Dielectric study of the phase transitions in [P(CH3)4]2CuY4 (Y = Cl, Br)

Phase transitions in [P(CH 3 ) 4 ] 2 Cu Y 4 ( Y = Cl, Br) have been studied by dielectric measurements. In [P(CH 3 ) 4 ] 2 CuCl 4 , a slight break and a discontinuous jump on the dielectric constant vs. temperature curve are seen at the normal-incommensurate and the incommensurate-commensurate phase transitions, respectively. A small peak of dielectric constant along the b -direction exists just above the incommensurate-to-commensurate transition temperature. The anisotropic dielectric anomalies of [P(CH 3 ) 4 ] 2 CuBr 4 at phase transitions were measured along the three crystallographic axes. The pressure-temperature phase diagram of [P(CH 3 ) 4 ] 2 CuCl 4 was determined. The initial pressure coefficients of the normal-to-incommensurate and the incommensurate-to-commensurate transition temperatures are 0.19 K/MPa and 0.27 K/MPa, respectively. The incommensurate phase in [P(CH 3 ) 4 ] 2 CuCl 4 disappears at a triple point which exists at 335 MPa and 443 K. The stability and the pressure effects of the inc...