Crystal structure of La4Mg3W3O18layered oxide

The crystal structure of A-site deficient La4Mg3W3O18 perovskite has been solved by x-ray powder diffraction in combination with group theoretical analysis. Above 700 K, the crystal structure is orthorhombic (space group Ibam; 2ap × 4ap × 2ap type superstructure) and presents a sequence of [LaO]–[Mg1/2W1/2O2]–[La1/3O]'–[Mg1/2W1/2O2]–[LaO]– [Mg1/2W1/2O2]–[La1/3O]''–[Mg1/2W1/2O2] layers stacked along the b axis. The lanthanum ions and the vacancies in the [La1/3O]' and [La1/3O]'' layers are ordered and form rows along the c axis. A half-period shift along the a direction between these layers leads to a quadrupling of the primitive perovskite unit cell in the b direction. The ordering of the vacancies in the lanthanum poor layers is connected with the ionic ordering between Mg2+ and W6+ in the neighbouring [Mg1/2W1/2O2] blocks. Around 700 K, due to an anti-phase rotation of the octahedra, a continuous phase transition mediated by the Γ2+ irreducible representation from orthorhombic (Ibam, a0b0c0) to monoclinic (C 2/m, a0b0c−) symmetry takes place.