Ferroelastic phase transition in BiVO4: III. Thermodynamics

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Abstract
For pt.II see ibid., vol.16, p.2455 (1983). The temperature and pressure dependences of the elastic and strain-related properties of the 'pseudo-proper' ferroelectric material BiVO4 have been determined using mean-field theory. In the tetragonal paraelastic phase the elastic constants c11-c12, c66 and c16 are all predicted to behave anomalously near Tc although none are expected to vanish. From analysis of the acoustic properties of BiVO4 zero-velocity pure transverse acoustic modes are expected to propagate along the ferroelastic domain wall directions at Tc. Predicted relationships between pressure and temperature-dependent properties agree well with experimental observations. The theory also accurately predicts the ratio of secondary and primary spontaneous strains in the ferroelastic phase.