On the magnetic structure and magnetic behaviour of the most distorted member of the series of RNiO3 perovskites (R = Lu)

Abstract

The crystal structure of LuNiO₃ perovskite has been examined below RT and across T_N = 125 K by neutron powder diffraction. In this temperature region (2–298 K), well below the metal–insulator transition this oxide exhibits at T_MI = 599 K, this material is insulating and characterized by a partial charge disproportionation of the Ni valence. In the perovskite structure, defined in the monoclinic P2₁/n space group, there are two inequivalent Ni sites located in alternating octahedra of different sizes. The structural analysis with high-resolution techniques (λ = 1.594 Å) unveils a subtle increase of the charge disproportionation as temperature decreases, reaching δ_eff = 0.34 at 2 K. The magnetic structure has been investigated from low-T NPD patterns collected with a larger wavelength (λ = 2.52 Å). Magnetic peaks are observed below T_N; they can be indexed with a propagation vector k = (½, 0, ½), as previously observed in other RNiO₃ perovskites for the Ni sublattice. Among the three possible solutions for the magnetic structure, the first one is discarded since it would correspond to a full charge ordering (Ni²⁺ + Ni⁴⁺), with magnetic moments only on Ni²⁺ ions, not compatible with the structural findings assessing a partial charge disproportionation. The best agreement is found for a non-collinear model with two different moments in Ni1 and Ni2 sites, 1.4(1) μ_B, and m 0.7(1) μ_B at 2 K, the ordered magnetic moments lying on the a–c plane. This is similar to that found for YNiO₃. In complement, the magnetic and thermal properties of LuNiO₃ have been investigated. AC susceptibility curves exhibit a clear peak centered at T_N = 125 K, corresponding to the establishment of the Ni antiferromagnetic structure. This is corroborated by DC susceptibility and specific heat measurements. Magnetization vs. field measurements confirm that the system is antiferromagnetic down to 2 K, without any further magnetic change. This linear behavior is also observed in the paramagnetic regime (T > T_N).