Dielectric study of the pressure effect on the structural phase transitions in [P(CH3)4]2CuBr4 crystal

Effect of hydrostatic pressure on the successive phase transitions in tetramethylphosphonium tetrabromocuprate, [P(CH3)(4)](2)CuBr4, was studied by dielectric measurements. The transition temperature (406 K at 0 MPa) from the incommensurate phase (phase II) to commensurate phase (III) linearly increases with increasing pressure with a slope of 0.23 K/MPa. The incommensuate II phase seems to disappear at about 250 MPa. The transition temperature from the commensurate phase (III) to the low-temperature monoclinic phase (IV) (197 K at 0 MPa) increases linearly with the slope of 0.060 K/MPa. Any evidence for the existence of pressure-induced phases was not observed. The results are compared with the pressure-effects on the phase transitions in corresponding tetramethylammonium compound of [N(CH3)(4)](2)CuBr4 and discussed.