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 α-form were observed. The phase transitions from γ-BaGa₂O₄ (a _γ = 18.6143(2) Å = 23a _α, c _γ = 8.6544(1) Å = c _α, S.G. P6₃) to β-BaGa₂O₄ (a _β = 2a _α, c _β = c _α S.G. P6₃) and to the new δ-polymorph having a monoclinic structure (a _δ = c _α, b _δ = 2a _α + b _α, c _δ = 2b _α and β ≈ 92°, S.G. P2₁/c) were induced by electron beam irradiation. High resolution electron microscopy (HREM) observations allow to establish the close similarity between the structures of δ-BaGa₂O₄ and β-SrGa₂O₄. The γ→β and β→δ transitions involve a rearrangement of oxygen atoms in the BaO layers together with a tilting distortion of the tetrahedral framework. The microstructure of the δ-phase is characterised by the presence of numerous translation and orientation domains.