Charge Ordering, Commensurability, and Metallicity in the Phase Diagram of the LayeredNaxCoO2

The phase diagram of nonhydrated ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ has been determined by changing the Na content $x$ using a series of chemical reactions. As $x$ increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator (at $x=\frac{1}{2}$), then to a “Curie-Weiss metal” (around 0.70), and finally to a weak-moment magnetically ordered state ($x>0.75$). The unusual properties of the state at $\frac{1}{2}$ (including particle-hole symmetry at low $T$ and enhanced thermal conductivity) are described. The strong coupling between the Na ions and the holes is emphasized.