Crystal Structure of La6Mg4Ta2W2O24 Oxide:  A Representative of a Novel A3 n B‘2 n B‘ ‘2 n O12 n Homologous Series with n = 2

• Published in: Chemistry of materials 2006-08, Vol.18 (16), p.3843-3849
• Main Authors: Khalyavin, Dmitry D, Lopes, Augusto B, Senos, Ana M. R, Mantas, Pedro Q
• Format: Article
• Language: English
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm060916q

The crystal structure of a new A-site deficient La6Mg4Ta2W2O24 perovskite oxide has been solved in the space group I2/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], [Mg1/2(Ta/W)1/2O2], and [La1/2O] 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 Mg2+ and Ta5+/W6+, results in a 2a p × 8a p × 2a p type superstructure. An antiphase octahedral tilting around the b axis reduces the symmetry from orthorhombic Ibam to monoclinic I2/a. The number of equivalent [La1/2O] layers, n, contained in a unit cell of the parent untilted crystal structure allows a new homologous series, A3 n B‘2 n B‘ ‘2 n O12 n , with the common Ibam space group and 2a p × 4na p × 2a p type superstructure to be identified. The La6Mg4Ta2W2O24 oxide is a representative with n = 2.