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

Abstract

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 ${T}_{\mathrm{MI}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}582\mathrm{K}$ it changes to a monoclinic insulator due to a charge disproportionation $(2{\mathrm{Ni}}^{3+}\ensuremath{\rightarrow}{\mathrm{Ni}}^{3+\ensuremath{\delta}}+{\mathrm{Ni}}^{3\ensuremath{-}\ensuremath{\delta}})$ 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 $[\mathbf{k}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(1/2,0,1/2)]$ with unequal moments at Ni1 and Ni2 octahedra.