Ferroelastic phase transition in BiVO₄: III. Thermodynamics

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 T_c 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 T_c. 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.