Preparation, crystal structures and rapid hydration of P2- and P3-type sodium chromium antimony oxides

Two new Na_x[Cr_(1+x)/2Sb_(1−x)/2]O₂ compounds have been prepared by solid-state reactions in argon. Their structures have been determined by the X-ray Rietveld method. Both new phases together with NaCrO₂-based solid solution comprise brucite-like layers of edge-shared (Cr,Sb)O₆ octahedra but differ by packing mode of the layers and coordination of the interlayer Na⁺ ions. A P3 phase exists at x≈0.5-0.58. It is rhombohedral (R3?m), a=2.966, c=16.937 Å at x≈0.58, with 29% Na⁺ occupancy of trigonal prisms. A P2 phase exists at x≈0.6-0.7. It is hexagonal (P6₃/mmc), a=2.960, c=11.190 Å at x≈0.7, with 37% and 33% Na⁺ occupancy of two non-equivalent trigonal prisms. Both P2 and P3 phases rapidly absorb moisture in air; packing mode is preserved, the a parameter changes slightly but c increases by 24-25%. Very high sodium ion conductivity is predicted for both P2 and P3 anhydrous phases.