Spontaneous Strain of Mixed Crystal System [N(CH₃)₄]₂ZnBr₄-[P(CH₃)₄]₂ZnBr₄

Abstract

Spontaneous strain accompanied by the ferroelastic phase transition of the mixed crystal system [N 1- x P x (CH 3 ) 4 ] 2 ZnBr 4 ( x =0, 0.05, 0.15, 0.44, 0.75, 0.85, 0.93, 1) was studied by X-ray diffraction. The deviation angle Δβ of the monoclinic angle β from 90° showed characteristic temperature dependence, where the sign of Δβ was chosen so that the configuration of ions in the ferroelastic phase was the same for all compositions. As the value of x increased to 0.15, the maximum value of Δβ increased and the slope in the low-temperature region decreased. At x =0.44, Δβ was almost temperature independent. Absolute values were very small at x =0.75. At x =0.85 it was negative throughout, and above the concentration it approximated the typical temperature dependence of [P(CH 3 ) 4 ] 2 ZnBr 4 . The transition temperature increased with increasing x . The phase transition changed from second order to first order one between x =0.85 and 0.93. Structure analysis revealed that the position of ions changed only slightly in the ferroelastic phase in each compound.