Effects of epitaxial strains on spontaneous polarizations and band gaps of alkaline-earth-metal oxides MO (M = Mg, Ca, Sr, Ba)

Abstract

The spontaneous polarization and band gap of the alkaline-earth-metal binary oxides MgO, CaO, SrO and BaO under epitaxial strain are investigated using a first principles density functional theory. The main findings are summarized as follows: (1) the polarization can reach up to ∼100μC/cm2 at the compressive strain of ɛs=−8%, which is much larger than those of most typical ferroelectric perovskite oxides. For all the studied four systems, the variation of the polarization displays the extremely similar behavior that the polarization value increases almost linearly under the compressive or tensile strain. (2) For MgO, the optical band gap increases remarkably under the compression, and reaches its maximum at ɛs=−10.74%. Beyond this range, the band gap gradually decreases. For the other three heaviest oxides, the band gaps undergo a decrease for both types of strain. Our calculated results are in good agreement with the available experimental and other theoretical data.