Phenomenological theory of phase transitions in epitaxial BaxSr(1-x)TiO3 thin films

TL;DR

This paper develops a thermodynamic phenomenological model for BaxSr(1-x)TiO3 thin films grown on cubic substrates, predicting complex phase behavior and new phases induced by epitaxial strain and composition.

Contribution

It introduces a modified Landau-Devonshire model for BST-x thin films, accounting for high-temperature behavior and epitaxial strain effects, with detailed phase diagrams and new phase predictions.

Findings

Multiple phases converge at low Ba content near 47 K.

Epitaxial strain significantly alters phase stability and transitions.

New phases appear in thin films not present in bulk materials.

Abstract

A phenomenological thermodynamic theory of BaxSr(1-x)TiO3 (BST-x) thin films epitaxially grown on cubic substrates is developed using the Landau-Devonshire approach. The eighth-order thermodynamic potential for BT single crystal and modified fourth-order potential for ST 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 = 47 K, x = 0.028) 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.