IMF2017

All the known ferroelectric phase transitions in perovskite crystals are of the first order. However, in thin perovskite films on substrates the same phase transitions look continuous and somewhat smeared. There is no consensus about the reason of this difference. Most of the researchers seem to agree with conclusion of Pertsev et al. (1998) that partial clamping by the substrate converts the first order transition in BaTiO₃ and PbTiO₃ crystals into a second order one. Alternatively, Roytburd ( 2014) argues that constrains on first order transitions provoke two-phase states, not homogeneous second order transition. However, his arguments are relevant only for thick films on substrates while thin films are of the main current interest and the focus of our attention. We demonstrate existence of another mechanism of inhomogeneity: loss of elastic stability due to anomalies of elastic moduli at what would be second order transitions in films on substrates if the assumption about homogeneity were correct. The stability loss occurs independently of the film thickness. Also, we numerically check for inhomogeneity near phase transitions in thin films of various thicknesses accounting for elasticity boundary conditions. Inhomogeneous structures similar to smeared two-phase states are observed in a broad interval of film thicknesses.