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Abstract
We present first evidence for the existence of a ferroelastic phase in [N(CD3)4]2ZnCl4, which is the true lock-in phase in this compound. The incommensurate to commensurate phase transition of [N(CH3)4]2CuCl4. which is of the same type, is also examined. In both cases, ferroelastic domains have been observed under polarizing microscope and by X-ray topography, and the specific heat anomalies at the transitions are presented. The problem of the mechanism for the structural change from incommensurate to commensurate ferroelastic phase is discussed. These results are compared to those obtained by X-ray topography on the nucleation of the ferroelastic phase in an isostructural compound, [N(CH3)4]2ZnCI4, where the phase transition occurs between two commensurate phases (q = 2/5-ferroelectric to q = 1/ 3-ferroelastic).