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

Pressure-temperature phase diagrams of {N(CH 3 ) 4 } 2 CoCl 4 and {N(CH 3 ) 4 } 2 ZnCl 4 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 d T i v - v /d p =50±3°C/kbar and d T v - v i /d p =-54±3°C/kbar, respectively.