Reversible phase transitions in Na2S under pressure: a comparison with thecation array in Na2SO4

The structural behavior of the antifluorite Na2S, disodium sulfide, has bee n studied under pressure up to 22 GPa by in situ synchrotron X-ray diffract ion experiments in a diamond anvil cell at room temperature. At approximate ly 7 GPa, Na2S undergoes a first phase transition to the orthorhombic antic otunnite (PbCl2) structure (Pnma, Z = 4). The lattice parameters at 8.2 GPa are a = 6.707 (5), b = 4.120(3), c = 8.025 (4) Angstrom. At approximately 16 GPa, Na2S undergoes a second transition adopting the structure of the Ni 2In-type (P6(3)/mmc, Z = 2), The lattice parameters at 16.6 GPa are a = 4.3 76 (18). c = 5.856 (9) Angstrom. Both pressure-induced phases have been con firmed by full Rietveld refinements. An inspection of the cation array of N a2SO4 reveals that its Na2S subarray is also of the Ni2In-type. This featur e represents a new example of how the cation arrangements in ternary oxides correspond to the topology of the respective binary compounds, We discuss analogies between the insertion of oxygen and the application of pressure.