Lifshitz-point critical and tricritical behavior in anisotropically stressed perovskites

The antiferrodistortive phase transitions of some perovskite crystals, driven first order by strongly cubic order-parameter fluctuations, are shown to become continuous at finite anisotropic stresses favoring ordering of one order-parameter component. The associated critical and tricritical behavior will be that of one of a spectrum of uniaxial Lifshitz systems, its precise character reflecting the form of the anisotropy in the soft-mode dispersion. Our predictions are in substantial accord with recent experiments on RbCaF3.