Crystal structure and elementary properties of NaxCoO2 (x=0.32, 0.51, 0.6, 0.75, and 0.92) in the three-layer NaCoO2 family

Abstract

The crystal structures of ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ phases based on three-layer $\mathrm{Na}\mathrm{Co}{\mathrm{O}}_2$, with $x=0.32$, 0.51, 0.60, 0.75, and 0.92, determined by powder neutron diffraction, are reported. The structures have triangular $\mathrm{Co}{\mathrm{O}}_2$ layers interleaved by sodium ions, and evolve with variation in Na content in a more complex way than has been observed in the two-layer ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ system. The phases with highest and lowest Na content studied ($x=0.92$ and 0.32) are trigonal, with three $\mathrm{Co}{\mathrm{O}}_2$ layers per cell and octahedral Na ion coordination. The intermediate compositions have monoclinic structures. The $x=0.75$ compound has one $\mathrm{Co}{\mathrm{O}}_2$ layer per cell, with Na in octahedral coordination and an incommensurate superlattice. The $x=0.6$ and 0.51 phases are also single layer, but the Na is found in trigonal prismatic coordination. The magnetic behavior of the phases is similar to that observed in the two-layer system. Both the susceptibility and the electronic contribution to the specific heat are largest for $x=0.6$.