Abstract
The polymorphic behaviour of the adamantane-type compound MnGa 2 Se 4 is reinvestigated by X-ray power diffraction and electron microscopy techniques carried out on annealed and quenched samples. The low-temperature modification (γ-phase) is conformed to crystallize in the defect tetragonal thiogallate structure (space group I4) with lattice parameters a = 5.677(1) A and c = 10.762(2) A. Samples quenched from hith temperature (750 °C) show the coexistence of two ternary phases both having the MnGa 2 Se 4 composition but different crystal structures: the main phase is still tetragonal with slightly different lattice parameters (γ'-phase); the secondary phase exhibits an orthorthombic cell with with approximate dimensions a = 13.5 A, b = 8.0 A, c = 6.5 A and possible space group Pna2 1 (e-phase). This latter polymorph is identified as the β-ZnAl 2 S 4 -type form of MnGa 2 Se 4 . TEM analyses performed on the e-phase show the presence of ferroelastic twins, confirmed by electron diffraction. The connections among the three structures are shortly discussed.
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