Incommensurately modulated crystal structure of α' (O′3)-type sodium cobalt oxide NaxCoO2 (x ∼ 0.78)

A single-phase sample of alpha' (O'3)-type layered sodium cobalt oxide NaxCoO2 (x similar to 0.78) was prepared and its incommensurately modulated crystal structure was analyzed using the (3+1)-dimensional superspace approach to the powder neutron diffraction data. The crystal structure of the cobaltate is accurately described based on the superspace group C2/m(alpha 0 gamma)00, wherein the positions of Na atoms are most significantly modulated in the monoclinic a direction to form an ordered arrangement. Such a displacive modulation causes a quasi-periodic shift of Na atoms from the centers of the NaO6 polyhedra between undulated CoO2 sheets, changing the form of the NaO6 polyhedron from an octahedral coordination (O) to a trigonal prismatic (P) one, via an intermediate capped trigonal prismatic NaO7 coordination (C). At the positions where the Na atoms are most significantly shifted, the neighboring Na atoms are located at almost touching distances. However, the occupation factor of Na atoms becomes zero at such positions, yielding Na-deficient sites V-Na, sandwiched either between C and P, or C and C-type polyhedra.

Automated summary

The study analyzed the crystal structure of alpha'-type layered sodium cobalt oxide NaxCoO2 using the (3+1)-dimensional superspace approach, revealing that the Na atoms undergo displacive modulation resulting in a transformation of NaO6 octahedra coordination to trigonal prismatic coordination through capped trigonal prismatic NaO7 coordination.