Theoretical Temperature-Pressure Phase Diagram for {N(CH ₃) ₄} ₂ZnCl ₄

Abstract

A phenomenological approach for construction theoretical phase diagrams with multiple phase transitions including incommensurate transitions is developed for the case when a special triple point, a Lifshitz-type point, is assumed to be present in the experimental diagram for {N(CH 3 ) 4 } 2 ZnCl 4 crystal family. The theoretical temperature-pressure phase diagram for {N(CH 3 ) 4 } 2 ZnCl 4 is plotted and is found to be in agreement with the experimental phase diagram. The approximations and assumptions made in the construction of the diagrams are discussed.

Theoretical Temperature–Pressure Phase Diagram for {N(CH3)4}2CuBr4

“…The theoretical approach to calculation of the T-P phase diagrams for TMATC-M crystals was elaborated in our previous paper. 4) This approach is based on the phenomenological description of a Devil's staircase 5) and on the assumption that the phase diagram contains a special triple point, the Lifshitz-type (LT) point, which was theoretically introduced. 6) The LT-point appeared on the T-P phase diagram due to the assumed existence of two minima of the soft optical branch: one at the center and the other at an arbitrary point of the Brillouin zone.…”

“…4. First we construct the diagram on the plane of dimensionless coefficients D and A of the thermodynamic potentials (see below).…”