Influence of cooling rate on the structure and composition of NaxCoO2 (x ∼ 0.65)

Abstract

The influence of cooling rate on the composition, structure and magnetic behaviour of Na_xCoO₂ (x ∼ 0.65) is reported. Slow cooled and quenched samples are shown to have different compositions: x = 0.63 and x = 0.68, respectively. On slow cooling, exsolution of sodium occurs and appears as a separate phase, Na₂CO₃. Whereas the slow-cooled sample has the expected hexagonal symmetry, P6₃/mmc, a distortion to orthorhombic symmetry (Cmcm, a = 2.83333(4) Å, b = 4.92297(7) Å, c = 10.8948(2) Å) is observed for the quenched material. High resolution neutron powder diffraction reveals displacement of sodium ions from their ideal positions for both materials. The different structure and composition are reflected in the Raman spectra and magnetic behaviour of the phases.