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

The structural behavior of the antifluorite Na(2)S, disodium sulfide, has been studied under pressure up to 22 GPa by in situ synchrotron X-ray diffraction experiments in a diamond anvil cell at room temperature. At approximately 7 GPa, Na(2)S undergoes a first phase transition to the orthorhombic anticotunnite (PbCl(2)) structure (Pnma, Z = 4). The lattice parameters at 8.2 GPa are a = 6.707 (5), b = 4.120 (3), c = 8.025 (4) A. At approximately 16 GPa, Na(2)S undergoes a second transition adopting the structure of the Ni(2)In-type (P6(3)/mmc, Z = 2). The lattice parameters at 16.6 GPa are a = 4.376 (18), c = 5.856 (9) A. Both pressure-induced phases have been confirmed by full Rietveld refinements. An inspection of the cation array of Na(2)SO(4) reveals that its Na(2)S subarray is also of the Ni(2)In-type. This feature 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.