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It is hypothesized that the transformation of NaNH 4 tartrate from the prototypic phase to the ferroelectric and ferroelastic phase is caused by instability of a pair of conjugate lattice vibration modes with the wave normal in the orthorhombic b direction and with the wave length equal to 3 b . Then the strange dielectric anomaly of NaNH 4 tartrate which has been observed by Jona et al. Takagi et al. is deduced naturally. Also, peculiar elastic and piezoelectric anomalies to be observed are deduced.
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Kêitsirô Aizu (1971) studied this question.
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