Isolation of Two-Dimensional 2:1 Cation-Ordered Perovskite Units by Anion Vacancy Ordering in Ba₆Na₂Nb₂P₂O₁₇

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.