Transition from a phase-segregated state to single-phase incommensurate sodium ordering in Na_xCoO_2 with x \approx 0.53

TL;DR

This study uses synchrotron X-ray diffraction to investigate sodium ordering in Na_xCoO_2 near x=0.53, revealing a temperature-dependent transition from incommensurate to phase-segregated states with distinct superstructures.

Contribution

It provides detailed insights into the temperature-driven transition from incommensurate to phase-separated sodium ordering in Na_xCoO_2 around x=0.53.

Findings

Incommensurate sodium ordering exists between 220 K and 430 K.

Reversible phase segregation occurs below 220 K into two superstructures.

The transition from incommensurate to commensurate phases is intrinsic near x=0.5+0.03.

Abstract

Synchrotron X-ray diffraction investigations of two single crystals of Na_xCoO_2 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 x 6a x c hexagonal supercell. We argue that the commensurate-to-incommensurate transition is an intrinsic feature of samples with Na concentrations x = 0.5 + d with d ~ 0.03.