystals, paratellurite and tellurite [1]. -

;s2Tea0s 1000 Fig.

800 600 Wavenumber

400 (cm -~ )

10. Raman spectra of M2Te205 (M

=

200

Li and Cs) c~stals.

contains isolated T e 2 0 ~- ions [18]. Each tellurium atom forms a T e O 3 tp and has two NBOs by resonating between Te---O and T e - O - bonds. The distance between one of the NBOs and adjacent tellurium atom of neighboring Te2 O2- ion is in the range of 0.268-0.270 nm. Two intense, sharp peaks are observed at 725 and 776 cm -1, and a broad peak is observed at 260 cm-1 in the Raman spectrum of Cs2T%O 5 crystal. The intense peaks are assigned to T e - O - stretching vibrations. A weak interaction between one of the NBOs and tellurium atom of adjacent Te2 O2ion will cause the T e - O - stretching vibration at 725 cm-1 to shift to a lower wavenumber region, as is the case for Na2TeO3 crystal. The peak at 260 cm -1 is assigned to a bending mode of T e O 3 tp having NBOs. a-Li2Te205 crystal [21] has (Te202-)= sheet structure, in which TeO3+ 1 polyhedra having T e - O - bond are connected at the three vertices. The Raman spectrum of this crystal shows two intense peaks at 645 and 785 cm-1. The peak at 785 cm-1 is assigned to T e - O - stretching vibra-

4.1.3 Cs2Te4O 9 crystal In the Raman spectrum of Cs2Te409 crystal (fig. 11), two intense peaks are observed at 672 and 728 cm -1, and a less intense peak at 470 cm -1. According to Loopstra and Goubitz [18], Cs2Te409 crystal contains two different tellurium atoms, Te(1) and Te(2), whose coordination states

T. Sek&a et al. / Raman spectra of MO1/2-TeO 2 (m = Li, Na, K, Rb, Cs and Tl) glasses

'28/

T fO3 ~o C

Cs2Te40s

1000

800 800 Wavenumber

400 200 (cm-~)

Fig. 11. Raman spectra of Cs2Te409 crystal.

are TeO3+ t and TeO3+ 2 polyhedra, respectively. However, we think that the coordination state of Te(2) is not TeO3+ 2 polyhedron but T e O 3 tp having a weak interaction because of the reasons discussed below. (a) The relationship between the bond length and the bond strength for various tellurite crystals is given by Philippot [22] with the equation, S = 1.333(r/0.1854) -5.2, where r is the bond length (nm) and S is the bond strength. The T e - O distances and calculated bond strengths in C s 2 T e 4 0 9 crystal are shown in table 2. In the case of Te(2), the bond strengths for two longer T e - O bonds are very small compared with those for three shorter bonds. It seems that the two longer bonds between Te(2) and oxygen atoms can be neglected, although weak interactions may exist. Table 2 T e - O bond lengths and bond strengths in Cs2Te40 9 crystal T e - O bond length (nm)

T e - O bond strength

Te(1)-O(3) 0(2) 0(4) 0(2')

0.1854 0.1870 0.2020 0.2315

1.333 1.275 0.853 0.420

Te(2)-O(1) 0(4) 0(5) 0(3) O(1')

0.1842 0.1956 0.1978 0.2403 0.2513

1.378 1.009 0.952 0.346 0.274

The numerals of atoms are taken from ref. [18].

137

(b) The composition dependence of the molar volume gives information about the coordination state of the individual cations. The molar volume of C s 2 T e 4 0 9 crystal is larger t h a n the interpolated value with polymorphic forms of T e O 2 crystals and Cs2Ye205 crystal. We suggest that Te(2) has a small coordination number because an increase of the coordination number of tellurium atom causes a decrease of the molar volume. (c) The glass formation range of the CSOl/2T e O 2 system is smaller than that of other alkali tellurite glasses. Cs2Te40 9 crystals precipitate when the melt containing 25-40 mol% CsO1/2 is cooled. We suggest that an addition of Cs + ion results in cleavage of T e - O - T e linkage and the glass formation is hindered in this composition range. We assume that the coordination state of Te(2) is T e O 3 tp for the above-mentioned reasons. C s 2 T e 4 0 9 crystal is composed of Te(1)O3+ 1 polyhedra having T e = O - bond and of Te(2)O 3 tp's having T e - O bond, and has a (Te402-)= ladderlike s t r u c t u r e c o n t a i n i n g Y e ( 1 ) i i i + i - O - - Te(1)in+i , T e ( 1 ) m + i - O - T e ( 2 ) n i and Te(2)ii IO - T e ( 2 ) i n linkages. The NBOs may interact weakly with adjacent Te(2). The Raman peak at 728 c m - t is assigned to the stretching vibrations of Te=O and T e - O - bonds. These vibrations are shifted to a lower wavenumber region because of the weak interaction. The peak at 672 cm-1 in C s 2 T e 4 0 9 crystal, compared with the peak at 649 cm-1 in paratellurite, is assigned to the antisymmetric vibration of Teill+ I- O - - - T e l I I + I linkage. The peak at 470 cm-1 is assigned to symmetric stretching (and bending) vibrations of Tern+ IO- - -Te ii1+ i and Te lI1+ I(nI)- O - T e nI linkages. 4.1.4. TeO 2 glass The Raman spectrum of T e O 2 glass is also deconvoluted into five peaks in the wavenumber range 420-880 cm -1. Our previous work based o n the normal vibrational analysis of o~-TeO 2 crystal showed that the five peaks are assigned to us2TeO 4 ( a n d

Vs2+asTeO4) , Usl+asTeO4, UslTeO4,

uasTeO4 and 6stTeO4 (and 6s2TeO4), respectively, and that the former four and the other are assigned to vasTe-eqO~-Te and vsTe-eqO~-Te , respectively [1]. It was indicated that, in T e O 2

138

T. Sekiya et aL / Raman spectra o f M O 1 / 2 - T e O 2 ( m = Li, Na, I~ Rb, Cs and Tl) glasses

"~ O~

'~1 O~

,.# 0~',

' ~

~

~

~ 7

oo~ LLo +

+

I

I I

E

I

I

0 ~

-4

O~ 7

7

6

6

.4

7~

.=. 'S

Z

0

..=

0 © I

'.or +

?v

II

o~ +',1~

£

-[

o7

L7

L~

I

E

+

e-,

~.~0 •= ~ m

oo 0 ~ eaj~

~ ~

~

~~

c~ a3

~

T. Sekiya et al. / Raman spectra of MO1/2-Te02 (M = Li, Na, K, Rb, Cs and Tl) glasses

glass, T e O 4 tbp's are formed by most of tellurium atoms and connected at vertices by forming T e eqOax-Te linkages [1]. The five peaks correspond to the peaks A - E observed in the spectra of binary glasses. We suggest that vibrations of the continuous network of T e O 4 tbp's contribute to intensities of five peaks observed in the spectra of binary glasses.

4.1.5. Assignment of Raman peaks observed in spectra of binary glasses We assume that a peak observed in the spectrum of a binary glass with the same wavenumber as a peak not only in a binary crystal but also in T e O 2 glass is due to the same structure and vibrational mode. A relationship between Raman peaks observed in the spectra of binary glasses (peaks A - E and at about 270 cm-1) and those of alkali tellurite crystals and T e O 2 glass are given in table 3. In the spectra of M O 1 / 2 - T e O 2 (M = Li, Na, K, Rb and Cs) glasses, the deconvoluted peaks are assigned on the basis of the assignments of binary crystals and T e O 2 glass as follows. (a) At the frequency of peak A (about 780 cm-1), the vibration of the continuous network composed of T e O 4 tbp's is overlaid with peak A'. The peak A' is assigned to the T e - O - stretching vibration of TeO3+ 1 polyhedra or T e O 3 tp's. In this case, NBO has little interaction from adjacent tellurium atoms. (b) The peak B at about 720 cm-1 is divided into two parts. One is due to peak B' and the other due to the vibration of the continuous network composed of T e O 4 tbp's. The peak B' is assigned to stretching vibrations between tellurium and NBO of TeO3+ 1 polyhedron and T e O 3 tp in which the NBO interacts with adjacent tellurium atoms. The frequencies of T e - O and Te--O stretching vibrations are independent of the coordination state of tellurium atoms in TeO3+ 1 polyhedra and T e O 3 tp's, and are dependent on the surroundings around the NBOs. In silicate glass, the stretching vibration of NBO is localized [23]. We assume that a weak interaction between adjacent tellurium and NBO weakens T e - O - and Te=O bonds and causes the stretching vibrations of both bonds to shift toward a

139

lower wavenumber region, when NBO is adjacent to other tellurium atom. We suggest that the existence of peak B' is due to NBOs interacting with adjacent tellurium atoms. The peak B' is hardly possible to be assigned to T e - O - stretching vibration of distorted T e O 2- ions observed in Li2TeO 3 crystal, although it appears in the same wavenumber region as peak B', as is seen in table 3. Because, in binary glasses, no peak is observed in the frequency range of bending vibrations of distorted T e O ~ - ion. (c) The peak C at about 665 cm-1 is assigned to the antisymmetric vibrations of T e - O - T e linkages constructed by two unequivalent T e - O bonds. T h e T e i v - e q O a x - T e l v , Teni+i(iii )O---TeIii+i, Teiv-eqO---Telii+ I and T e i v a~O-Tem+I0n~ linkages are examples of this type of T e - O - T e linkages. Generally, in binary tellurite crystals, either or both of tellurium atoms in this type of T e - O - T e linkage must form T e O 4 tbp or TeO3+ 1 polyhedron [18,21,24-28]. We suggest that the existence of peak C indicates a continuous network structure containing T e O 4 tbp's and TeO3+ 1 polyhedra. Therefore, the intensity of peak C will probably reflect the fraction of T e O 4 tbp's and T e O 3 + 1 polyhedra. (d) The peak D at about 615 cm -1 also includes the peak D ' and reflects the vibration of the continuous network composed of T e O 3 tbp's. The position of peak D ' corresponds to that of the peak observed at 633 cm -1 in Cs2Te40 9 crystal. (e) The peak E at about 450 cm-1 is assigned to the symmetric stretching (and bending) vibrations of T e - O - T e linkages, which are formed by vertex-sharing of TeO 4 tbp's, TeO3+l polyhedra and T e O 3 tp's. We suggest that the existence of peak E indicates a continuous network consisting of T e O n (n = 4, 3 + 1 and 3) polyhedra. (f) The broad peak observed at about 270 cm -1 is assigned to the bending vibrations of T e O 3 tp's having NBOs. The appearance of this peak together with peaks A' or B' indicates the existence of T e O 3 tp having NBOs. The assignments the Raman peaks for the TIO1/2TeO 2 glasses are the same as those for alkali tellurite glasses. With increasing T101/2 content, the peaks assigned to stretching vibra-

140

T. Sekiya et aL / Raman spectra of MO1/TTeO 2 (M = Li, Na, K, Rb, Cs and Tl) glasses

tions between tellurium and NBOs are shifted to lower wavenumber because of the partial covalency of T1 + ion [29]. The Raman spectrum of 6 6 T 1 0 1 / z ' 3 4 T e O 2 glass is similar to that of T12TeO 3 crystal. The peaks at 320, 680 and 725 cm -a are assigned to /"2, /'3 and /'1 of T e O zion, respectively. The peak at 100 cm -1, is assigned to the vibration of T10 n (n < 4) polyhedron [30]. 4.2. Structure o f binary M O 1 / 2 - T e O 2 glasses

T e O 2 glass has a network structure in which T e O 4 tbp's are composed of tellurium atoms connected at vertices by Te-eqOax-Te linkages. The composition dependences of the Raman peak intensities are due to changes of basic structural units of coordination polyhedra of tellurium atoms. The glasses having low alkali content have a continuous network structure composed of T e O 4 tbp's and TeO3+ 1 polyhedra sharing vertices. Since the peak at 270 cm-1, due to vibration of T e O 3 tp having NBOs, is absent in the spectra of the glasses, when the alkali content is less than 20 mol%, we assume that the fraction of TeO3+ 1 polyhedra having T e - O - bond increases with increasing alkali content in this compositional range. A part of these NBOs will interact weakly with adjacent tellurium atoms. The composition dependence of the intensity of peak E indicates a decrease in T e - O - T e linkages and a cleavage of the continuous network composed of T e O 4 tbp's and T e O 3+ 1 polyhedra. In glasses containing 2 0 30 mol% alkali oxides, T e O 3 tp's having NBO's are formed in the continuous network. The decrease in the intensity of peak E and the increase in the intensity of peak A' indicate a cleavage of the continuous network. The composition dependence of the intensity of peak C indicates a decrease in the fraction of T e O 4 tbp's and TeO3+ 1 polyhedra. The appearance of the peak at 270 cm -1 in the glasses containing 20 mol% alkali oxide indicates the formation of T e O 3 tp having NBO's. The increase of intensity of peak B' to 30 mol% alkali oxide indicates that the fraction of NBOs interacting weakly with adjacent tellurium atoms increases with increasing alkali content. It

seemg that the TeO 3 tp's are included in the continuous network until alkali content reaches 30 mol%. We think that the NBOs incorporated into the continuous network interact weakly with adjacent tellurium atoms and that the NBOs of isolated structural units have little interaction. When the alkali content is more than 30 mol%, the fraction of NBOs interacting weakly with adjacent tellurium atoms decreases. The composition dependences of intensities of peaks A' and B' indicate that isolated structural unit, T e 2 0 ~ion, coexists with the continuous network. We assume that the fraction of T e O 4 tbp's decreases along with the decrease in the fraction of T e - O Te linkages. The glasses containing nearly 50 mol% alkali component are composed of a continuous network constructed by sharing vertices of TeO3+ 1 polyhedra and T e O 3 tp's, and of isolated structural units, such as isolated T e 2 0 2and TeO32- ions. Few T e O 4 tbp's will coexist in such glasses. The glass formation becomes difficult with increasing fractions of isolated structural units and NBOs. Based on the similar shapes of their Raman spectra, the structures of xTIO1/2 • (100-x)TeO 2 (0 < x < 30) glasses are similar to those of alkali tellurite glasses. In the glasses containing more than 40 mol% T101/2, Raman peaks, assigned to stretching vibration between tellurium and NBOs at about 750 cm-1 and to bending vibration of T e O 3 tp having NBOs at about 280 cm -1, have large intensities. We suggest that the fraction of T e O 3 tp's having NBOs in the T 1 0 1 / 2 - T e O 2 glasses is larger than that in binary alkali tellurite glasses. When T101/2 content is larger than 50 mol%, the glasses consist of isolated structural units, such as Te2 O2- and TeO32- ions. As mentioned above, almost all tellurium atoms will form isolated TeO32- ions in 66T101/2"34TeO 2 glass. A wide glass formation range of this system may be due to a role of s2-type ion, such as T1 + and Pb 2+ [29,31]. A part of T1 + ions participates in the glass network and the other behaves as a modifier [30]. The present authors propose a new mechanism for the structural changes of the tellurite glasses on the basis of three structural assumptions.

T. Sekiya et al. / Raman spectra of MO1/2-ZeO 2 (M = Li, Na, IV, Rb, Cs and Tl) glasses

(i) T e - O - T e linkages formed in the tellurite glasses are classified into two groups: one contains Teiv-eqOax-Teiv, Tem+i(m)-O---Teni+i, Te w - eq O - - -Te iii + i and Te:v- a~O-TeIH +I(Im linkages formed by connecting long Te-O bond (> 0.20 nm) and short T e - O bond ( < 0.20 nm), and the other contains Teiii+i(m)-O-Tem+Km) linkages formed by connecting two middle T e - O bonds (~ 0.20 nm). (ii) NBO is formed in the shortest T e - O bond. (iii) The change of coordination state of tellurium atom from 4 through 3 + 1 to 3 results from the large electron-donating ability of NBO. The production of TeO 3 tp is accelerated by an additional resonance energy between T e - O - and Te=O bonds. Our mechanism differs from the ones proposed by Yoko et al. [7] who take account of only the effect of electron-donating ability of NBO. The structural change from TeO 4 tbp to TeO3+ 1 polyhedron having T e - O - bond is illustrated in fig. 12. A small amount of MO1/2 added to TeO 2 glass breaks the Te-eqOax-Te linkage and, then, we imagine that two types of NBOs, Te-eqO- and Te-~xO- bonds are created as shown in the illustration (fig. 12(I), (II)). Since the TeO 4 tbp having Te-ax O - bond is unstable, it is distorted into TeO3+: polyhedron; the Te~,O- bond shortens and, as a result, the other Te-axO bond elongates, forming a Te---O bond (fig. 12(III)). The question remains whether the TeO 4 tbp having Te-eqO- bond is distorted into TeO3+ 1 polyhedron or remains in a non-distorted configuration. This type of TeO 4 tbp seems to be less stable than TeO3+ 1 polyhedron having T e - O - bond because the increase of the electron-donating ability in Te-eqO- bond will weaken the Te-axO bonds. We assume that this type of TeO 4 tbp remains in a less-distorted configuration in the glasses having low alkali content and is highly distorted in the glasses having high alkali content. Since three vertices of this type of TeO 4 tbp will be shared with other TeO 4 tbp's in the glasses having low alkali content, we assume that the distortion attendant with a change of T e - O bond strengths of surrounding TeO 4 tbp's is forbidden. The increase of Raman intensity for the peak C, observed in the range 0-5

141 2MOv2

(I)

4D(ll) M~

"-c>

4Z> (III)

Tu®M ~ C ~ L

O :Oxygen Atom • :Telturium Atom M :Modifier Atom ¢ ® :Etectric Charge -*s~-:Shortening of Bond ÷ L * :Elongating

~:5hort --:Long -

-

-

:

Bond (_<2.0~) Bond (>2.0~) (> 2.2,~1

Length

of Bond Length

Fig. 12. M e c h a n i s m for the structural change induced by the addition of MO1/2 c o m p o n e n t into the network composed of T e O 4 tbp's.

mol% alkali content, may indicate the existence of TeO 4 tbp. The vibration of TeO 4 tbp corresponding to peak C is the stretching mode in reverse phase for two Te-eqO and two Te-~xO bonds [1]. The different polarizability changes for

T. Sekiya et al. / Raman spectra of mO1/2-TeO 2 (m = Li, Na, I~ Rb, Cs and Tl) glasses

142

the Te-eqO and Te-~xO bonds will contribute to the change of peak intensity. Since the polarizability change of T e - e q O - bond is larger than that of bridging Te-eqO bond, the peak intensity will increase along with the formation of TeO 4 tbp having T e - e q O - bond. In the glasses having higher alkali content, change of this type of TeO 4 tbp into TeO 3 + 1 polyhedron having T e - O - bond will be produced by a cleavage of the continuous network and an increase of the fraction of TeO 3 + 1 polyhedra formed in the continuous network. The formation of TeO 3 tp having Te=O bond is shown in fig. 13. This mechanism is similar to

M® L

o®A

(Vl)

..... o.. ve

.....

_~..-.(~

M® ,4

(VII)

Cleave

® ~,_~__

(m) %

( I V ) ~~ "T" M®

M®

.-(3

Cleave

>I

,

-

-

-

-

l

"~

~

a

v

~

U

....

~ M®

( vii; )

.%'" (v)

e l : o x y g e n Atom • :Tellurium Atom M :Modifier Atom ®®:Electric Charge

~ --=

Short Bond (<_2.0~,) Long Bond (>2.0~,) (>2.2]k) Double Bond

Fig. 14. Mechanism for the structural change from the network composed of TeO3+ I polyhedra into isolated Te2 O 2 ion: the arrow -* indicates the transfer of electron.

O :Oxygen Atom • :Tellurium Atom M :Modifier Atom ®®:Electric Charge

~:Short Bond (<--'2.0~,) --:Long Bond (>2.0/~) --- : (>2.2]k) ~ :Double Bond

Fig. 13. Mechanism for the structural change from TeO3+ 1 polyhedron into TeO 3 tp: the arrow --~ indicates the transfers of electron and modifier ion.

that proposed by Yoko et al. [7]. When the Te- - -O bond of TeO3+ 1 is elongated and cleaved by the transfers of electron and modifier ion, as is illustrated in fig. 13(IV), TeO 3 tp having Te=O bond is forme0 (fig. 13(V)). This type of TeO 3 tp is

T. Sekiya et al. / Raman spectra of MO1/e-TeO: (M = Li, Na, K~ Rb, Cs and Tl) glasses

found only in C s 2 T e 4 0 9 crystal. We assume that this structural unit is formed in a glass which contains a modifier ion having large ionic radius. The structural change from TeO3+ 1 polyhedron to T e O 3 tp is illustrated in fig. 14. When the T e - - - O bond in TeO3+ 1 polyhedron is elongated and cleaved, a terminal T e O 3 tp is formed which has two NBOs (fig. 14 (VI), (VII)). When the Te-- -O bonds of neighboring T e O 3+ 1 polyhedra are cleaved, an isolated Te2 O2- ion is formed (fig. 14 (VIII)). Further addition of alkali oxide causes the T e I I I - O - T e m linkage of Te2 O2- ion to break and results in the formation of isolated T e O 2- ion (this is not shown in the figure). The T e O 3 tp which has two or three NBOs is stabilized by the resonance between Te=O and T e - O bonds. It is noted that the properties of these glasses, i.e., molar volume, glass transition temperature and thermal expansion coefficient, change almost linearly with increasing MO1/2 content (figs. 6-8). This change is explained by the coordination change of tellurium atom taking place successively f r o m T e O 4 tbp through TeO3+ 1 polyhedron to T e O 3 tp with increasing MO1/2 content.

5. Conclusions

The intensity-normalized Raman spectra of alkali tellurite glasses containing equal amount of alkali oxide are similar to each other. With increase of alkali content, the coordination state of tellurium atom in binary glasses changes from T e O 4 tbp through TeO3+ I polyhedron to T e O 3 tp and NBOs increase. In the glasses containing nearly 50 mol% alkali oxide, isolated structural units, such as T e 2 0 2- and TeO32- ions, coexist with a continuous network constituted by T e O 3__1 polyhedra and T e O 3 tp's. The thallium tellurite glasses have similar structure. In the 66T101/2. 34TEO 2 glass, most of tellurium atoms are in a form of isolated T e O ~ - ion. A new hypothesis is proposed for a mechanism for the basic structural changes in the tellurite glasses on the basis of three structural assumptions. First, two types of T e - O - T e linkages are present in glasses: one is formed by connecting long T e - O bond ( > 0.20

143

nm) and short T e - O bond ( < 0.20 nm) and the other is formed by the connection of two middle T e - O bonds (-- 0.20 nm). Second, NBO is formed in the shortest T e - O bond. Last, the change of coordination state of tellurium atom from 4 through 3 + 1 to 3 results from the large electron-donating ability of NBO. The production of T e O 3 tp is accelerated by an additional resonance energy between T e - O - and Te=O bonds.

References [1] T. Sekiya, N. Mochida, A. Ohtsuka and M. Tonokawa, J. Ceram. Soc. Jpn. 97 (1989) 1435. [2] N. Moehida, K. Takahashi, K. Nakata and S. Shibusawa, Yogyo-Kyokai-Shi 86 (1978) 316. [3] S. Noev, V. Kozhukharov, I. Gerasimova, K. Krezhov and B. Sidzhimov, J. Phys. C.-Solid State Phys. 12 (1979) 2475. [4] T. Nishida, S. Saruwatari and Y. Takashima, Bull. Chem. Soc. Jpn. 61 (1988) 4093. [5] T. Yoko, K. Kamiya, H. Yamada and K. Tanaka, J. Am. Ceram. Soc. 71 (1988) C70. [6] K. Tanaka, T. Yoko, H. Yamada and K. Kamiya, J. Non-Cryst. Solids 103 (1988) 250. [7] T. Yoko, K. Kamiya, K. Tanaka, H. Yamada and S. Sakka, J. Ceram. Soc. Jpn. 97 (1989) 289. [8] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th Ed. (Wiley, New York, 1986). [9] I. Nakagawa, T. Onishi, H. Takahashi, M. Misumi and T. Fujiyama, Jikken-Kagaku-Koza (compiled by Chem. Soc. Japan), Zoku 10 (Maruzen, Tokyo, 1964) ch. 8. [10] T. Shimanouchi, M. Tsuboi and T. Miyazawa, J. Chem. Phys. 35 (1961) 1597. [11] H.C. Urey and C.A. Bradley, Phys. Rev. 38 (1931) 1969. [12] H. Siebert, Z. Anorg. Allg. Chem. 275 (1954) 225. [13] T, Shimanouchi and I. Suzuki, J. Chem. Phys. 42 (1965) 296. [14] M. Tasumi, Kagaku-no-Ryoiki, Zokan 115 (1977) 23. [15] F. Folger, Z. Anorg. Allg. Chem. 411 (1975) 103. [16] R. Masse, J.C. Guitel and I. Tordjman, Mater. Res. Bull. 15 (1980) 431. [17] H.J. Thiimmel and R. Hoppe, Z. Naturforsch. 29b (1974) 28. [18] B.O. Loopstra and K. Goubitz, Acta Crystallogr. C42 (1986) 520. [19] B. Frit and D. Mercurio, Rev. Chim. Miner. 17 (1980) 192. [20] M. Arnaudov, V. Dimitrov, Y. Dimitriev and L. Markova, Mater. Res. Bull. 17 (1982) 1121. [21] D. Cachau-Herreillat, A. Norbert, M. Maurin and E. Philippot, J. Solid State Chem. 37 (1981) 352. [22] E. Philippot, J. Solid State Chem. 38 (1981) 26.

144

T. Sekiya et al. / Raman spectra o f m O l / 2 - T e O 2 ( m = Li, Na, I~ Rb, Cs and Tl) glasses

[23] T. Furukawa, K.E. Fox and W.B. White, J. Chem. Phys. 75 (1981) 3226. [24] O. Lindqvist, Acta Chem. Scand. 22 (1968) 977. [25] H. Beyer, Z. Kristallogr. 124 (1967) 228. [26] D. Hottentot and B.O. Loopstra, Acta Crystallogr. C39 (1983) 320. [27] H. Hanke, Naturwiss. 53 (1966) 273. [28] J. Galy and O. Lindqvist, J. Solid State Chem. 27 (1979) 279.

[29] J.F. Baugher and P.J. Bray, Phys. Chem. Glass, 10 (1969) 77. [30] H. Hosono, H. Kawazoe, J. Nishii and T. Kanazawa, J. Non-Cryst. Solids 51 (1982) 217. [31] M. Leventhal and P.J. Bray, Phys. Chem. Glass. 6 (1965) 113.