Nippon Seramikkusu Kyokai GakujutsuRonbunshi 97 [12] 1435-40 (1989) 1435

Normal Vibrations of Two Polymorphic forms of TeO2 Crystals and Assignments of Raman Peaks of Pure TeO2 Glass

Takao SEKIYA, Norio MOCHIDA, Atsushi OHTSUKA and Mamoru TONOKAWA†

( Division of Materials Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, 156, Tokiwadai, Hodogaya-ku, Yokohama-shi 240)

TeO2結 晶 の 二 つ の 多 形 の 基 準 振 動 の 計 算 とTeO2ガ ラ ス の ラ マ ン ピ ー ク の 帰 属

関 谷 隆 夫 ・持 田 統 雄 ・大 塚 淳 ・殿 川 衛 † (横浜国立 大学工学部物質工学科, 240横 浜市保土 ケ谷区常盤台156)

Raman spectra of paratellurite, tellurite and pure TeO2 glass were measured. The normal vibration of paratellurite and tellurite was analyzed. The spectrum of pure TeO2glass were deconvoluted into symmetric Gaussian functions. The normal vibrations of paratellurite are exactly described as combined representations of movement of oxygen atom in Te-eqOax-Te linkage and vibrations of TeO4 trigonal bipyramid (tbp). Compared the resolved Raman peaks of pure TeO2glass with nor mal vibration of paratellurite, all Raman peaks from 420 to 880 cm-1 are assigned to the vibrations of TeO4 tbp and Te-eqOax-Te linkage. The antisymmetric stretching vibrations of Te-qeOax-Te linkage have relatively large intensities to symmetric stretching vibrations of Te-eqOax-Te linkage. In pure TeO2 glass, TeO4 tops are formed by most of tellurium atoms and connected at vertices by the Te-eqOax-Te linkages. [ReceivedMay 18, 1989; AcceptedAugust 22, 1989]

Key-words: Normal vibration, Raman spectrum, Paratellurite, Tellurite, Pure TeO2glass

1. Introduction network modifiers, were added to TeO2. But pure Tellurium dioxide is one of the typical glass TeO2 glass structure was not investigated. Re forming oxides. The TeO2-based glasses have cently, it was reported that tellurium dioxide wide glass forming range and several useful itself became glassy state when a small batch of properties such as low melting temperature, high its melt was cooled rapidly.7) refractive index and good transmittance of infra Vibrational spectroscopy is one of the most

red.1)-5) powerful techniques for studying the structure of

The coordination states of TeO3 Te4+ ions are glass. For the purpose of analysis of the structure TeO4 trigonal bipyramid (tbp), trigonal pyramid of pure TeO2 glass, Raman spectra of two

and these intermediate states in crystals contain polymorphic forms of crystalline TeO2 (para ing TeO2. The structure of TeO4 tbp is described tellurite and tellurite) were investigated. Raman

as that one equatorial site of the Te sp3d hybrid spectrum of pure TeO2 glass was measured and

orbitals is occupied by a lone pair of electrons and deconvoluted into the symmetric Gaussian func

the other two equatorial and two axial sites are tions. The resolved Raman peaks were investi

occupied by oxygen atoms. In the TeO3 trigonal gated with the results of normal vibration ana pyramid, one of the Te sp3 hybrid orbitals is lysis. occupied by a lone pair of electrons.

For the structure of TeO2-based glasses, 2. Experimental procedure Mochida et al.6) concluded from study of IR 2.1 Preparation of specimens

spectra that the coordination states of part of Te Tellurite (ƒÀ-TeO2) was used as raw materials . atoms changes from TeO4 tbps to TeO3 pyramids Paratellurite (ƒ¿-TeO2) was prepared by melting when mono- and di-valent cations, that is typical of TeO2 and then crystallizing. Pure TeO2 glass

was prepared by melting of 0.1-0 .2g TeO2 batch †Now Foundation for Promotion of Material Science and in Pt crucible and then cooling by dipping Technology of Japan, 3-11-1, Kamisoshigaya, Setagaya crucible bottom into ice-cold water ku, Tokyo 157 . The speci 現 在: (財) 材 料 科 学 技 術 振 興 財 団, 157東 京 都 世 田 谷 区 上 men was confirmed to be glassy state by X-ray

祖 師 谷3-11-1 diffraction pattern and DTA analysis . Normal Vibrations of Two Polymorphic Forms of TeO2 Crystals and 1436 Assignments of Raman Peaks of Pure TeO2 Glass

2.2 Measurement of Raman spectra tellurite. Raman spectra were measured in the wavenum The program of optimization was made for NEC ber range from 20 to 1200 cm-1 using Ar+ laser PC-9801 series computer in our laboratory .

(514.5nm, 200mW) and R-800 laser Raman spectrophotometer with third monochrometer sup 3. Results and discussion

plied by Japan Spectroscopic Co., Ltd. 3.1 Paratellurite, ƒ¿-TeO2 Raman spectrum of paratellurite is shown in (JASCO). For pure TeO2 glass, polarized spec tra, I•V and I•Û, were measured and depolarization Fig. 1.

ratio, ƒÏ, was determine as following equation. The crystal data of paratellurite have been

given by Lindgvist10): tetragonal, P41212-D44, a where I•V and I•Û are the intensities of scattered

lights, when polarization vector of scattered beam

are parallel and perpendicular to that of incident

beam, respectively. The spectrum was separated

in the wavenumber range from 420 to 880cm-1 by

a curve fitting method using the symmetric Gaus

sian function as a profile function. Peak posi

tions, intensities, half widths and depolarization

ratios were optimized.

2.3 Analysis of normal vibrations The analysis of normal vibration was carried

out with the mass corrected Cartesian symmetric

displacement coordinate method. Simple Urey-

Bradley force field was used as internal potential field.

For paratellurite, Raman peaks had be already

assigned by Pine et al.8) The force constants of stretching of valence bonds, K, bending of

bonds, H, and repulsion between nonbonded atoms, F, were set as Table 1 and optimized by the brute force method. The initial values of force constants of stretching for the optimization were estimated from that of TeO32- ion,9) and the other force constants were assumed to be zero. No reports for assignment of Raman peaks of tellur ite, exists. Using optimized force constants, Fig. 1. Raman spectra of two polymorphic forms of normal vibrations were tentatively calculated for TeO2 crystals, paratellurite and tellurite.

Table 1. Force constants and calculated and observed frequencies of paratellurite. T. SEKIYA et al. 1437

A1 species (Vs1TeO4, VasTe-eqOax-Te) A1 species (δs1TeO4, VsTe-eqOax-Te) Vcalc.=668cm-1 Vcalc.=449cm-1 Vobs Vobs.=649cm-1 .=392cm-1

B1 species (VasTeO4, VasTe-eqOax-Te) B2 species (δs2TeO4, VsTe-eqOax-Te Vcalc.=645cm-1 ) Vcalc .=440cm-1 Vobs.=589cm-1 Vobs .=415cm-1

Fig. 2. Normal vibrations of paratellurite. The compo B2 species (Vs2TeO4, VasTe-eqOax-Te) nents of atomic displacement vector are indicated by Vcalc.=750cm-1 x, y and z. Vobs.=786cm-1

to electrostatic interactions. The optimization of =4 .812•ð, c=7.615•ð and z=4. The structure of paratellurite is a three-dimensional network force constants was carried out for A1, B1 and B2 built up from the TeO4 tbp, sharing vertices and species to disregard LO-TO split. containing only Te-eqOax-Te linkages. The force constants and calculated frequency

The Bravais cell of paratellurite contains four values are listed in Table 1. The agreement

TeO2 units resulting in 36 phonon branches with between the calculated and observed values is

ƒ¡= 4A1+5A2+5B1+4B2+9E. Of these 1A2+ reasonably good. If Coulonb interaction is consi

1E comprise the acoustic modes. A1, B1, B2 and dered as a long range force, the difference

E species are Raman active. A2 and E species between observed and calculated frequency which are infra-red active are split to longitudinal values will be smaller. But simple Urey-Bradley

(LO) and transverse (TO) optical modes owing force field is sufficient for our purpose of analysis Normal Vibrations of Two Polymorphic Forms of TeO A 2 Crystals and 1438 ssignments of Raman Peaks of Pure TeO 2 Glass

of vibrational types of TeO4 tbp and its applica Table 2. Calculated and observed frequencies of tel tion for pure TeO2 glass. lurite.

The vibrational modes for paratellurite , which have relatively large frequencies, are shown in

Fig. 2. These modes are exactly described as

combined representations of the vibrations of

TeO4 tbp and the movement of bridging oxygen

atom in Te-eqOaX-Te linkages.

In A1 species at 668cm-1, two equatorial bonds

stretch in the same phase, two axial bonds in the

same phase and two types of bonds in the reverse

phase. This mode is assigned to vs1 TeO4 from the atomic displacement of TeO4 tbp. In B1 species at

645cm-1, two equatorial bonds stretch in the reverse phase and two axial bonds vibrate in the

same manner. It is assigned to vas TeO4. The B2 species at 750cm-1 is assigned to vs2TeO4 , where all the Te-O bonds of each TeO4 tbp stretch in the

same phase. The modes at 696cm-1 and 653cm-1

belong to E species. The former mode contains

two types of atomic displacement of TeO4 tbp . tentatively appropriated for those of tellurite . One is regarded as atomic displacement of TeO4 The observed and calculated frequencies of Ra tbp in vs2 TeO4 and the other is in vas TeO4 . In the man active modes are listed in Table 2. case of the latter mode, one is in vs1 TeO4 and the The symmetry assignment of observed Raman other is in VasTeO4 (The schemes of these modes peaks is not clear. The suitability of appropria are omitted). Therefore, they are assigned to tion of the force constants of paratellurite is vs2+asTeO4 and vs1+asTeO4, respectively. These doubtful because tellurite contains Te2O2 four- five vibrational modes are assigned to antisym membered ring due to edge sharing of TeO4 tpbs . metric vibrations of Te-eqOax-Te linkage . We avoid the detailed discussion about this In A1 species at 442cm-1, two bending of crystal. But the calculated frequencies of the Oax-Te-axO and Oeq-Te-egO bonds of TeO4 tbp peaks which have relatively large intensities in occur in the same phase. It is assigned to simple stretching model and belong to Ag mode , ƒÂs1TeO4. The B2 species at 440cm-1 is assigned resemble to observed frequencies . In the view to ƒÂs2TeO4. All Te-O bonds are twisting , but point of the vibrations of TeO4 tbp which are re direction of twisting of equatorial bonds is oppo garded as the vibrational modes assigned to anti site to that of axial bonds. These two modes are symmetric stretching vibration of Te-eqOax-Te assigned to symmetric stretching vibrations of linkage, some of the vibrational modes of tellurite Te-eqOax-Te linkage. correspond to those of paratellurite, although the 3.2 Tellurite, ƒÀ-TeO2 movements of edge sharing atoms are restricted in Raman spectrum of tellurite is shown in Fig . 1. tellurite. The crystal data of tellurite have been given by 3.3 Pure TeO2 glass

Beyer11): orthorhombic, Pbca-D2h15, a=12 .035•ð Polarized Raman spectrum of pure TeO2 glass

, b=5.464•ð, c=5.607•ð and z=8. A pair of is shown in Fig. 3 and the result of deconvolution the slightly distorted TeO4 tops is connected by a is given in Table 3 . The closest possible agree common edge to form Te2O6 unit. These units ment between calculated and observed intensities form infinite (Te2O4)•‡ sheets by sharing the was obtained , when the presence of five peaks at remaining four vertices. All the oxygen atoms in 773, 716, 659, 611, and 450cm-1 , was assumed in this structure form Te-eqOax-Te linkages the range from 420 to 880cm-1 . . For the purpose of studying the influence of The frequencies of the resolved five peaks agree with the frequencies of paratellurite edge sharing of TeO4 tops on the frequencies of . The Raman peak, vibrational frequencies of Raman four higher Raman peaks of pure TeO 2 glass are active modes were calculated . The vibrational assigned to ν「s2TeO4 (and νs2+asTeO4) , νs1+asTeO4, representation of tellurite is represented as ƒ¡= νs1TeO4, νasTeO4 of TeO4 tbp and assigned to

9Ag+9Au+9B1g+9B1u+9B2g+9B2u+9B3g+ antisymmetric stretching vibration of Te-eqOax-

9B3u. Ag, B1g, B2g and B3g species are Raman Te linkage. The other peak is assigned to active. The force constants of paratellurite were δs1TeO4 (and δs2TeO4) of TeO4 tbp, and assigned T. SEKIYA et al. 1439

Table 3. Result of deconvolution and assignments of peaks in pure TeO2 glass.

to symmetric stretching vibration of Te-eqOax-Te

linkage. No peaks corresponded to other structu

ral units are observed. It is indicated that TeO4

tbps are formed by most of tellurium atoms in pure TeO2 glass.

The antisymmetric stretching vibrations of

Te-eqOax-Te bonds in pure TeO2 glass have

relatively large intensities to symmetric stretch

ing vibrations of Te-eqOax-Te bonds . For the simple stretching model, the intensities of Raman

peaks are determined by the tensors (•Ýaik/•ÝQk)o, written in the following expression,

(∂ αik/∂Qp)。=Σn(∂αik/∂γn)。(∂γn/∂Qp)

where aik are components of the polarizability

tensor, ƒÁn are the changes of the bond lengths and

Qp are the p th normal coordinates. If it is assumed that each atom has an isotropic (scalar)

polarizability which depends on the distance of its nearest neighbouring dipole moment(or atom) , (•Ýa/•Ýr)o is inversely proportional to the fourth

(or third) power of the equilibrium distance.12) By this model, antisymmetric stretching vibration has little intensity in the structure built from MOx polyhedra sharing vertices.13),14) For examples, this is true for crystalline and vitreous SiO2 and Fig. 3. Polarized Raman spectra and result of decon

GeO2. The changes of polarizability are locally volution of pure TeO2 glass . The observed spectra of I•V compensated because of the reverse motions of and I•Û resolved Raman peaks (the full lines) and sum neighboring same kinds of M-O bonds. The of those peaks (the broken lines) are shown . The depolarization ratio, ƒÏ, is also shown in upper side difference of changes of polarizability between . The observed and calculated values are indicated by the two varieties of Te-O bonds is contributed to points and the full line, respectively. Raman intensity in case of Te-eqOax-Te bond, while those are reduced to zero in case of

Te-eqOeq-Te and Te-axOax-Te bonds. The exist paratellurite. The reasons are as follows. Para ence of Te-eqOeq-Te and Te-axOax-Te bonds is tellurite is crystallized by heating of pure TeO2 indefinable, but it was indicated that pure TeO2 glass. According to X-ray diffraction study,15) glass contains Te-eqOax-Te linkages, like TeO2 Te-Te peak which corresponds to Te2O6 unit , is crystals. not observed in 3.2•ð of the radial distribution

It was revealed from these results that pure function of pure TeO2 glass.

TeO2 glass is consisted of TeO4 tbp, linked by Te-eqOax-Te bonds. Though an additional in 4. Conclusions formation for pure TeO2 glass can not be obtained Raman spectra of paratellurite, tellurite and from Raman spectroscopy , we think that the pure TeO2 glass were measured. The normal structure of pure TeO2 glass is the network of vibration of paratellurite and tellurite was analy TeO4 tbp sharing vertices similar to that of zed. The spectrum of pure TeO2 glass was

Normal Vibratiolls of Two Polymorphic Forms of TeO2 Crystals and 1440 Assignments of Raman Peaks of Pure TeO2 Glass deconvoluted into symmetric Gaussian functions at vertices by the Te-eqOax-Te linkages . taking account of optimizing of depolarization ratio. The following results were obtained. References 1) J. E. Stanworth, Nature, 169, 581-82 (1952).

(1) The normal vibrations of paratellurite 2) J. E. Stanworth, J. Soc.Glass Tech., 36, 217-41 (1952). are exactly described as combined representations 3) J. A. James and J. E. Stanworth, J. Soc. Glass Tech., of movement of oxygen atom in Te-eqOax-Te 38, 421 T-24 T (1954). 4) J. E. Stanworth, J. Soc. Glass Tech., 38, 425 T-35 T linkage and vibrations of basic structural unit, (1954). that is TeO4 tbp. 5) M. Imaoka and K. Satake, Seisan-Kenkyu,9, 505-09

(2) The resolved Raman peaks at 773, 716, (1957). 6) N. Mochida, K. Takahashi, K. Nakata and S. Shibusa 659, 611 and 450cm-1 in pure TeO2 glass agreed wa, Yogyo-Kyokai-Shi,86, 317-26 (1978). with the frequencies of paratellurite. The five 7) P. T. Sarjeant and R. Roy, J. Am. Ceram. Soc., 50, peaks are assigned to ƒËs2TeO4 (and ƒËs2+asTeO4), 500-03 (1967).

ƒËs1+asTeO4, ƒËs1TeO4, ƒËasTeO4, ƒÂs1TeO4 (and 8) A. S. Pine and G. Dresselhaus, Phys. Rev., B5, 4087-93 (1972). ƒÂs2TeO4), respectively. The four higher modes 9) H. Shibert, Z. anorg. allg. Chem., 275, 225-40 (1954). are assigned to antisymmetric stretching vibra 10) O. Lindqvist, Acta Chem. Scand., 22, 977-82 (1968). tions of Te-eqOax-Te linkage and the other is to 11) H. Beyer, Z. Krist., 124, 228-37 (1967). symmetric stretching vibration of Te-eqOaX-Te 12) M. W. Wolkenstein, Compt. Rend. Acad. Sci. URSS,32, 185-88 (1941) linkage. The intensities of antisymmetric stretch 13) T. Furukawa, K. E. Fox and W. B. White, J. Chem. ing vibrations of Te-eqOax-Te linkage are re Phys., 75, 3226-37 (1981). latively large to those of symmetric stretching 14) N. Mochida, K. Sakai and K. Kikuchi, Yogyo-Kyokai- vibrations of Te-eqOax-Te linkage. Shi, 92, 164-72 (1984). 15) T. Yoko, H. Takeuchi, K. Kamiya and K. Tanaka, The (3) In pure TeO2 glass, TeO4 tbps are 28 th Symposium on Glass Abstracts, Tokyo (1987) formed by most of tellurium atoms and connected p. 67.