Abstract
A brief review is given for the experimental results of hydrostatic pressure effect on the lock-in transitions in various tetramethylammonium tetrahalogenometallic compounds, {N(CH3)4}2XY4 (X: Mn, Fe, Co, Ni, Cu, Zn; Y: Cl, Br). The pressure-temperature phase diagrams of the chloride compounds with X = Mn, Fe, Co, Zn can be arranged in a unified reduced p-T phase diagram. Detailed observations are described as to the stabilization of the ferroelectric phase in the p-T space for the -MnCl4, -FeCl4, and -ZnCl4 compounds. The phase transition in the -ZnBr4. -MnBr4, and -CoBr4 compounds corresponds to the high pressure state in the chloride compounds. The deuteration effect on the phase transitions at high pressure is discussed.

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