Multicritical point in structurally incommensurate

Abstract

The x-ray-diffraction experiment of [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{CuCl}}_{4}$ under hydrostatic pressure has revealed the existence of a special confluent point, denoted by L* in the pressure-temperature (p-T) phase diagram, among the prototypical normal phase, the incommensurate phase, and the commensurate phase characterized by the wave vector q=c*/3. With increasing pressure, the incommensurate phase with qc*/3 decreases its own stable temperature range and vanishes at L*. The incommensurate region with q\ensuremath{\gtrsim}c*/3 appears at L* and widens with increasing pressure. On the p-T phase diagram the second-order normal-incommensurate and the first-order incommensurate-commensurate phase lines meet at L*. The wave vector in both incommensurate regions goes continuously to c*/3 and the jump of the satellite intensity on the first-order incommensurate-to-commensurate phase transition becomes small, as the L* point is approached. These experimental facts show that the L* point is a multicritical point in incommensurate systems. These features of the obtained phase diagram are different from those of the universal one for [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$M${\mathrm{Cl}}_{4}$ (M=Mn, Fe, and Zn) reported previously. \textcopyright{} 1996 The American Physical Society.