VIBRATIONAL DYNAMICS OF RbAg4I5 BY RAMAN SPECTROSCOPY

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VIBRATIONAL DYNAMICS OF RbAg4I5 BY RAMAN SPECTROSCOPY

E. Cazzanelli, A. Fontana, G. Mariotto, F. Rocca

To cite this version:

E. Cazzanelli, A. Fontana, G. Mariotto, F. Rocca. VIBRATIONAL DYNAMICS OF RbAg4I5 BY RAMAN SPECTROSCOPY. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-190-C6-192.

�10.1051/jphyscol:1981656�. �jpa-00221592�

JOURNAL DE PHYSIQUE

CoZZoque C6, suppZ6ment au nO1 2, Tome 42, ddcembre 1981 page C6- 190

VIBRATIONAL DYNAMICS OF RbAg415 BY RAMAN SPECTROSCOPY

E. Cazzanelli, A. Fontana, G. Mariotto and F. Rocca*

Dipartimento d i Fisica, Universita' d i Trento e Unit6 GNSM-CUR, Trento, I t a l y

" ~ s t i t u t o per Za Ricerca S c i e n t i f i c a e TeenoZogica, Trento, I t a l y

Abstract.- We have measured the temperature dependence of Raman scattering of superionic RbAgq15 from 2 0 ' ~ to melting point. We interpret our data in terms of a vibrational density of states.

The group compounds like MAgqIg ( M = Rb, K, NH..) are well known for their proper- ties of ionic conductivity at room temperature. For this reason they have been stu- died recently and compared with more famous compounds like AgI, CuI, and CuBr which structure is much more simple but with the "disvantage" to become superionic conduc- tors at higher temperatures The other cause of interest of RbAgqIg crystal is the presence of two phase transitions: the first at 120°K, the second at 208"~. Af- ter the former transition the RbAgq15 crystal is in superionic phase: this transi- tion is associated to a discontinuity in ionic conductivity o(T) (3). The last tran- sition takes the crystal to an other phase (yet superionic) and it is associated with a discontinuity of dO(T)/dT (4).

Both transitions are associated with a discontinuity in specific heat. The 208'~ phase transition is a second order transition and it has been designated a disorder-disorder transition because neither the a nor the 6-phase can be ordered (5).

In spite of such transitions the vibratianal dynamics measured by Raman spectro- scopy does not change significately in these three phases

(apart from the di- sappearance at 12@Kof asmall peak centered at 23 cm-I among more of thirty peaks ( 7 ) and the appearing of a low quasi-elastic tail); while in AgI crystals we have a strong change in the shape of spectrum, with the appearing of large bands in $-phase and a strong quasi-elastic scattering in superionic phase (8). Finally we have no ef- fects at 208°K transition. IJe have accurately measured Raman scattering in RbAgq15 as a function of temperature from liquid helium temperature to melting point.

Raman spectra were excited by He-Ne 6328ilaser line with 5ml.I power on samples.

The low power and defocalized radiation were necessary to avoid photosensibility ef- fects. The scattered light is dispersed by a double monochromator Jobin Yvon (2200 lines per mm); such double monochromator has a very large rejection of stray light:

this rejection has allowed us to measure Raman spectra 2 cm-1 from laser line. Our unoriented samples were of pale yellow colour and high optical quality. They were grown from HI solution in E.T.H. in Ziirich Laboratory and kindly provided us by Her- bert Looser.

At liquid helium temperature the spectrum appears with a large number of peaks which are superimposed mainly in two zones centered at 30 cm-I and 100 cm-I respecti- vely (Fig. 1).

The large number of peaks is not surprising because the RbAgqIg in its a-phase has twelve molecules in unit cell ( 3 ) .

The spectrum shape and the peak frequencies do not change with increasing tempe-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981656

rature. Essentially we have a progressive broadening of peaks until the spectrum re- duces to two large bands centered at the same above mentioned frequencies, showing a low unharmonicity but,too, that vibrational dynamics is not changing. Concurrently, the integrated Raman intensity between 3 cm-I to 13 cm-I range changes. Such inten- sity is reported in fig. 2, normalized by the Bose-Einstein population factor. We note that such low frequency scattering is very low and constant up to about ~o'K, then it increases linearly unti1300K through the two phase transitions (T = 120°K, 208OK). Above such temperature it tends to saturate. At the same time the integra- ted total Raman intensity has no change. In figure 3 we reports IR(T) where

200 cm-1 I (T) = I

R dm

n(w,T) + 1

where I ( w , T) is Raman intensity at w frequency and T temperature, n(w, T)+1 is Bose-Einstein factor. Thus the data show that the total Raman intensity behaves as first order scattering until melting point. The different behaviour of Raman da- ta reported by D.A. Gallagher and M. Klein near melting point, can be explained if we take in account the broadening of peaks centered at 30 cm-l: there is no change

in intensity but a broadening of peaks.

A possible explanation of the noted effects can be obtained if we take into account the RbAg41g structure in its three phase (y, 8 , a at increasing tempera- ture ( 3 ) and the h ~ g h ionic conducibility.

The absence of low frequency contribution to the normalized spectral density for temperatures below * 8 0 ' ~ indicates that RbAg415 at such temperatures is orde- red, at least as for as Raman spectroscopy is concerned. Upon increasing temperatu-

Fig. 1 : RbAgqI5 Raman spectrum at 2 2 O ~ . Solid line is experimental datum with 0.5

cm-I resolution. The points are the fit with 30 gaussians.

C6-192 JOURNAL DE PHYSIQUE

re, as the remaining spectral

features broaden the low fre- quency controbution starts to increase. This is an indica- tion of incipient disorder in the system.

The effect of disordering will be to break wave vector conservatior~ rule. Thus the

low frequency scattering can be interpreted as a density of

states contribution from the

acoustic vibrational branches. Fig. 2 : Integraid Raman scattering between 3 cm-l -

In RbAgqIg, such contri- 13 cm-' range as a function of temperature.

bution is the only one that can The intensity is normalized by the approprig be clearly isolated, since the te Bose-Einstein population factor.

remaining part of the spectrum - ^-

- - - - -

-

- -

- - - - -7

change smoothly from the super- i l r u ) r I

position of zone center normal 1 ^I

mode peaks of the ordered sy- '

stem to a vibrational density I

of states which can hardly be di-

stinguished from the ordered =...*..* .. . . ... . . ....

.. .

. ^I

situation, given the large num- /

ber of atoms in the unit cell (120) and the consequent flat- ; ^I

ness of dispersion curves. I

Thus the low frequency scatte-

ring intensity can be used as a

probe of the progressive disor- 1

100 200 300 400 K

dering of the RbAgqIg structure.

Fig. 3 : Integrated total Raman scattering as a fun- Such local disordering starts

ction of temperature in the 3 cm-l - 200 in the y phase and continues

cm-l .range.

through the two following phase

transitions. In this sense RbAg415 can be considered to be always disordered, apart from the low temperature part of phasey(T <80°~), and its Raman spectrum for

~ 2 8 0 ' ~ is best described as a vibrational density of states.

The authors wish to thank prof. M.P. Fontana for valuable suggestions and dis- cussions.

This work is partially supported by CNK contract number 80.00816.11.

1) W. Van Goo1 (editor): Fast Ion Transport in Solids, North Holland,Amsterdam,l973 2) G.D. Mahan, W.L. Roth (editors): Superionic Conductors,Plenum Press,New York,1976 3) S. Geller (editor): Solid Electrolytes, Springer Verlag, Berlin 1977

4) R. Vergas, M.B. Solomon, C.P. Flynn, Physical Rev. Lett. 37, 1550 (1976) 5) S. Geller, Phys. Rev. e, 4345 (1976)

6) D.A. Gallagher and M.V. Klein, Phys. Rev. E, 4282 (1979)

7) G. Burns, F.H. Dacol, M.W. Shager, Solids St. Corn. 19, 287 (1976)

8) G. Mariotto, A. Pontana, E. Cazzanelli and M.P. Fontana, Phys. Stat. Sol. b 101,

341 (1980)

9) R. Shuker and R.W. Gammon, Phys. Rev. Lett. 2, 222 (1970)