Crystal Structure of La₆Mg₄Ta₂W₂O₂₄ Oxide: A Representative of a Novel A_3nB‘_2nB‘ ‘_2nO_12n Homologous Series with n = 2

The crystal structure of a new A-site deficient La 6 Mg 4 Ta 2 W 2 O 24 perovskite oxide has been solved in the space group I 2/ a ( a = 7.8711(10) Å, b = 31.9057(10) Å, c = 7.8762(10) Å, and β = 90.08(1)°) from X-ray powder diffraction data. The highest symmetry structural motif which does not involve octahedral tilting is characterized by the Ibam space group and represents a sequence of [LaO], [Mg 1/2 (Ta/W) 1/2 O 2 ], and [La 1/2 O] layers stacked along the b axis. The lanthanum ions and the vacancies in the lanthanum poor layers are ordered and form rows along the c direction. There are two types of these layers which are shifted between each other in a half period along the a axis. The alternation of the lanthanum poor layers with the lanthanum rich ones, along with the rock salt type cation ordering between Mg 2+ and Ta 5+ /W 6+, results in a 2 a p × 8 a p × 2 a p type superstructure. An antiphase octahedral tilting around the b axis reduces the symmetry from orthorhombic Ibam to monoclinic I 2/ a . The number of equivalent [La 1/2 O] layers, n, contained in a unit cell of the parent untilted crystal structure allows a new homologous series, A 3 n B‘ 2 n B‘ ‘ 2 n O 12 n, with the common Ibam space group and 2 a p × 4 na p × 2 a p type superstructure to be identified. The La 6 Mg 4 Ta 2 W 2 O 24 oxide is a representative with n = 2.