Design of ternary alkaline-earth metal Sn(ii) oxides with potential good p-type conductivity

Abstract

Oxides with good p-type conductivity have been long sought after to achieve high performance all-oxide optoelectronic devices. Divalent Sn(II) based oxides are promising candidates because of their rather dispersive upper valence bands caused by the Sn-5s/O-2p anti-bonding hybridization. There are so far few known Sn(II) oxides being p-type conductive suitable for device applications. Here, we present via first-principles global optimization structure searches a material design study for a hitherto unexplored Sn(II)-based system, ternary alkaline-earth metal Sn(II) oxides in the stoichiometry of MSn₂O₃ (M = Mg, Ca, Sr, Ba). We identify two stable compounds of SrSn₂O₃ and BaSn₂O₃, which can be stabilized by Sn-rich conditions in phase stability diagrams. Their structures follow the Zintl behaviour and consist of basic structural motifs of SnO₃ tetrahedra. Unexpectedly they show distinct electronic properties with band gaps ranging from 1.90 (BaSn₂O₃) to 3.15 (SrSn₂O₃) eV, and hole effective masses ranging from 0.87 (BaSn₂O₃) to above 6.0 (SrSn₂O₃) m₀. Further exploration of metastable phases indicates a wide tunability of electronic properties controlled by the details of the bonding between the basic structural motifs. This suggests further exploration of alkaline-earth metal Sn(II) oxides for potential applications requiring good p-type conductivity such as transparent conductors and photovoltaic absorbers.