Elastic Anomaly and Phase Transition of BaTiO3

Abstract

The acoustic-phonon velocities of a single crystal of BaTi03 have been measured as a function of pressure up to 8 GPa, including the transition pressure p c =2.35 GPa, by means of Brillouin scattering technique. Only one elastic eigenmode shows an anomalous decrease on approaching p c and the elastic anisotropy increases. The elastic anomaly of cubic BaTiC>3 is interpreted well by introducing a new tensorial order parameter which describes the anisotropy. The cubic-to-tetragonal phase transition in BaTiC>3 is shown to be triggered by the fluctuation of the anisotropy, not the polarization itself.PACS numbers: 77.80.Bh, 62.50.+p, 62.65.+k, 78.35,+c Studies on the cubic-to-tetragonal phase transition in BaTiC>3 have been extensively done, 1 ' 2 using a variety of experimental methods. The result of an infrared measurement by Luspin, Servoin, and Gervais showed that the frequency co s of the soft phonon mode varies from 1350 to 520 K as G)}azT-T c , but becomes independent of temperature below 520 K to T c , 3 and suggested the existence of a crossover from displacive to order-disorder type of transition. 4 The recent results of hyper-Ramanscattering measurements, which were done independently by Inoue and Akimoto 5 and by Vogt, Sanjurju, and Rossbroich, 6 were in conflict with each other as to the interpretation of the phase transition, i.e., whether the mechanism of the phase transition is of the orderdisorder type or of the displacive type. Thus the controversy on the mechanism of the phase transition is still unresolved.A measurement of inelastic neutron scattering showed the existence of a strong anisotropy in the spectra near the phase transition. 7 The anisotropy seems to be related to the elastic anisotropy. Although the elastic anomaly of BaTiC>3 was observed in the cubic phase near the phase transition, 8 the contribution of the elastic anisotropy to the phase transition was not elucidated since the experiments on the elastic properties were partly performed within the limited region close to the phase transition. Thus no systematic study of the elastic properties of BaTiC>3 has been hitherto performed in relation to the elastic anisotropy. If the anisotropy plays an important role in the phase transition, the studies on the elastic properties will give important information on the mechanism of the phase transition since the anisotropic fluctuation should be of relaxational type.By means of the Brillouin scattering technique in combination with a diamond anvil cell, we have performed the first systematic study of the elastic properties of BaTiOs for a wide region including the phase-transition point, as a function of pressure at room temperature. We have found that the elastic anisotropy increases anomalously on approaching the phase transition. The purpose of this paper is to give new insight into the phase-transition mechanism of BaTiC>3 on the basis of the elastic anomaly accompanying the phase transition.The 6-cut plates were prepared from a melt-grown BaTiOs single crystal (NEC Co. Ltd.) which has ...

Finite strain Landau theory of high pressure phase transformations

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First-principles computation of second-order elastic constants and equations of state for tetragonal BaTiO3

“…These variations in computational and experimental values of elastic constants and bulk moduli [24,26,28,6,21] have been constantly observed in a variety of experimental results. The causes of variation in experimental and computational values of elastic constants and bulk modulus may not be completely known.…”

A bibliographical review of the high pressure studies of ferroelectric phase transitions

“…Brillouin scattering measurements on BaTiO, in the cubic phase have been carried out at high pressure, and the variation of elastic constants C,, and C,, determined as a function of pressure up to 110 kbar (Fischer and Polian, 1987). Brillouin scattering studies under pressure also indicate that the elastic anisotropy increases anomalously on approaching the cubic-tetragonal transition (Ishidate and Sasaki, 1989). In the case of TGSe, the polarization relaxation time as function of pressure has been determined by Brillouin scattering (Yamashita and Tatsuzaki, 1982).…”