Ferroelastic thin films under epitaxial strain relax by forming ordered domain patterns. Domains crucially affect the functional responses of the thin film and, thus, understanding the domain formation mechanisms is of much interest. The thermodynamic laws that determine the size of these domains are well established. However, only recently we are able to grow thin films with enough control in order to test in which cases do ferroelastic thin films behave according to thermodynamics. Indeed, the observed domain widths are often not statistically spread around the expected equilibrium value, nor they are typically related to the presence of random defects. These deviations are generally explained by kinetic effects, but determining such kinetic mechanisms is a challenge. We will show the evolution of ferroelastic domain structures in epitaxial BaTiO3 films grown on NdScO3 substrates during the phase transition between two ferroelastic phases. We observe that the equilibrium domain structure is achieved by halving of the domain widths sequentially as the temperature is decreased, as previously predicted [1]. We will show that this type of domain kinetics can also be inferred from other published reports in different materials. This knowledge can impact in the design of domain wall (DW) functionality: since the DW conductivity reflects the thermodynamic equilibrium oxygen vacancy concentration in the films, walls formed at different stages will show different conductivity. Thus the relative magnitude of conductivity at different DWS can be used to “date” DWs.
[1] E.K.H. Salje, Phase Transitions in Ferroelastic and Co-elastic Crystal, Cambridge University Press (1993).