Using Raman scattering experiments we have investigated the rotational and translational modes in K1−x(NH4)xI mixed crystals for ammonium concentrations x=0.005, 0.07, 0.16, and x=0.46 and in a fully deuterated isomorph with x(D)=0.42. The A1g+4Eg and T2g Raman responses have been measured at atmospheric pressure in the paraelectric plastic phase (T=295 K) and in the orientational glassy phase (T=7 K). In the low‐frequency region ν=5–600 cm−1, seven Raman active components could be identified. These bands are entirely due to the admixture of anisotropic NH+4 to pure KI. Their contributions result either from reorientations of the molecules between equivalent directions and rotational‐like excitations around these preferential orientations, or from translational modes of the whole Brillouin zone. In this peculiar case, orientational disorder of the ammonium lifts wave vector conservation of the scattering processes. An interpretation of the different Raman allowed transitions is proposed. The composition and polarization dependences as well as the effect of deuteration for these modes are discussed with respect to earlier reports.

1.
N. G. Parsonage and L. A. K. Staveley, Disorder in Crystals (Oxford University, Oxford, 1978).
2.
C.
Bostoen
,
G.
Coddens
, and
W.
Wegener
,
J. Chem. Phys.
91
,
6337
(
1989
). Claude Bostoen, Ph.D., University of Antwerpen, Belgium, 1991 (unpublished).
3.
J.-F.
Berret
,
F.
Bruchhäuser
,
R.
Feile
,
C.
Bostoen
, and
S.
Haussühl
,
Solid State Commun.
74
,
1041
(
1990
).
4.
I.
Fehst
,
R.
Böhmer
,
W.
Ott
,
A.
Loidl
,
S.
Haussühl
, and
C.
Bostoen
,
Phys. Rev. Lett.
64
,
3139
(
1990
).
5.
W.
Joosen
,
H.
Fleurent
,
C.
Bostoen
,
D.
Schoemaker
, and
S.
Haussühl
,
Phys. Rev. B
43
,
11542
(
1990
).
6.
J.-F.
Berret
,
C.
Bostoen
,
J.-L.
Sauvajol
,
B.
Hennion
, and
S.
Haussühl
,
Europhys. Lett.
16
,
91
(
1991
).
7.
I. Fehst, S. L. Hutton, R. Böhmer, A. Loidl, and S. Haussühl, Proceedings of the 7th European Meeting on Ferroelectricity, Dijon 1991.
8.
Y.
Ozaki
,
K.
Maki
,
K.
Okada
, and
J. A.
Morrison
,
J. Phys. Soc. Jpn.
54
,
2595
(
1985
);
Y.
Ozaki
,
J. Phys. Soc. Jpn.
56
,
1017
(
1987
).,
J. Phys. Soc. Jpn.
9.
A. B.
Gardner
,
T. C.
Waddington
, and
J.
Tomkinson
,
J. Chem. Soc. Faraday Trans. 2
73
,
1191
(
1977
).
10.
A.
Heidemann
,
J.
Howard
,
K. J.
Lushingston
,
J. A.
Morrison
,
W.
Press
, and
J.
Tomkinson
,
J. Phys. Soc. Jpn.
52
,
2401
(
1983
).
11.
W.
Dultz
and
H.
Ihlefeld
,
J. Chem. Phys.
58
,
3365
(
1973
).
12.
M.
Couzi
,
J. B.
Sokoloff
, and
C. H.
Perry
,
J. Chem. Phys.
58
,
2965
(
1973
).
13.
H. D.
Hochheimer
,
E.
Spanner
, and
D.
Strauch
,
J. Chem. Phys.
64
,
1583
(
1976
).
H. D.
Hochheimer
,
M. L.
Shand
,
C. T.
Walker
, and
A.
Huller
,
J. Chem. Phys.
71
,
5008
(
1979
).,
J. Chem. Phys.
14.
T.
Schräder
,
A.
Loidl
, and
T.
Vogt
,
Phys. Rev. B
39
,
6186
(
1989
).
15.
J.-F.
Berret
,
J.-L.
Sauvajol
, and
G.
Cohen-Solal
,
Europhys. Lett.
13
,
273
(
1990
), and references therein.
16.
J.-F. Berret, C. Bostoen, J.-L. Sauvajol, and B. Hennion (to be published, 1992).
17.
L.
Genzel
and
W.
Bauhofer
,
Z. Phys. B
28
,
13
(
1976
).
18.
J. H.
Fertel
and
C. H.
Perry
,
Phys. Rev.
184
,
874
(
1969
).
19.
G.
Dolling
,
R. A.
Cowley
,
C.
Schittenhelm
, and
I. M.
Thorson
,
Phys. Rev.
147
,
577
(
1966
).
20.
N.
Vagelatos
,
J. M.
Rowe
, and
J. J.
Rush
,
Phys. Rev. B
12
,
4522
(
1975
).
21.
J.
Tomkinson
,
B. A.
Dasannacharya
,
P. S.
Goyal
, and
R.
Chakravarthy
,
J. Chem. Soc. Faraday Trans.
87
, (
1991
).
22.
A. R.
Sharp
and
M. M.
Pintar
,
J. Chem. Phys.
53
,
2428
(
1970
).
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