Evidence of Ni(III) disproportionation in the T1NiO3 perovskite lattice through neutron powder diffraction and Mossbauer spectroscopy

TlNiO3 has been prepared under high oxygen pressure. This perovskite has been characterized by high-resolution neutron powder diffraction (NPD), Mossbauer spectroscopy, and magnetic measurements. The crystal structure was refined in the monoclinic P2(1)/n space group, containing two crystallographic sites for Ni3+, in such a way that expanded and contracted NiO6 octahedra alternate along the three crystallographic directions. In this model the agreement factors are significantly better than those obtained after the refinement of the structure in the conventional space group Pbnm (orthorhombic symmetry). TlNiO3 becomes antiferromagnetically ordered below T-N = 104 K. The low-temperature (2 K) magnetic structure, also studied from NPD data and characterized by a propagation vector k = (1/2, 0, 1/2), is similar to that observed for YNiO3, and it is compatible with this two-sites model. A Mossbauer study of (TlNi0.98Fe0.02O3)-Fe-57 confirms the existence of two octahedral sites for Fe-57(3+). All these results are in agreement with a Ni3+ disproportionation (2Ni(3+)-->NP(3+alpha) + Ni(3-alpha)+) phenomenon, already suggested for other ANiO(3) (A = rare earths or Y) perovskites through neutron diffraction data.