Supertetragonal BaSn O 3 -induced giant ferroelectricity in SrTi O 3 / BaSn O 3 superlattices

The perovskite $\mathrm{BaSn}{\mathrm{O}}₃$ has an Sn $s$-orbital conduction band minimum, which makes it of interest as a transparent-conducting oxide parent compound but also contraindicates the ferroelectric instability characteristic of the related compound $\mathrm{BaTi}{\mathrm{O}}₃$. In this work, we studied the effect of (001) compressive strain on $\mathrm{BaSn}{\mathrm{O}}₃$ using first-principles methods. We found that, with low compressive strain, symmetry breaking takes cubic $\mathrm{BaSn}{\mathrm{O}}₃$ to a nonpolar tetragonal state, with a first-order phase transition to a hidden highly polarized ferroelectric supertetragonal state at \ensuremath{\sim}\ensuremath{-}5%. Based on the facts that the mismatch of the lattice constant in experiment between $\mathrm{BaSn}{\mathrm{O}}₃$ and $\mathrm{SrTi}{\mathrm{O}}₃$ is \ensuremath{\sim}\ensuremath{-}5.2% and coherent growth of $\mathrm{BaSn}{\mathrm{O}}₃$ on $\mathrm{SrTi}{\mathrm{O}}₃$ has been experimentally realized for $\mathrm{BaSn}{\mathrm{O}}₃$ layers thinner than 3 unit cells, we studied a series of $\mathrm{SrTi}{\mathrm{O}}₃/\mathrm{BaSn}{\mathrm{O}}₃$ superlattices with 1 or 2 unit cells of $\mathrm{BaSn}{\mathrm{O}}₃$ and several unit cells of $\mathrm{SrTi}{\mathrm{O}}₃$. We found that the superlattices are ferroelectric with large polarizations. We propose that the origin of ferroelectricity in the superlattices is the mechanical and electrical coupling of the $\mathrm{BaSn}{\mathrm{O}}₃$ and $\mathrm{SrTi}{\mathrm{O}}₃$ layers, with the polarized supertetragonal state of $\mathrm{BaSn}{\mathrm{O}}₃$ induced by compressive strain from the $\mathrm{SrTi}{\mathrm{O}}₃$ layers and polarization of the $\mathrm{SrTi}{\mathrm{O}}₃$ layers by the polar $\mathrm{BaSn}{\mathrm{O}}₃$ layers. Due to the distinctive electronic states in the $\mathrm{BaSn}{\mathrm{O}}₃$ layers, the realization of ferroelectricity holds promise for the design of electronic devices.