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

A new six-layer perovskite-related structure Ba6Na2Nb2M2O17 (M = P, V), which consists of cubic (c) BaO3 layers and oxygen-deficient pseudocubic (c′) BaO2 layers stacked in the sequence c′ccccc, is presented. In Ba6Na2Nb2M2O17, 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 NbO6 octahedral layers are sandwiched by two single NaO6 octahedral layers, which, in turn, connect with two isolated MO4 tetrahedral layers. Both oxides are derived from the 2:1 ordered perovskite structure (e.g., Ba3ZnTa2O9) by ordered removal of O atoms in every sixth BaO3 layer. Both materials exhibit a relative permittivity of ∼20−23, Q × f0 values of ∼7800−10600 GHz, and negative temperature coefficients of the resonant frequency of ∼−23 to −7 ppm/°C.