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

The structural behavior of the antifluorite Na₂S, 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, Na₂S undergoes a first phase transition to the orthorhombic anticotunnite (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) Å. At approximately 16 GPa, Na₂S undergoes a second transition adopting the structure of the Ni₂In-type (P6₃/mmc, Z = 2). The lattice parameters at 16.6 GPa are a = 4.376 (18), c = 5.856 (9) Å. Both pressure-induced phases have been confirmed by full Rietveld refinements. An inspection of the cation array of Na₂SO₄ reveals that its Na₂S subarray is also of the Ni₂In-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.