Long-range charge order in the low-temperature insulating phase of PrNiO3

We report the observation of a static long-range 2Ni(3+)-> Ni3+delta+Ni3-delta charge redistribution in PrNiO3 below T-MI=130 K by means of high-resolution powder neutron diffraction. In contrast to previous structural studies, these present data show that the symmetry of the low-temperature insulating phase is not orthorhombic but monoclinic P2(1)/n. The two available Ni sites host contracted and expanded NiO6 octahedra which alternate along the three pseudocubic perovskite axes, suggesting the existence of a Ni3+delta/Ni3-delta charge order similar to that reported for the nickelates with heavier lanthanide ions. The difference between the average Ni-O distances in the two Ni sites at T/T-MI=0.08 is 0.052(2) A, which is almost two times smaller than the value reported for LuNiO3 at T/T-MI=0.50 [0.084(4) A]. This result suggests a progressive decrease in delta by approaching the itinerant limit and gives further support to the charge disproportionation mechanism as an origin of the metal-insulator transition in the whole RNiO3 family.