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

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

Feb 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993pepi...76...17z&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 76, Issue 1-2, p. 17-34.

Physics

29

Scientific paper

The crystal structure of NaMgF3 perovskite is observed to transform directly from orthorhombic (Pbnm) to cubic (Pm3m) at a temperature of Tc = 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 Tc. 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.

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