High-temperature structural evolution ofRNiO3(R=Ho,Y, Er, Lu)perovskites: Charge disproportionation and electronic localization

Abstract

The structural changes of polycrystalline $R{\mathrm{NiO}}_{3}$ perovskites $(R=\mathrm{Ho},$ Y, Er, and Lu, prepared under high-oxygen pressure) across the metal-insulator (MI) transition ${(T}_{\mathrm{MI}}$ ranging between 573 and 600 K) have been studied by high-resolution neutron- and synchrotron diffraction techniques. In the insulating (semiconducting) regime, below ${T}_{\mathrm{MI}},$ the perovskites are monoclinic, space group ${P2}_{1}/n,$ and contain two chemically different Ni1 and Ni2 cations, as a result of the charge disproportionation (CHD) of ${\mathrm{Ni}}^{3+}$ cations. Above ${T}_{\mathrm{MI}}$ the samples become orthorhombic, space group Pbnm. Upon heating across ${T}_{\mathrm{MI}},$ the CHD vanishes and there is an abrupt convergence of the two sets (Ni1 and Ni2) of three Ni-O bond lengths, in the monoclinic-insulating phase, to three unique Ni-O distances in the orthorhombic-metallic phase. An unexpected expansion of the b unit-cell parameter is observed across the electronic transition. This effect, not reported for the former members of the series, is the consequence of an extremely anisotropic rearrangement of Ni-O bonds across the transition. The distortion of the ${\mathrm{NiO}}_{6}$ octahedra reaches its maximum value at temperatures close to ${T}_{\mathrm{MI}}.$ The evolution of the mean 〈Ni-O〉 bond lengths suggests a gradual increase of the degree of CHD (i.e., the difference between the average size of ${\mathrm{Ni}1\mathrm{O}}_{6}$ and ${\mathrm{Ni}2\mathrm{O}}_{6}$ octahedra) on increasing temperature in the insulating regime, to reach a maximum at 60--80 K below the MI transition.