Ca2FeOsO6/Sr2FeOsO6 superlattice: Multiferroicity above room temperature with giant hybrid-improper ferroelectric polarization and high photovoltaic efficiency

Abstract

Using first-principles calculations, the structure and electronic properties of the room-temperature ferrimagnetic ${\mathrm{Ca}}_{2}{\mathrm{FeOsO}}_{6}/{\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ superlattice are investigated. We show that the superlattice hosts hybrid-improper ferroelectricity despite the fact that bulk ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ realizes an ${a}^{0}{a}^{0}{c}^{\ensuremath{-}}$ tilting pattern of the O octahedra. The magnitude is comparable to that of conventional ferroelectric materials and is found to increase under both compressive and tensile strain. In contrast to competing superlattices, a ferrimagnetic critical temperature above room temperature is realized. An indirect-to-direct band-gap transition is observed between $+$1% and $+$2% strain, coming along with localization of the valence and conduction states on different transition-metal sublattices, which enables efficient electron-hole separation upon photoexcitation. The potential gradient due to the ferroelectric polarization supports the electron-hole separation and a spectroscopic limited maximum efficiency of 27% confirms excellent potential in solar cell applications. The tunable room-temperature ferroelectricity, high critical temperature of the ferrimagnetic ordering with high magnetization, and favorable photoabsorption properties of the ${\mathrm{Ca}}_{2}{\mathrm{FeOsO}}_{6}/{\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ superlattice open up a broad range of technological applications.