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Abstract
The orthorhombic-monoclinic phase transition at Tc=287K in [N(CH3)4]2 ZnBr4 crystals has been studied with an X-ray diffractometer and an optical polarizing microscope. Temperature dependence of the spontaneous shear strain x5s=(π/180)(β-90°) has been measured in the wide range of temperature from Tc down to 44K, and found to take positive values just below Tc, and after taking a maximum value gradually decrease to zero at Tz=123K, and take negative values below Tz. The rotation angle Φ(T) of optical indicatrix around the b-axis rapidly increases just below Tc, and shows a change of slope around 230K. The rotation angle does not vanish, but takes a finite value of about 33° at the compensation temperature Tz. It is shown that the results can be explained on the basis of a ferridistortive model in which spontaneous strain consists of two kinds of sublattice strains with opposite signs and different magnitudes.