Electronic and structural properties of SnO under pressure

Pressure-induced changes in the electronic and structural properties of tin monoxide are examined by means of ab initio density-functional calculations. Also, the pressure shifts of the $A_{1g}$ and $E_g$ zone-center phonon modes are derived. The results are compared to recent experimental high-pressure data as well as to previous calculations for ambient conditions. In agreement with earlier findings we observe that the Sn-$5s$ lone pair is not inert but hybridizes with the O-$p$ states. Differences in that respect between SnO and PbO are shown to be a “relativistic dehybridization effect” caused by the large mass-velocity downshift of the Pb-$6s$ states. SnO is a small-gap (indirect) semiconductor at ambient pressure, but an insulator-metal transition occurs as pressure is applied. The transition is estimated to occur around 5 GPa. The gap depends sensitively on the distance between the layers $d{E}_{\text{gap}}∕d\ln (c∕a)\approx 21\mathrm{eV}$.