The phase diagram of non-hydrated NaxCoO2 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 = 1 2 ) 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 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.Research on oxide conductors has uncovered many interesting electronic states characterized by strong interaction, which include unconventional superconductivity, and charge-or spin-ordered states [1,2]. Recently, attention has focussed on the layered cobaltate Na x CoO 2 . At the doping x ∼ 2 3 , Na x CoO 2 exhibits an unusually large thermopower [3]. Although the resistivity is metallic, the magnetic susceptibility displays a surprising Curie-Weiss profile [4], with a magnitude consistent with antiferromagnetically coupled spin-1 2 local moments equal in number to the hole carriers [5]. The thermopower at 2.5 K is observed to be suppressed by an in-plane magnetic field [5]. This implies that the enhanced thermopower is largely due to spin entropy carried by strongly correlated holes (Co 4+ sites) hopping on the triangular lattice. When intercalated with water, Na x CoO 2 ·yH 2 O becomes superconducting at or below 4 K [6] for 1 4 < x < 1 3 [7,8,9]. These experiments raise many questions. Is the Curie-Weiss state at 2 3 continuous with the 1 3 state surrounding superconductivity? Are commensurability and charge-ordering effects important? To address these questions, we have completed a study of the phase diagram of non-hydrated Na x CoO 2 . As x increases from 0.3 to 0.75, we observe a series of electronic states, the most interesting of which is an insulating state at x = 1 2 that involves charge ordering of the holes together with the Na ions. We identify details specific to the triangular lattice, especially in the metallic state from which the superconducting composition evolves, and comment on recent theories.Starting with powder or single-crystal samples with x ∼ 0.75, we vary x by specific chemical deintercalation of Na (Fig. 1, caption). Powders of Na 0.77 CoO 2 were made by solid-state reaction of stoichiometric amounts of Na 2 CO 3 and Co 3 O 4 in oxygen at 800 C. Sodium deintercalation was then carried out by treatment of samples in solutions obtained by dissolving I 2 (0.2 M, 0.04 M) or Br 2 (1.0 M) in acetonitrile. After magnetic stirring for five days at ambient temperature, they were washed with copious amounts of acetonitrile and multiple samples were tested by the ICP-AES method to determine Na content. Unit-cell parameters were determined by powder X-ray diffraction (XRD) with internal Si standards. For the transport studies, we first grew a boule (with x = 0.75) in an optical furnace by the floating-zone technique. Crystals cleaved from the boul...

The interplay of magnetic interactions, the dimensionality of the crystal structure and electronic correlations in producing superconductivity is one of the dominant themes in the study of the electronic properties of complex materials. Although magnetic interactions and two-dimensional structures were long thought to be detrimental to the formation of a superconducting state, they are actually common features of both the high transition-temperature (Tc) copper oxides and low-Tc material Sr2RuO4, where they appear to be essential contributors to the exotic electronic states of these materials. Here we report that the perovskite-structured compound MgCNi3 is superconducting with a critical temperature of 8 K. This material is the three-dimensional analogue of the LnNi2B2C family of superconductors, which have critical temperatures up to 16 K (ref. 2). The itinerant electrons in both families of materials arise from the partial filling of the nickel d-states, which generally leads to ferromagnetism as is the case in metallic Ni. The high relative proportion of Ni in MgCNi3 suggests that magnetic interactions are important, and the lower Tc of this three-dimensional compound-when compared to the LnNi2B2C family-contrasts with conventional ideas regarding the origins of superconductivity.

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB2, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions). Here we show that the addition of electrons to MgB2, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.

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