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Abstract
The phase transition and the domain structures in ferridistortive crystals of [A(CH3)4]2XBr4 type compounds, where A denotes N or P atom, and X denotes Zn, Co or Mn atom, have been studied with a polarizing microscope. A distinct phase front between the high and the low temperature phases is observed just at the transition temperature Tc in [P(CH3)4],XBr4 crystals, indicating that the phase transition is of the first order. When observed on b-plates in this family, a-domains consisting of narrow strips parallel to the a-axis appear more frequently and seem to be more stable than c-domains consisting of narrow strips parallel to the c-axis in thick plates. The average density of a-walls (parallel to the ab-plane) in [P(CH3)4]2>XBr4 crystals is found to be smaller than that in [N(CH3)4]2XBr4 crystals in thick plates. It is found that the average density of a-walls in [N(CH3)4]2XBr4 crystals is larger as thickness of sample plate is larger.