Pressure-induced Pb-Pb bonding and phase transition in Pb₂SnO₄

High-pressure single-crystal to 20 GPa and powder diffraction measurements to 50 GPa, show that the structure of Pb₂SnO₄ strongly distorts on compression with an elongation of one axis. A structural phase transition occurs between 10 GPa and 12 GPa, with a change of space group from Pbam to Pnam. The resistivity decreases by more than six orders of magnitude when pressure is increased from ambient conditions to 50 GPa. This insulator-to-semiconductor transition is accompanied by a reversible appearance change from transparent to opaque. Density functional theory-based calculations show that at ambient conditions the channels in the structure host the stereochemically-active Pb 6s² lone electron pairs. On compression the lone electron pairs form bonds between Pb²⁺ ions. Also provided is an assignment of irreducible representations to the experimentally observed Raman bands.