STRUCTURAL POLYMORPHISM IN MNGA2SE4

The polymorphic behaviour of the adamantane-type compound MnGa2Se4 is reinvestigated by X-ray powder diffraction and electron microscopy tec hniques carried out on annealed and quenched samples. The low-temperat ure modification (gamma-phase) is confirmed to crystallize in the defe ct tetragonal thiogallate structure (space group <I(4)over bar>) with lattice parameters a = 5.677(1) Angstrom and c = 10.761(2) Angstrom. S amples quenched from high temperature (750 degrees C) show the coexist ence of two ternary phases both having the MnGa2Se4 composition but di fferent crystal structures: the main phase is still tetragonal with sl ightly different lattice parameters (gamma'-phase); the secondary phas e exhibits an orthorhombic cell with approximate dimensions a = 13.5 A ngstrom, b = 8.0 Angstrom, c = 6.5 Angstrom and possible space group P na2(1), (epsilon-phase). This latter polymorph is identified as the be ta-ZnAl2S4-type form of MnGa2Se4. TEM analyses performed on the epsilo n-phase show the presence of ferroelastic twins, confirmed by electron diffraction. The connections among the three structures are shortly-d iscussed.