Ferroelastic phase transition and anomalous elastic and thermal properties of betaine borate ( CH3) 3NCH2COO· H3BO3

Abstract

The temperature and pressure derivatives of the elastic constants of orthorhombic betaine borate, ( CH 3) 3 NCH 2 COO· H 3 BO 3, have been determined by measuring temperature and stress induced shifts of resonance frequencies of thick plates at ca. 15 MHz in the range between 140 and 300 K and 0 and 3 kbar. The elastic ‘shear’ resistance c 44 exhibits a value as low as 0.0492×10 10 Nm -2 at 293 K. With decreasing temperature c 44 approaches zero at ca. 142.5 K, indicating an acoustic soft mode behaviour connected with a ferroelastic phase transition. The softening of c 44 is described in a good approximation by c 44( T) p=0 = a log T/ T 0 with a=0.0663×10 10 N m -2 and T 0 = 139.5 K. Further, c 44 decreases with increasing pressure according to the linear relation c 44( p) T=293 K = 0.0492−0.184×10 -4 p (p in bar, c 44 in 10 10 Nm -2). All other elastic constants show a quite normal temperature and pressure dependence. At 293 K the transition is induced by a pressure of 2.65 kbar. The transition temperature T c depends linearly on pressure according to T c = 142.5+0.0568 p ( p in bar, T c in K). Passing through the transition no discontinuous change of the lattice constants is observed. The three principal coefficients of thermal expansion and the pressure derivatives of the dielectric constants exhibit discontinuities at the transition. The transition is of strongly second order.