Phase transitions and strain-induced ferroelectricity inSrTiO3epitaxial thin films

A Landau-Ginsburg-Devonshire-type theory is used to describe the mechanical substrate effect on equilibrium states and phase transitions in ${\mathrm{SrTiO}}_{3}$ epitaxial thin films. The misfit strain-temperature phase diagram of ${\mathrm{SrTiO}}_{3}$ films is developed taking into account the existence of two coupled instabilities (antiferrodistortive and ferroelectric) in this crystal. It is shown that ${\mathrm{SrTiO}}_{3}$ films remain paraelectric down to 0 K only in a narrow range of small negative misfit strains between $\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ and $\ensuremath{-}2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}.$ Outside this ``paraelectric gap,'' the 2D clamping and straining of the film by the substrate leads to the appearance of ferroelectricity in ${\mathrm{SrTiO}}_{3}$ films. The temperature of the ferroelectric transition increases rapidly outside the aforementioned misfit strain range.