Anomalous lattice expansion of coherently strained<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>SrTiO</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>thin films grown on<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>Si</mml:mi><mml:mo>(</mml:mo><mml:mn>001</mml:mn><mml:mo>)</mml:mo></mml:mrow></mml:math>by kinetically controlled sequential deposition

X-ray diffraction has been used to study the epitaxy and lattice expansion of thin films grown coherently on by kinetically controlled sequential deposition. The coherent growth of on Si produces films with greater in-plane compressive strain than previously attained, %. The measured expansion of the out-of-plane lattice constant exceeds the prediction of the bulk elastic constants of by nearly 100%. This expansion agrees with recent theoretical predictions and experimental findings of room-temperature ferroelectricity in films under epitaxial mismatch strain. Our first principles density functional calculations determine an energetically favorable interfacial-defect and surface-charge structure that allows the ferroelectric polarization in these ultrathin films.