Low-temperature phase diagrams: non-linearities 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 enhance the stability of the high-symmetry phase, reducing the observed transition temperature. The effect on phase diagrams of temperature versus some other control parameter (e.g. pressure or chemical composition) is described. A simple low-temperature extension of Landau theory is used, where the extent of quantum mechanical effects is characterized by a saturation temperature theta(S). The theory is used to model the phase diagrams of the mineral anorthoclase (theta(S) = 271 K), the ferroelectric materials KH2PO4 (theta(S) = 49 K), SrTiO3 (theta(S) = 20 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 extent to which changes in the hard phonon modes influence the transition mechanism.