Structure and charge order in the antiferromagnetic band-insulating phase of NdNiO3

We present a high-resolution synchrotron powder-diffraction investigation on NdNiO3 that confirms the presence on long-range charge order below the metal-insulator transition previously reported at T-MI=205 K. The monoclinic symmetry of the insulating state is unambiguously proved. A detailed description of the true crystal structure in the antiferromagnetic phase (T-MI=T-N) was obtained that proves the occurrence of two octahedral sites below T-MI. Antiferromagnetic NdNiO3 presents long-range charge order with delta approximate to 0.2-0.3 < 1. The breathing distortion observed in the octahedra reduces the repulsion U at Ni sites, avoids Jahn-Teller effect, and allows charge segregation. The phonon-assisted partial charge disproportionation is a common mechanism in RNiO3 oxides. With the exception of LaNiO3, the compounds of the whole family of RNiO3 perovskites must be considered band insulators with a charge ordered ground state at low temperature.