Ferroelastic phase transition and elastic properties of orthorhombic betaine hydrogen maleinate, (CH 3) 3NCH 2COO·(CH) 2(COOH) 2

Abstract

Crystals of the title compound undergo a ferroelastic transition at about 194 K. The transition has been detected by the measurement of temperature derivatives of elastic constants employing ultrasonic methods. Two elastic shear stiffnesses, c 44 and c 66, decrease strongly approaching the transition temperature from above. c 66 exhibits a sharp minimum at the transition, whereas c 44 reaches still smaller values down to 0.012 × 10 10Nm -2. Below 194 K elastic waves connected with c 44 are strongly attenuated indicating the formation of ferroelastic domains with monoclinic symmetry. The temperature dependence of c 44 is approximated by c 44=a· log( T T 0 ) with a = 0.446 × 10 10Nm −2 and T 0 = 184 K. The constants c 44 and c 66 even possess negative pressure derivatives. At 293 K the transition is induced by a pressure of 3495 bar. The transition is of strongly second order, as no spontaneous variation of the lattice constants is observed.