Electron diffraction and Raman scattering evidence of a symmetry breaking at the metal-insulator transition of ${\mathrm{NdNiO}}_3$

${\mathrm{NdNiO}}_3$ is studied by electron diffraction and by Raman scattering, and drastic changes are observed at the metal-insulator transition. These features are explained in the framework of a charge disproportionation of nickel, which is at variance from earlier results, but consistent with recent studies on smaller rare earth nickelates. The Raman spectra in the insulator phase are discussed with respect to the elpasolite arrangement. It is shown that several frequencies directly probe the charge transfer, which leads us to conclude that this parameter evolves over about 50 K below the transition.