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

• The Born effective charges and structure distortions of MgO, CaO, SrO and BaO under strains are calculated.
• The dependence of the spontaneous polarizations of MgO, CaO, SrO and BaO on strains is illustrated.
• The trends in band gap value changes of MgO, CaO, SrO and BaO are predicted.

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 ∼100 μC/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.

Calculated band gap as a function of ɛs. The equilibrium band gap is indicated by the arrow.Figure optionsDownload as PowerPoint slide