Long-range charge order in the low-temperature insulating phase ofPrNiO3

Abstract

We report the observation of a static long-range $2{\text{Ni}}^{3+}\to {\text{Ni}}^{3+\delta }+{\text{Ni}}^{3-\delta }$ charge redistribution in ${\text{PrNiO}}_3$ below $T_{\text{MI}}=130\text{K}$ 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 $P{2}_1/n$. The two available Ni sites host contracted and expanded ${\text{NiO}}_6$ octahedra which alternate along the three pseudocubic perovskite axes, suggesting the existence of a ${\text{Ni}}^{3+\delta }/{\text{Ni}}^{3-\delta }$ 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 $T/T_{\text{MI}}=0.08$ is $0.052\left(2\right)\text{Å}$, which is almost two times smaller than the value reported for ${\text{LuNiO}}_3$ at $T/T_{\text{MI}}=0.50$ $\left[0.084\left(4\right)\text{Å}\right]$. 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 $R{\text{NiO}}_3$ family.