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Submitted on 1 Jan 1981
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HYPER-RAMAN SCATTERING STUDY OF
FERROELECTRIC PHASE TRANSITIONS IN SrTiO3
AND BaTiO3
K. Inoue, N. Asai
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C6, supptgrnent au n o 12, Tome 42, de'cembre 1981 page C6-430
HYPER-RAMAN SCATTERING STUDY OF FERROELECTRIC PHASE TRANSITIONS IN SrTi03 AND BaTi03
K . Inoue and N. A s a i
Department o f Physics, PacuZty of S c i e n c e , Shizuoka U n i v e r s i t y , 836 Ohya, Shizuoka 422, Japan.
Abstract.- We have directly observed the "ferroelectric" EU and
A2u soft modes in SrTiO3 separately in a temperature range from
50 K to
4.5
K by the hyper-Raman scattering method. Variation of the respective mode frequencies with temperature is measured and is compared to that of the dielectric constant. Further, it is found that the intensity of hyper-Rayleigh scattering lightdrastically increases as the temperature is lowered till
4.5
K.As for BaTiO
,
the hyper-Raman spectra in the cubic phasehave been measure2 with special emphasis on the spectral profile of the seemingly overdamped mode. The present result, however, seems to contradict the existence of the soft mode.
1. Introduction.- All phonon modes classified into 3FlU+ F2u in the 1
cubic phase (Oh) of SrTi03 and BaTiO are not Raman-active but hyper- 3
Raman-active at the Brillouin zone center. In the tetragonal phase 4or
X 15
KO-
Y . Temperature I K )
-
5
a - Fig. 2. A plot of the observedZ
W .
r
--
mode frequencyLdsversus temperature40 - in SrTiO3. The inset shows a com-
parison of& with 5 - I ( solid
lines ) in ref.
4.
Frecpercy Sh~ft I cm-'1
Fig. 1. Examples of Stokes hyper- Raman spectra in SrTiO
3'
(Dqh) of SrTiO below 105 K, the EU and soft modes stemmed from 3
the lowest F -TO mode are still hyper-Raman-active. Consequently,the lu
hyper-Raman scattering ( abbreviated as HRS ) should provide a useful
tool for studying the "ferroelectric" soft mode, if any.
2. Experiment.- A standard Q-switched N ~ ~ ~ Y A G laser was employed at a
repetition rate of 400 pps to excite the HRS at a level of peak power less than 5 kW. The sample of SrTi03 was prepared in such a form as
allowing to observe the phonons having the wavevector q parallel to
one of the crystallographic axes even in the multi-domain state below
105 K. The form of the BaTiO sample is such that the phonons of ql
//
3
<loo> or
<
110) can be measured in the cubic phase above 1 2 8 O ~ . The HR spectra of both cases were measured in the right-angle geometry by using a home-made detection system with a gated photon-counter. 3. Results and Discussion.- a) SrTi03- Examples of the unpolarized Stokes HR spectra below 100 cm-I are shown in Fig. 1,where the hori- zontal scale is plotted as the shift relative to the harmonic frequen-1
cy of the laser 2 ~ 3 , . It is found that around 50 K the soft mode starts
to split into two spectral lines,the separation of which becomes
larger as the temperature T is lowered. The higher- and lower frequen-
cies can be assigned to those of the A2U and EU modes respectively, on the basis of a comparison with HR spectra2 observed in the forward direction. This assignment agrees with an anticipation by pressure-
induced Raman scattering? The result on the mode frequency d, versus T
Temperature I K )
Fig.3. A plot of the intensity
of the hyper-Rayleigh scatter-
ing light, 12~,versus T in
SrTi03.
Fig. 4. Examples of hyper-Raman
spectra for g#( 100, in BaTiO
.
The inset shows that an overdimped
Lorentzian shape ( solid line ) can
C6-432 JOURNAL DE PHYSIQUE
Table I Comparison of d,and A 3 in SrTiO
3 Table I1 Mode frequency in BaTi03 ( 16o0c )
present work anticipation in ref. 1
( 5 . 7 K ) ref.3 ( 2 K ) ( 8 . 0 K ) mode U, (em-')
'Cjr
cm-')*q em-') Os ( em-') ~d ( cm-'1 TO-1--
LC-1M-2
]
1856
'
Eu 10.3 1.3 9.1
<
0.5 LC-2 462A2U 19.3 0.8 19.1 (not split) To-3 490
is shown in Fig. 2. The spectral line-width Ah3 of both modes is found
to become narrower as T is lowered. Values at 5.7 K and 200 K are
shown as an example in Table I. A comparison of the result dS versus T
with the existing data4 of the dielectric constants and f + ~ versus T
is made through the LST relation (Fig. 2). A considerable discrepancy
between both curves is recognized at low temperatures. However, the
-
present values at 5.7 K are very consistent with the deduced ones? Next,it is found that the relative intensity 12&,of the hyper-
a
Rayleigh scattering light rapidly incre.ses as T is lowered ( Fig. 3).
The inverse plot of 12devs T resembles very much that of & recently
reported? indicating another manifestation of the quantum effect.
b) BaTiO - Examples of the HR spectra for q(.Vf.lOO>.in the cubic phase
3
are shown only in a range below 100 cm-'in Fig.
4.
Those are measuredwith all experimental conditions unchanged except the sample tempera-
ture. The phonon frequencies observed are listed in Table 11. It has
turned out that the spectrum does not exhibit any remarkable variation with T including the integrated intensity. Similar spectra are also observed for qj//<llO>phonon. From this evidence it follows that the of the soft optic mode related to the cubic-tetragonal phase transition might be negative in the present work. More detailed study on this problem is in progress.
The authors acknowledge the partial support by the Grant-in-Aid
for Scientific Research from the Ministry of Education ( Japan ) .
References
1. P. A. Flewry and J. M. Worlock, Phys. Rev.
~,
613 (1968) ; Y. Yamada and G. Shirane, J. Phys. Soc. Jpn.26,
396 (1969).2. K. Inoue, N. Asai and T. Sameshim, J. Phys. Soc. Jpn.
9,
1291 (1981). 3. H. Uwe and T. Sakudo, Phys. Rev. ,&.I 271 (1976).4.
