  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.
