Crystal structure and elementary properties of Nax Co O2 (x=0.32, 0.51, 0.6, 0.75, and 0.92) in the three-layer NaCo O2 family

The crystal structures of Nax Co O2 phases based on three-layer NaCo O2, with x=0.32, 0.51, 0.60, 0.75, and 0.92, determined by powder neutron diffraction, are reported. The structures have triangular Co O2 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 Nax Co O2 system. The phases with highest and lowest Na content studied (x=0.92 and 0.32) are trigonal, with three Co O2 layers per cell and octahedral Na ion coordination. The intermediate compositions have monoclinic structures. The x=0.75 compound has one Co O2 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.