Phenomenological theory of phase transitions in epitaxial BaxSr1-xTiO3 thin films

A phenomenological thermodynamic theory of BaxSr1-xTiO3 (BST-x) thin films epitaxially grown on cubic substrates is developed using the Landau-Devonshire approach. The eighth-order thermodynamic potential for BaTiO3 (BT) single crystal and modified fourth-order potential for SrTiO3 single crystal were used as starting potentials for the end members of the solid solution with the aim to develop potential of BST-x solid solution valid at high temperatures. Several coefficients of these potentials for BT were changed to obtain reasonable agreement between theory and experimental phase diagram for BST-x (x>0.2) solid solutions. For low Ba content we constructed the specific phase diagram where five phases converge at the multiphase point (T-N2=48 K, x=0.029) and all transitions are of the second order. The concentration-misfit strain phase diagrams for BST-x thin films at room temperature and temperature-misfit strain phase diagrams for particular concentrations are constructed and discussed. Near T-N2 coupling between polarization and structural order parameter in the epitaxial film is modified considerably and large number of new phases not present in the bulk materials appear on the phase diagram.