Neutron and synchrotron diffraction data provide the first observation of changes in the crystal symmetry at the metal-insulator transition in $R{\mathrm{NiO}}_3$ perovskites. At high temperatures, ${\mathrm{YNiO}}_3$ is orthorhombic and metallic but below $T_{\mathrm{MI}}=582\mathrm{}\mathrm{K}$ it changes to a monoclinic insulator due to a charge disproportionation $(2{\mathrm{Ni}}^{3+}\to {\mathrm{Ni}}^{3+\delta }+{\mathrm{Ni}}^{3-\delta })$ that develops at the opening of the gap. We report the presence of two alternating ${\mathrm{NiO}}_6$ octahedra with expanded (Ni1) and contracted (Ni2) Ni-O bonds and a magnetic structure $[\mathbf{k}=(1/2,0,1/2\left)\right]$ with unequal moments at Ni1 and Ni2 octahedra.

A layered sodium manganese oxide, Na_0.6MnO₂, stable at temperatures above 800 °C, was synthesized by using a sol–gel method that employs Mn(acac)₃ (ac = acetylacetonate), Na₂CO₃ and propionic acid to form the resin framework. This layered bronze possesses a hexagonal, P2-type structure, in which the distortion associated with Mn³⁺ is hardly perceptible. It reacts slowly, though reversibly, with atmospheric moisture, which causes the interlayer spacing in the structure to increase by ca. 2.5 Å, through intercalation of water molecules into the interlayer gap occupied by Na⁺ ions. The anhydrous material was tested as a cathode in sodium cells. Although the electrochemical intercalation of Na⁺ occurs in two steps, the host retains its main structural features, with a slight tendency in the interlayer spacing to contract as the sodium content increases. The similarity between the discharge and charge profiles of the first cycles reveals a quasi-reversible nature in the intercalation process and that the cell can deliver a constant specific capacity of ca. 140 A h kg⁻¹ at 0.1 mA cm⁻² when cycled in a voltage window of 3.8–2.0 V. However, the continuous strains and distortions resulting from the insertion and extraction of Na⁺ ions cause the host structure to gradually collapse and yield an amorphous material after the first eight cycles. This leads to a progressive reduction of the cell capacity, irrespective of the specific voltage window used.