Dielectric Study of the Pressure Effect on the Phase Transitions in Deuterated Crystal [N(CD₃)₄]₂CuCl₄

Abstract

Effect of hydrostatic pressure on the phase transitions in deuterated crystal [N(CD₃)₄]₂CuCl₄ have been studied by dielectric measurements. The pressure–temperature ( p–T) phase diagram was obtained and compared with that of the hydrogen compound. By deuteration the boundary region between the atomspheric-pressure incommensurate phase (phase II: q<(1/3)/c₀^*) and the presssure-induced incommensurate phase (phase II': q>(1/3)c₀^*) shifts toward lower pressure side. (q: the modulation wave vector, c₀^*: the reciprocal unit cell vector in the normal phase) Deuteration shifts the pressure-induced ferroelectric phase to lower pressure by about 40 MPa. The result shows that the effect of deuteration on the phase stability in [N(CH₃)₄]₂CuCl₄ is equivalent to application of hydrostatic pressure. This deuteration effect is opposite tendency to that of other [N(CH₃)₄]₂XCl₄-type crystals.