Successive Phase Transitions in {N(CH3)4}2CuCl4

Successive phase transitions in {N(CH 3 ) 4 } 2 CUCl 4 single crystal were studied by the measurements of the angle between two extinction positions, thermal expansions, dielectric constants and ultrasound velocities, and also by the weissenberg photographs. It has turned out that {N(CH 3 ) 4 } 2 CUCl 4 undergoes the phase transitions successively from the prototypic phase (Pmcn), to the incommensurate phase, to the ferroelastic commensurate monoclinic phase (P12 1 /c1) and then to another monoclinic phase (P112 1 /n) at T 1 =24°C, T 2 =18°C (on heating) and T 3 = -10°C, respectively. The phase transition sequence of {N(CH 3 ) 4 } 2 CuCl 4 , including the incommensurate-commensurate (ferroelastic) transition, is discussed in terms of the Landau-type free energy.