Charge Disproportionation inRNiO3Perovskites: Simultaneous Metal-Insulator and Structural Transition inYNiO3

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.