| Abstract: | The polymorphic behaviour of the adamantane-type compound MnGa2Se4 is reinvestigated by X-ray powder diffraction and electron microscopy techniques carried out on annealed and quenched samples. The low-temperature modification (γ-phase) is confirmed to crystallize in the defect tetragonal thiogallate structure (space group I4-) with lattice parameters a = 5.677(1) Å and c = 10.761(2) Å. Samples quenched from high temperature (750 °C) show the coexistence of two ternary phases both having the MnGa2Se4 composition but different crystal structures: the main phase is still tetragonal with slightly different lattice parameters (γ′-phase); the secondary phase exhibits an orthorhombic cell with approximate dimensions a = 13.5 Å, b = 8.0 Å, c = 6.5 Å and possible space group Pna21 (ε-phase). This latter polymorph is identified as the β-ZnAl2S4-type form of MnGa2Se4. TEM analyses performed on the ε-phase show the presence of ferroelastic twins, confirmed by electron diffraction. The connections among the three structures are shortly discussed. |
