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

Two new Na x [Cr (1+ x)/2 Sb (1− x)/2 ]O 2 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 NaCrO 2-based solid solution comprise brucite-like layers of edge-shared (Cr,Sb)O 6 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 ( R3̄ 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 ( P6 3/ 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. Polyhedral presentation of layered structures of Na x [Cr (1+ x)/2 Sb (1− x)/2 ]O 2· (Cr,Sb)O 6 octahedra are green. Part of the sodium prisms are open to show short distances between sodium sites which cannot be occupied simultaneously. [Display omitted] ► Two new layered Na x [Cr (1+ x)/2 Sb (1 −x)/2 ]O 2 phases, P2 and P3, have been prepared in argon. ► Interlayer O–O distances are larger than in Na x ( M,Ti)O 2 analogs ( M=Cr,Ni,Li). ► As a result, both rapidly hydrate in air with c-axis expansion of 24–25%. ► Bottleneck radii are also larger, and this predicts very high Na +-ion conductivity.