~ Solid State Communications, Vol.64,No.4, pp.455-457, 1987. 0038-1098/87 $3.00 + .00 Printed in Great Britain. Pergamon Journals Ltd.

TEMPERATURE DEPENDENCE OF THE POLARIZATION OF THE DOMINANT RAMAN LINES IN B203 AND

(B203)0.84(Na20)0.16 GLASSES.

M.A. RAMOS, S. VIEIRA AND J.M. CALLEJA(+)

Dpto de Fisica de la Materia Condensada, C-Ill (+) Dpto. de Fisica Aplicada, C-IV

Facultad de Ciencias, Universidad Aut6noma de Madrid

Cantoblanco, 28049-Madrid, Spain.

(Received June lO, 1987 by M. Cardona)

We report the polarization behaviour of the dominant Raman lines in pure vitreous boric oxide (806 cm -I) for two different densities of the sample and in the sodium borate glass (B203)0.84(Na20)0.16 (803 and 772 cm-l), in a temperature interval ranging from 4.2 K up to room temperature. A strong increase of the depolarized scattering for these Raman lines with decreasing temperature has been observed. A tentative explanation of this effect, in terms of medium-range order and angular correlations is suggested.

One of the best accepted models of the structure of vitreous boric oxide (v-B203) is that proposed many years ago by Krogh-Moe [i]. According to it, the v-B203 structure consists mainly of planar hexagonal "boroxol" rings (B303) interconnected, at the boron atoms, by bridging oxygens to the rest of the network, in which some other structures such as equilateral triangles (BO 3) exist.

Within the boroxol rings [2], every angle has a very precise value of ~ = 120 ° . The bridging oxygen angles 8 (the B-O-B angles between planar rings and/or triangles) show a certain distribution around a most probable value <0> m 130 ° . A dihedral angle 6 (randomly distributed, in principle) specifies the relative orientation of two neighbouring structural units - triangles or rings. Available data of X-ray diffraction [3] are consistent with this geometrical structure. In addition, Pifiango et al. [4] have shown from Raman Scattering experiments that, in samples subjected to "stabilization processes", the number of boroxol rings per molar volume depends on the sample density, higher density corresponding to larger number of rings. By "stabilization process" [4,5] is meant an annealing at a constant temperature for a time long enough to achieve a stationary density.

The structure of alkali borate glasses has been studied by several other authors [6-8] who have shown that the addition of alkali oxide to B203 makes the boron atom coordination change from triangular to tetrahedral, producing the appearance of diborate, triborate or tetraborate groups. In the case of addition of Na20 tetraborate groups are present in the glass [6,7] together with boroxol rings.

The main feature of the Raman spectrum of v-B203 is an intense, polarized and very sharp peak at m 806 cm -I which has been assigned [6-11] to the syrmnetric breathing mode of the oxygen atoms in the boroxol rings. The large amplitude of the peak is consequently ascribed to the big number of these rings present per unit volume.

The narrowness of the peak (full width at half maximum (FWHM) m 15 cm -I ) shown in some Raman spectra [9,10] has been explained [2,11] by the high degree of decoupling of the boroxol ring from the rest of the amorphous network. This is a result of the specific symmetry of these 3-fold rings and the fact that these modes involve very little motion of the boron atoms [2,ii].

In the alkali borate glasses, another Raman line appears around 770-780 cm -I , which is assigned [6-8] to the breathing mode of the six-membered rings with both BO 3 and BO 4 units and has properties similar to those of the boroxol ring peak. Progressive addition of alkali oxide increases the intensity of the new peak, decreases that of the boroxol ring peak and shifts both slightly to lower frequencies. At the same time they become less sharp. For

(B203)0.84(Na20)0.16, the two peaks have approximately the same intensity and are situated at 772 and 803 cm -I .

We have performed measurements of the Raman spectrum of v-B203 and (B203)0.84(Na20)0.16 in a temperature interval ranging from 4.2 K up to room-temperature in order to try to better understand the relation between the vibration frequencies and the structure of these glasses. The sample of pure boric oxide was made from B203 powder (Merck, Suprapur), by heating at

455

456 THE POLARIZATION OF

250 ° C for 24 h and then at i000 ° C for one more day in a platinum crucible. Both processes were carried out in vacuum. By means of a stabilization treatment at 200°C for 166h in vacuum,we achieved a density of Pl = 1.85 g/ec. After recording the Raman spectra the density of this sample was changed to ~2 = 1.81 g/cc by a stabilization process at 312 ° C for 24 h in

vacuum. The sample of (B203)0.84(Na20)0.16 was prepared from raw products Merck pro-analysis

B203 and CO3Na 2. All Raman spectra were obtained in a

right-angle scattering configuration, the scattering plane being horizontal. A Krypton laser operating on the 6471 ~ line with an incident power of % IW was used. A low pressure of helium exchange gas was kept in the sample chamber of the cryostat to allow for temperature homogeneity. The scattered light was collected and analyzed by a double monochromator and a photon counting system. A Dove prism was used in order to fit the horizontal image of the light path across the sample into the vertical entrance slit of the monochromator. The spectral slit width was less than 2 cm -I

The polarization degree of the laser beam after crossing the sample was measured simultaneously to the Raman data.

In this communication, we present results on the degree of polarization of the 806 cm -I peak in pure B203 and on the 772 and 803 cm -I peaks in

(B203)0.84(Na20)0.16 . Fig i shows the IVH/I w

ratio versus temperature for pure v-B203. (H stands for horizontal and V for vertical with respect to the scattering plane). In Fig. 2 some original Raman Spectra at different temperatures are shown. Our results indicate that the depo- larization ratio IVH/I w for the 806 cm -I peak increases drastically with decreasing tempera- ture. This is a consequence of a strong increase in IVH, since I w remains approximately constant at any temperature. No distinctions are made in Fig. I between the experimental points for densities 0l and P2 because no differences were observed in their behaviour. At 300 K the depolarization ratio is ~ 0.05, in good agreement with the value reported by Galeener et al. [9].

~m

0 . 4

>. >

F~I \ I >

0 . 3

0 . 2

0 . 1

O

O O

+ + +

+

+ o

0 O ++ + + +

o o °

O , , i , , , 0 5 0 l O 0 1 5 0 200 250 3 0 0

TEMPERRTURE (K)

Fig. l.- Plot of the depolarization ratio IVH/IVv versus temperature for v-B203 . o: measured for decreasing temperature +: for increasing temperature.

THE DOMINANT RAMAN LINES Vol. 64, No. 4

N

m

Z i

Z

E I ¢ 1

0 83O

" I I I 1300 K :

- 120 K

3 0 0 K

820 810 8()0 790 7 8 0

FREQUENCY (cm"] Fig. 2. Some typical W and VH Raman Spectra at different temperatures. The shown spectra were taken with the same sensitivity.

At low temperatures the IVH/Ivv ratio approaches a value ~ 0.3. A hysteresis-like behaviour of the depolarization ratio can also be observed in Fig.l.

No changes were observed in the polarization degree of the laser beam after crossing the sample, which could be related to the variation of the depolarization ratio of the Raman line shown in the figure. This excludes the possibility for this variation to be due to strain induced depolarization of the incident light.

We want to mention other experimental facts which could be relevant to understand the observed depolarization change:

i. The peak frequency (806 ± 2 cm -I ) is independent both on the temperature and the density of the sample.

2. The FWHM ranges a~proximately from 10 cm -I at 4.2 K up to 13 cm -I at room temperature (for the two different densities). For a given temperature and density, the peak widths are the same for W and V}{ scattering.

These experimental facts (the sharpness of the peak and the negligible variation of its frequency with temperature) suggest that the observed depolarization change cannot be due neither to a deformation of the rings nor a dynamical coupling between them. They also seem to indicate that the boroxol ring structure is independent of the density; changing the density of the samples (by means of stabilization processes) merely alters the amount of boroxol rings.

An even stronger low temperature depolarization was observed for both the 772 and

the 803 cm -I lines in (B203)0.84(Na20)0.16 ,

Vol. 64, No. 4 THE POLARIZATION OF THE

since the depolarization ratio ranged from the room temperature value m 0.04 to a low temperature ratio m 0.45 for the two peaks. Besides, these peaks showed features (temperature independent frequency, ordinary low temperature sharpening, etc) similar to those mentioned above for V-B203. So the six membered ring structures seem to experience no variation independently of the coordination of the boron atoms.

On the other hand this depolarization could be explained by a change with temperature of the coupling coefficient CVH(~) [12] which relates the Raman IV} { intensity with the vibrational density of states. Martin and Galeener [13] have shown that such a change (i.e. a deviation of the IVH spectra with respect to the density of states) could occur when correlation between the relative orientations of neighbouring units takes place. These authors give analytic expressions for the polarized and depolarized Raman intensities of the vibrational excitations of a network glass. They found that the depolarized Raman spectrum involves only bond self-correlations and is proportional to the density of states multiplied by smoothly varying functions (CVH(~)), as it is usually accepted, assuming that no angular correlation exist between neighbouring rings. But when short-and medium-range angular correlations are present, such as preferred dihedral angles, appreciable differences in the depolarized spectrum with respect to the density of states will occur. This kind of mechanism could explain the observed enhancement of IVH for the dominant Raman lines in v-B203 and

(B203)0.84(Na20)0.16- In fact, glasses are generally supposed

[14] to have many available configurations with similar energy for T < T , Tg being the glass transition temperature. T~is can produce finite displacements of atoms or groups of atoms which do not necessarily increase the internal energy. The presence of these low-energy modes down to temperatures near 0 K is the basis of the widely accepted tunneling-state models [14,15], which

DOMINANT RAM_AN LINES 457

take account of the anomalous low-temperature properties in glasses.

We believe that, when temperature drops, the boroxol rings (or other kinds of rings for alkali borate glasses) and the bridging units can easily change their relative orientations, since the stiff rings appear not to change with temperature. So, the "freezing" of the structure can give a special (energetically favoured) distribution of dihedral angles, increasing the angular correlation between neighbouring rings.

Indeed, experiments on acoustic loss [16] in borate glasses indicate structural relaxations in the temperature interval of our measurements. In order to explain similar phenomena in glassy SiO2, Anderson and B6mmel [17] suggested that the relaxation is caused by oxygen atoms which have two equilibrium positions in directions transverse to the Si-O bond directions. A similar mechanism has been suggested for the borate glasses [16]. We speculate that this relaxation process can alter the correlation between rings in borate glasses and be the origin of the observed hysteretio depolarization behaviour.

In summary, we have reported a strong depolarization of the 806 cm -I (v-B203), 803 and 772 cm -I ((B203)0.84(Na20)0.16) Raman peaks when temperature drops, which is due to an increase of the depolarized VH-scattering. Since other features assigned to the rings such as the frequency of the peaks, the W-scattering intensity or the narrowness of the line (at low temperature the peak is even sharper) do not change, we are led to suggest a change in the distribution of dihedral angles with temperature as the origin of the depolarization increase.

Acknowledgments.

The authors wish to thank E. Martinez and E.S. Pifiango for helpful discussions. This work has been partially supported by the CAICyT under grant N ° 2924/84.

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i. J. Krogh-Moe, J. Non Cryst. Solids ~, 269 (1969).

2. F.L. Galeener and M.F. Thorpe, Phys. Rev. B 28, 5802 (1983) and references therein.

3. R.L. Mozzi and B.E. Warren, J. Appl. Cryst. ~, 251 (1970)

4. E.S. Pifiango, S. Vieira, and J.M. Calleja, J. Non-Cryst. Solids, 44, 387 (1981).

5. F.C. Everstein, J.M. Stevels and H.I. Waterman, Phys. Chem. Glasses l, 123 (1960)

6. T.W. Bril, Thesis. Philips Res. Rep. Suppl. No. 2 (1976).

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12. R. Shuker and R.W. Gan~non, Phys. Rev. Lett. 25, 222(1970).

13. R.M. Martin and F.L. Galeener, Phys. Rev. B 23, 3071 (1981).

14. W.A. P h i l l i p s , J.Low Temp. Phys. ~,351(1972). 15. P.W. Anderson, B.I. Halperin and C.M. Warma,

Philos. Mag. 25, i (1972). 16. J.T. Krause and C.R. Kurkjian, Borate

Glasses, (Edited by L.D. Pye, V.D. Frechette and N.J. Kreidl), Vol. 12, p. 577. Plenum, New York (1978).

B6mmel, J. Am. Ceram. 17.-O.L. Anderson and H.E. Soc. 38, 125 (1955).