The x-ray-diffraction experiment of [N(${\mathrm{CH}}_3$${)}_4$${]}_2$${\mathrm{CuCl}}_4$ under hydrostatic pressure has revealed the existence of a special confluent point, denoted by L* in the pressure-temperature (p-T) phase diagram, among the prototypical normal phase, the incommensurate phase, and the commensurate phase characterized by the wave vector q=c*/3. With increasing pressure, the incommensurate phase with q<c*/3 decreases its own stable temperature range and vanishes at L*. The incommensurate region with q≳c*/3 appears at L* and widens with increasing pressure. On the p-T phase diagram the second-order normal-incommensurate and the first-order incommensurate-commensurate phase lines meet at L*. The wave vector in both incommensurate regions goes continuously to c*/3 and the jump of the satellite intensity on the first-order incommensurate-to-commensurate phase transition becomes small, as the L* point is approached. These experimental facts show that the L* point is a multicritical point in incommensurate systems. These features of the obtained phase diagram are different from those of the universal one for [N(${\mathrm{CH}}_3$${)}_4$${]}_2$M${\mathrm{Cl}}_4$ (M=Mn, Fe, and Zn) reported previously. © 1996 The American Physical Society.

• Received 15 April 1996

DOI:https://doi.org/10.1103/PhysRevB.54.6915