Abstract
Dynamical properties of mechanical twins in (NH 4 ) 2 SO 4 have been studied using a polarizing microscope both in the paraelectric and ferroelectric phases. It has been found that both {011} and {031} twins can be introduced into an originally untwinned crystal, and twin planes once introduced can propagate through the crystal by application of mechanical stress. The threshold stress necessary to move twin planes was found to increase with decreasing temperature, but did not show any abrupt jump at the ferroelectric transition point. It is suggested by considering twinning condition that propagation of twin planes is accompanied with the reorientation of tetrahedral molecules SO 4 - .
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