Structural and electronic properties ofSrZrO3and Sr(Ti,Zr)O3alloys

Abstract

Using hybrid density functional calculations, we study the electronic and structural properties of ${\mathrm{SrZrO}}_{3}$ and ordered $\mathrm{Sr}(\mathrm{Ti},\mathrm{Zr}){\mathrm{O}}_{3}$ alloys. Calculations were performed for the ground-state orthorhombic $(Pnma)$ and high-temperature cubic $(Pm3m)$ phases of ${\mathrm{SrZrO}}_{3}$. The variation of the lattice parameters and band gaps with Ti addition was studied using ordered ${\mathrm{SrTi}}_{x}\phantom{\rule{0.16em}{0ex}}{\mathrm{Zr}}_{1\ensuremath{-}x}\phantom{\rule{0.16em}{0ex}}{\mathrm{O}}_{3}$ structures with $x=0$, 0.25, 0.5, 0.75, and 1. As Ti is added to ${\mathrm{SrZrO}}_{3}$, the lattice parameter is reduced and closely follows Vegard's law. On the other hand, the band gap shows a large bowing and is highly sensitive to the Ti distribution. For $x=0.5$, we find that arranging the Ti and Zr atoms into a $1\ifmmode\times\else\texttimes\fi{}1\phantom{\rule{4.pt}{0ex}}{\mathrm{SrZrO}}_{3}/{\mathrm{SrTiO}}_{3}$ superlattice along the [001] direction leads to interesting properties, including a highly dispersive single band at the conduction-band minimum (CBM), which is absent in both parent compounds, and a band gap close to that of pure ${\mathrm{SrTiO}}_{3}$. These features are explained by the splitting of the lowest three conduction-band states due to the reduced symmetry of the superlattice, lowering the band originating from the in-plane Ti $3{d}_{xy}$ orbitals. The lifting of the ${t}_{2g}$ orbital degeneracy around the CBM suppresses scattering due to electron-phonon interactions. Our results demonstrate how short-period ${\mathrm{SrZrO}}_{3}/{\mathrm{SrTiO}}_{3}$ superlattices could be exploited to engineer the band structure and improve carrier mobility compared to bulk ${\mathrm{SrTiO}}_{3}$.