Critical phenomena and phase transition of perovskite - data for NaMgF₃ perovskite. Part II

The crystal structure of NaMgF₃ perovskite is observed to transform directly from orthorhombic (Pbnm) to cubic (Pm3m) at a temperature of T_c = 765°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 θ and φ 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/mmmFmmm 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 λ-anomaly during the structural phase transition in NaMgF₃ perovskite, and are considered to be directly associated with the excess Gibbs free energy.