LOW FREQUENCY LIGHT SCATTERING SPECTRA OF AgI

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LOW FREQUENCY LIGHT SCATTERING SPECTRA OF AgI

W. Sasaki, Y. Sasaki, S. Ushioda, W. Taylor

To cite this version:

W. Sasaki, Y. Sasaki, S. Ushioda, W. Taylor. LOW FREQUENCY LIGHT SCATTERING SPECTRA OF AgI. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-181-C6-183. �10.1051/jphyscol:1981653�.

�jpa-00221589�

JOURNAL DE PHYSIQUE

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LOW FREQUENCY L I G H T SCATTERING SPECTRA OF A g I

W. Sasaki, Y. Sasaki, S. Ushioda and W. Taylor*

Abstract. The Rayleigh-Brillouin spectra of a- and B-AgI have been measured with a Pabry-Perot interferometer in a back scattering geometry. The longi- tudinal acoustic phonon frequency in the 8-phase gradually decreases with increasing temperature from 17.4 GHz at room temperature to 16 GHz just below the transition. At the transition temperature (145°~) the frequency of the LA-mode shifts suddenly to 13.5 GHz, and thereafter it remains independent of temperature. No peak corresponding to the TA phonon was observed. A broad peak was observed in the frequency region near 27 GHz where a conductivity anomaly was found by earlier infrared reflectivity measurements.

1. Introduction.- In the superionic phase of AgI liquid-like diffusive motions of the silver ions produce two central components in the inelastic light scattering spectra. In the same low frequency region, peaks due to acoustic phonons should appear, and according to theory (2'3) they may show interference with the diffusive motions of cation ions. The purpose of the present paper is to investigate the in- elastic light scattering spectra of a- and B-AgI in the frequency range of 0.1 GHz to 300 GHz. The frequency region near 30 GHz (1 cm-I) where an anomaly in the fre- quency dependence of the conductivity

was observed by the far-infrared measure-

~aent'~) has been explored to find a corresponding structure in the light scattering spectrum.

2. Experiment.- Single crystals of 8-AgI grown from a water solution(5) were cleaved parallel to the base plane with the c-axis of the wurtzite structure normal to the large hexagonal surface. This hexagonal plate shaped sample was mounted in an oven with one of the sides along the horizontal as indicated in the insert to Fig. 1.

What we call Y-axis is normal to one of the sides of the hexagon; Z-axis is parallel to the c-axis. The m e n temperature was controlled within 0.5'~. The incident light

(6328 A line of a multi-mode He-We laser) was directed along the c-axis and the back scattered light was analyzed by a triple-passed plane-parallel Fabry-Perot interfer- ometer with a fineness of 30. The frequencies of the observed Brillouin spectra were calibrated by comparison with the well-known Brillouin spectra of pure fused silica. (6)

3.

Results and Discussion.- Figure 1 shows the Brillouin spectra of 8-AgI (125'~) with the LA phonon peak located at 16.3 GHz. In the (YY) polarization configuration a clear peak is observed, while no peak is observed in the (XY) polarization con-

-- -

*permanent address

Department of Physics, University of Edinburgh )'~esearch supported by NSF, DMR 80-11435

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981653

C6- 182 JOURNAL DE PHYSIQUE

figuration as shown in the figure. According to the selection rule for the wurtzite

~tructure'~) this strong peak corres.ponds to the LA phonons propagating along the c-axis .

From the observed Brillouin frequency, v

17.35 GHz, and the refractive index, we find the sound velocity v

2.48 x 10 cm/sec. This value is reasonable 5 compared t~ the value of 2.17 r .105sc~isec obtained by extrapolating the neutron scattering data. (8) One of the elastic constants Cg3 was found to be Cj3

3.7 x 10" dyne/cm

. This value is comparable to the corresponding data for AgBr and AgCl .

- -

.-

S

4

-

k

ln Z W I- Z-

6 10 IS 2 C

FREQUENCY SHIFT ( G H z )

- - .- _,"

3 C

+

-

k

ln

z

W I- 5

10 IS 20

FREQUENCY SHIFT (GHz)

Fig. 1

Brillouin spectra of LA phonons Fig.

Brillouin spectra of LA phonons in 6-AgT.. The insert shows the crystal in a-AgI. The insert shows the crystal

orientation. orientation.

Figure 2 shows the Brillouin

spectra of a-Agl at 155'~. The frequency 1

of the cubic phase. (lo) Hence, under

Fig. 3

Temperature dependence of the LA our experimental condition (back scat- phonon frequency.

shift of the acoustic phonon decreases to 13.7 GHz at the transition. According to

16

the selection rule for a bcc crystal,

>

both the LA and the TA phonons should

W 14-

appear in these ~pectra,'~) but the TA

2

phonon peak is not found. After the

&to-a phase transition, the c-axis of

tering) the LA phonon propagates along

- ' - ^: ,

+-m

r Tc i

the C110'1 direction of the cubic phase, and can scatter light for the polarization configuration (Y,Y) but not for (Y,X).(7) By rotating the polarization of the in-

0 2 0 4 0 6 0 8 0 100 120 140 160 180

the wurtzite phase becomes the [ll~] axis

cident light, we found that the maximum intensity is obtained for the incident pol- arization at -45O from the vertical, i.e. (?x,Y) configuration. The intensity is significantly weaker when the incident light polarization is rotated to +45O, i.e.

for the (YX,Y) configuration. The reason for this polarization effect has not been

determined. However, we beliwe that the observed effect originates from multi- domain formation at the

transition which is first order.

Huberman and Martin,(2) and ~ubbaswam~(~) have predicted the existence of interactions between the diffusing A ~ + ions and the acoustic phonons. In our exper- iment, no clear indication of this interaction could be found from the observed spectral shape of the LA phonon peak. The fact that the TA phonon peak could not be found in our spectra may indicate a strong interaction between the diffusive motion of A ~ + and the TA phonons resulting in heavy damping. The absence of the TA phonons is probably related to the liquid-like behavior of the Ag ions in the +

ci-phase . ⁽¹¹⁾

Figure 3 shows the temperature dependence of the LA phonon frequency which decreases gradually from 17.5 GHz at room temperature to 16 GHz just below the transition temperature. The frequency suddenly shifts to 13.5 GHz at the transi- tion temperature T and remains constant above T . For our sample T was 1 4 5 ~ ~ .

In the frequency region about 30 GHz

1

=-1

some structures were found in the frequency dependent conductivity

which was derived from the infrared re- flectivity. (4912) We looked for a corresponding structure in the Brillouin spectra, and found a weak and broad peak at 27 GHz. However, the nature of this peak could not be determined definitively due to its low intensity.

- ¹

The diffusive central component with 3.8 cm width") could not be observed in this experiment, although we looked for such a feature.

4. References.

G. Winterling, W. Senn, M. Grimsditch and R. Katiyar: Proc. Intern. Conf. on Lattice Dynamics, ed. by M. Balkanski (Flammarion, Paris 1977).

B. A. Huberman, R. M. Martin: Phys. Rev. z, 1498 (1977).

K. R. Subbaswamy: Solid State Comm. 19, 1157 (1976).

P. ~rGesch, L. Pietronero, H. R. Zeller: J. Phys. C: 2, 3977, (1976).

M. E. Hills: J. Crystal Growth 1, 257 (1970).

S. W. Shapiro, R. W. Gammon, H.

Cisnerios: Appl. Phys. Lett. 9, 157 (1966) R. Vacher, L. Bayer: Phys. Rev. E, 639 (1972).

P. Bruesch, W. Buhrer, H. J. M. Smeets: Phys. Rev. E , 970 (1980).

K. H. Hellwage ed: LANDOLT-B~RNSTEIN Numerical Data and Functional Relation- ships in Science and Technology~II~/l (Springer-Verlag 1966).

D. R. Mills, C. M. Perrott, N. H. Fletcher.

Crystal Growth 6, 266 (1970).

M. J. Delaney, S. Ushioda: Phys. Rev. e, 1410 (1977).

H. Hayashi, M. Kobayashi, I. Yokota: Solid State Commun.: 2, 847 (1979).