Local distortion and chemical surroundings of NiO6 octahedra for Ni(III) oxides with K2NiF4-type structure

Ni(III) oxides with the K2NiF4-type structure, (A, A')(2)M'0.5Ni0.5O4 (A' = alkaline earth, A = rare earth, and M' = Mg, Zn, Ti, Al, Ga) have been prepared under high oxygen pressure (similar to 150 MPa). The evolution of the local distortion of NiO6 octahedra in such lattices was studied using X-ray diffraction analysis (XRD), electron paramagnetic resonance (EPR) and infrared (IR) spectroscopies. The observed distortions are discussed on the basis of covalency and relative strength of the axial [(A',A)-O] and equatorial(M'-O) competing bonds surrounding the NiO6 octahedron in the lattice. These results are related to the structural tolerance factor proposed by Poix.