Abstract
The paraelectric-to-ferroelectric phase transformation of (NH4)2Cd2(SO4)3 is conjectured to be caused by an instability of three degenerate pairs of conjugate lattice vibration modes with a wave length such that the condensation of each pair of conjugate waves should triple the primitive unit cell. This was explained in a previous paper with a model in which the spontaneous polarization vector arises parallel to the wave normal of the condensed soft waves. Reality of this model, however, seems difficult because of a certain severe requirement for its ferroelectricity. The present paper proposes a model in which Ps arises perpendicular to the wave normal. This model implies no such severe requirement.
