Co-Determination of Crystal Structures at High Pressure: Combined Application of Theory and Experiment to the AuGa2 Intermetallic analog to High-Pressure SiO2

The intermetallic compounds AuX₂ (X = In, Ga, Al) crystallize in the calcium fluorite (CaF₂) structure, making them analogs for SiO₂ at TPa pressures. A combination of high-pressure x-ray diffraction experiments and first-principles calculations reveals the sequence of crystal-structural phase transitions in AuGa₂ from cubic (Fm3m) to orthorhombic (Pnma) at 10 (± 4) GPa, and then to monoclinic (P2₁/n) at 33 (± 6) GPa. The post-cotunnite (P112₁/a) phase identified as the final step in the phase-transition sequence for other AX₂ compounds is found to be unstable. As the structural sequence followed by AuGa₂ is similar to that observed in ACl₂ (A = Pb, Sn) compounds it implies that AX₂ compounds iso-structural to PbCl₂ will acquire the monoclinic (P2₁/n) structure at extreme pressures. Neither theory nor experiment would have been adequate, on their own, in documenting this sequence of phases, but together they confirm a sequence differing from the Fm3m → Pnma → P6₃/mmc transitions predicted for CaF₂, and observed for the intermetallic analog compounds AuIn₂ and AuAl₂ under pressure. Experimental [and theoretical] values of zero-pressure volume and bulk modulus are 224.5 (± 0.2) [219.5] ų and 81 (± 5) [95] GPa; 204.0 (± 1) [204.8] ų and 112 (± 12) [96] GPa; and 192 (± 4) [201.1] ų and 157 (± 5) [99] GPa for the cubic, orthorhombic and monoclinic phases, respectively.