Transition from a phase-segregated state to single-phase incommensurate sodium ordering in gamma-NaxCoO2(x approximate to 0.53) (original) (raw)

Synchrotron X-ray diffraction investigations of two single crystals of sodium cobaltate NaxCoO2 from different batches with composition x = 0.525 − 0.530 reveal homogeneous incommensurate sodium ordering with propagation vector (0.53 0.53 0) at room-temperature. The incommensurate (qq0) superstructure exists between 220 K and 430 K. The value of q varies between q = 0.514 and 0.529, showing a broad plateau at the latter value between 260 K and 360 K. On cooling, unusual reversible phase-segregation into two volume fractions is observed. Below 220 K, one volume fraction shows the well-known commensurate orthorhombic x = 0.50 superstructure, while a second volume fraction with x = 0.55 exhibits another commensurate superstructure, presumably with a 6a×6a×c hexagonal supercell. We argue that the commensurate-to-incommensurate transition is an intrinsic feature of samples with Na concentrations x = 0.5 + δ with δ ≈ 0.03.