Pressure-Temperature Phase Diagrams of Ferroelectric {N(CH₃)₄}₂CoCl₄ and {N(CH₃)₄}₂ZnCl₄

Abstract

Pressure-temperature phase diagrams of {N(CH₃)₄}₂CoCl₄ and {N(CH₃)₄}₂ZnCl₄ were studied by means of dielectric measurements in the temperature and pressure region of −180∼50°C and 0∼2.0 kbar. Phases of Co- and Zn-salts are classified in a single pressure-temperature phase diagram; phases I, II, III, IV, V, VI and VII in Co-salt coincide with phases I, II, III, VII (pressure-induced), IV, V and VI in Zn-salt, respectively, by considering that the applied pressure p=0 bar (=1 bar) onto Co-salt corresponds to the p=550 bar to Zn-salt. The pressure coefficients of IV–V and V–VI phase transition temperatures in Zn-salt have extremely large values of dT_IV–V⁄dp=50±3°C/kbar and dT_V–VI⁄dp=−54±3°C/kbar, respectively.