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It is hypothesized that the transformation of (NH 4 ) 2 Cd 2 (SO 4 ) 3 from the prototypic phase to the ferroelectric and ferroelastic phase is caused by instability of triply degenerate pairs of conjugate lattice vibration modes with the wave normals along the cubic principal axes and with the wave length equal to three times the lattice constant. Then the strange dielectric anomaly of (NH 4 ) 2 Cd 2 (SO 4 ) 3 which has been observed by Ohshima and Nakamura is deduced naturally. Also, peculiar elastic and piezoelectric anomalies to be observed are deduced.
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Kêitsirô Aizu (1972) studied this question.
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