Evidence of Ni(III) disproportionation in the TlNiO3 perovskite lattice through neutron powder diffraction and Mössbauer spectroscopy
Résumé
TlNiO3 has been prepared under high oxygen pressure. This perovskite has been characterized by high-resolution neutron powder diffraction (NPD), Mössbauer spectroscopy, and magnetic measurements. The crystal structure was refined in the monoclinic P21/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 TN = 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 Mössbauer study of TlNi0.9857Fe0.02O3 confirms the existence of two octahedral sites for 57Fe3+. All these results are in agreement with a Ni3+ disproportionation (2Ni3+ → Ni(3+α) + Ni(3-α)+) phenomenon, already suggested for other ANiO3 (A = rare earths or Y) perovskites through neutron diffraction data.