LOW-TEMPERATURE PHASE-DIAGRAMS - NONLINEARITIES DUE TO QUANTUM-MECHANICAL SATURATION OF ORDER PARAMETERS

At low temperatures, the behaviour of structural phase transitions is modified by the influence of quantum fluctuations. Such fluctuations e nhance the stability of the high-symmetry phase, reducing the observed transition temperature.The effect on phase diagrams of temperature ve rsus some other control parameter (e.g. pressure or chemical compositi on) is described. A simple low-temperature extension of Landau theory is used, where the extent of quantum mechanical effects is characteriz ed by a saturation temperature theta(S). The theory is used to model t he phase diagrams of the mineral anorthoclase (theta(S) = 271 K), the ferroelectric materials KH2PO4 (theta(S) = 49 K), SrTiO3 (theta(S) = 2 0 K for the 20 K transition), KTaO3 (theta(S) = 20 K) and SbSI (theta( S) = 0 K). The saturation temperature theta(S) is related to the exten t to which changes in the hard phonon modes influence the transition m echanism.