H. Braeter et al., A microscopic model for the concentration dependence of the antiferrodistortive phase transition in mixed betaine phosphate/phosphite crystals, J PHYS-COND, 11(39), 1999, pp. 7481-7491
The concentration dependence of the high temperature antiferrodistortive (a
fd) phase transition temperature Te in single crystals of the mixed crystal
system betaine phosphate and betaine phosphite (BP1-xBPIx) is studied. New
experimental data obtained by H-2 nuclear magnetic resonance (NMR) spectro
scopy on deuterated single crystals of this system confirm the nonlinear be
haviour of T-C(x), in particular the maximum at intermediate concentrations
. A microscopic model is proposed which in extension of the treatment of a
small defect concentration makes use of the virtual crystal approximation (
VCA) to be applicable to the mixed crystal system DBP1-xDBPIx with finite c
oncentrations x. By this means the experimentally observed behaviour of the
afd phase transition temperature as a function of x is elucidated and the
nonlinear behaviour of T-C(x) is explained in agreement with the afd charac
ter of the transition.