Crystal Structure and Elementary Properties of NaxCoO2 (x = 0.32, 0.5, 0.6, 0.75, and 0.92) in the Three-Layer NaCoO2 Family

TL;DR

This study elucidates the crystal structures and magnetic properties of NaxCoO2 phases with varying sodium content, revealing complex structural evolution and magnetic behavior in the three-layer system.

Contribution

It provides detailed structural characterization of NaxCoO2 with different x values, highlighting differences from the two-layer system and identifying unique coordination and lattice arrangements.

Findings

Structures vary with Na content, including trigonal and monoclinic phases.

Magnetic susceptibility peaks at x=0.6, indicating strong magnetic interactions.

Na coordination changes from octahedral to trigonal prismatic across compositions.

Abstract

The crystal structures of the NaxCoO2 phases based on three-layer NaCoO2, with x=0.32, x=0.51, x=0.60, x=0.75 and x=0.92, determined by powder neutron diffraction, are reported. The structures have triangular CoO2 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 NaxCoO2 system. The highest and lowest Na containing phases studied (x=0.92 and x=0.32) are trigonal, with three CoO2 layers per cell and octahedral Na ion coordination. The intermediate compositions have monoclinic structures. The x=0.75 compound has one CoO2 layer per cell, with Na in octahedral coordination and an incommensurate superlattice. The x=0.6 and x=0.5 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.