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

The crystal structure of LuNiO perovskite has been examined below RT and across = 125 K by neutron powder diffraction. In this temperature region (2-298 K), well below the metal-insulator transition this oxide exhibits at = 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 2/ space group, there are two inequivalent Ni sites located in alternating octahedra of different sizes. The structural analysis with high-resolution techniques ( = 1.594 Å) unveils a subtle increase of the charge disproportionation as temperature decreases, reaching = 0.34 at 2 K. The magnetic structure has been investigated from low-T NPD patterns collected with a larger wavelength ( = 2.52 Å). Magnetic peaks are observed below ; they can be indexed with a propagation vector = (½, 0, ½), as previously observed in other RNiO 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 (Ni + Ni), with magnetic moments only on Ni 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) , and m 0.7(1) at 2 K, the ordered magnetic moments lying on the - plane. This is similar to that found for YNiO. In complement, the magnetic and thermal properties of LuNiO have been investigated. AC susceptibility curves exhibit a clear peak centered at = 125 K, corresponding to the establishment of the Ni antiferromagnetic structure. This is corroborated by DC susceptibility and specific heat measurements. Magnetization 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 ( > ).