Characterization of the structural transition in Na_0.75CoO ₂

In the Na_xCoO₂ chemical system, a two-phase region is found at ambient temperature near x = 0.75. The manner in which that two-phase region is formed on cooling from a uniform single phase at temperatures above 340 K is reported here. The driving force that induces the phase separation appears to be a difference in Na composition of the two phases, which, though small, is accompanied by substantial differences in the unit cell parameters. Hysteretic behaviour in the transition is observed. One of the phases exhibits negative thermal expansion in the narrow temperature region where the phases are in the process of separating.