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=205K . 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 δ≈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.