High-pressure phase transformations in MnAl2S4 and MnGa2S4

Phase transformations in MnAl2S4 and MnGa2S4 have been investigated at high pressure up to 95 kb and temperature up to 1000°C. The decomposition of MnAl2S4 to MnS with rock salt structure and the spinel phase at moderately high pressure has been observed. The lattice parameter for the spinel phase is shown to decrease with increasing pressure. It is interpreted due to the formation of nonstoichiometric spinel expressed by Mn1−xAl2S4−x. Above about 90 kb, an Al-rich unknown phase appears instead of spinel, which can not be identified on account of its extreme unstableness in air. Six polymorphs have been observed for MnGa2S4, designated as I ∼ VI in the order of increasing pressure. MnGa2S4 IV is identified to be identical with an atmospheric pressure phase of MnAl2S4 with the ZnIn2S4 structure. The crystal structures of MnGa2S4 I, II and III are thought to be due to the polytypism in the ZnIn2S4 structure. The identification of the other two polymorphs can not be made.