resumo
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) angstrom, b = 31.9057(10) angstrom, c = 7.8762(10) angstrom, and beta = 90.08(1)degrees) 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 [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) x 8a(p) x 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, A(3n)B'B-2n '' O-2n(12n), with the common Ibam space group and 2a(p) x 4na(p) x 2a(p) type superstructure to be identified. The La6Mg4Ta2W2O24 oxide is a representative with n = 2.
palavras-chave
DIELECTRIC-PROPERTIES; LANTHANUM TITANATE; PHASE-TRANSITION; CONDUCTIVITY; SYSTEM; DIFFRACTION
categoria
Chemistry; Materials Science
autores
Khalyavin, DD; Lopes, AB; Senos, AMR; Mantas, PQ
nossos autores
Ana Maria de Oliveira e Rocha Senos
Colaborador