Transmission electron microscopy study of polymorphism in barium gallate BaGa2O4

Polymorphism in barium gallate BaGa2O4 was studied using electron diffraction and high resolution electron microscopy. Three modifications of BaGa2O4 With structures closely related to the high-temperature alpha-form were observed. The phase transitions from gamma-BaGa2O4 (a(gamma) = 18.6143(2)Angstrom = 2root3a(alpha), c(gamma) = 8.6544(l)Angstrom = c(alpha), S.G. P6(3)) to beta-BaGa2O4 (a(beta) = 2a(alpha), c(beta) = c(alpha) S.G. P6(3)) and to the new delta-polymorph having a monoclinic structure (a(delta) = c(alpha), b(delta) = 2a(alpha) + b(alpha), c(delta) = 2b(alpha) and beta approximate to 92degrees, S.G. P2(1)/c) were induced by electron beam irradiation. High resolution electron microscopy (HREM) observations allow to establish the close similarity between the structures of delta-BaGa2O4 and beta-SrGa2O4. The gamma --> beta and beta --> delta transitions involve a rearrangement of oxygen atoms in the BaO layers together with a tilting distortion of the tetrahedral framework. The microstructure of the delta-phase is characterised by the presence of numerous translation and orientation domains.