CRITICAL PHENOMENA AND PHASE-TRANSITION OF PEROVSKITE DATA FOR NAMGF3 PEROVSKITE .2.

The crystal structure of NaMgF3 perovskite is observed to transform directly from orthorhombic (Pbnm) to cubic (Pm3m) at a temperature of T(c) = 765-degrees-C. Superlattice diffractions associated with in-phase and anti-phase octahedral tilts vanish simultaneously at the transition temperature. The intensities of the superlattice diffractions, the atomic displacements, and the octahedral tilts follow a Landau type of critical behavior as the temperature approaches T(c). The structural phase transition in NaMgF3 perovskite can be modelled as a tricritical ferroelastic phase transition. The octahedral tilts theta and phi represent the primary order parameters of the phase transition, and the coupling between these two order parameters are observed to be bi-quadratic. The spontaneous strain for the m3mFmmm ferroelastic species is derived in terms of lattice parameters and its relation to the ferroelastic species m3mF4/mmm and 4/mmm Fmmm are discussed. It is demonstrated experimentally that the coupling between the spontaneous strain and the octahedral tilts of perovskites is in a linear-quadratic form. Excess physical properties (thermal expansion and heat capacity, etc.) are observed to have a lambda-anomaly during the structural phase transition in NaMgF3 perovskite, and are considered to be directly associated with the excess Gibbs free energy.