Anomalous Phase Transition in Strained SrTiO₃ Thin Films

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We have studied the cubic to tetragonal phase transition in epitaxial SrTiO₃ films under various biaxial strain conditions using synchrotron X-ray diffraction. Measuring the superlattice peak associated with TiO₆ octahedra rotation in the low temperature tetragonal phase indicates the presence of a phase transition whose critical temperature is a strong function of strain, with TC as much as 50K above the corresponding bulk temperature. Surprisingly, the lattice constants evolve smoothly through the transition with no indication of a phase change. This signals an important change in the nature of the phase transition due to the epitaxy strain and substrate clamping effect. The internal degrees of freedom (TiO₆ rotations) have become uncoupled from the overall lattice shape.