title: Isolation of two-dimensional 2:1 cation-ordered perovskite units by anion vacancy ordering in Ba6Na2Nb2P2O17.
abstract: A new six-layer perovskite-related structure Ba 6Na 2Nb 2M 2O 17 (M = P, V), which consists of cubic (c) BaO 3 layers and oxygen-deficient pseudocubic (c') BaO 2 layers stacked in the sequence c'ccccc, is presented. In Ba 6Na 2Nb 2M 2O 17, two-dimensional slabs of the well-known 2:1 octahedral cation-ordered perovskite motif are isolated between layers of tetrahedral units formed by anion vacancy ordering: two consecutive NbO 6 octahedral layers are sandwiched by two single NaO 6 octahedral layers, which, in turn, connect with two isolated MO 4 tetrahedral layers. Both oxides are derived from the 2:1 ordered perovskite structure (e.g., Ba 3ZnTa 2O 9) by ordered removal of O atoms in every sixth BaO 3 layer. Both materials exhibit a relative permittivity of approximately 20-23, Q x f 0 values of approximately 7800-10600 GHz, and negative temperature coefficients of the resonant frequency of approximately -23 to -7 ppm/ degrees C.
authors.affiliation: Department of Chemistry, The University of Liverpool, UK.
authors.forename: Xiaojun
authors.initials: X
authors.lastname: Kuang
authors.forename: John B
authors.initials: JB
authors.lastname: Claridge
authors.forename: Tim
authors.initials: T
authors.lastname: Price
authors.forename: David M
authors.initials: DM
authors.lastname: Iddles
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Rosseinsky