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Abstract
Uniaxial pressure effect on structural phase transition at Ta = 222 K has been studied. The transition temperature Ta (σ) increases with increasing pressure σ perpendicular to the [001] axis. On the other hand, pressure along the [001] axis does not affect Ta . Based on experimental results, coupling coefficients between soft-mode amplitude and elastic stress have been evaluated. In the cases of pressure along [100] and [110] axes, divergent anomaly in ϵ c ′ at Ta (σ) has been observed. This can be interpreted as improper ferroelectricity due to a bilinear coupling between the non-polar soft-mode and the electric polarization under the pressure.