High-temperature structural evolution of RNiO3 (R = Ho, Y, Er, Lu) perovskites:: Charge disproportionation and electronic localization -: art. no. 094102

The structural changes of polycrystalline RNiO3 perovskites (R = Ho, Y, Er, and Lu, prepared under high-oxygen pressure) across the metal-insulator (MI) transition (T-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-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 Ni3+ cations. Above T-MI the samples become orthorhombic, space group Pbnm. Upon heating across T-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 NiO6, octahedra reaches its maximum value at temperatures close to T-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 Ni1O(6) and Ni2O(6) octahedra) on increasing temperature in the insulating regime, to reach a maximum at 60-80 K below the MI transition.