Distribution of nickel ions among octahedral and tetrahedral sites in NixZn1 –xAl2O4 spinel solid solutions

Abstract

The cation distribution in NiAl₂O₄ and in several Ni_xZn_{1 –x}Al₂O₄ solid solutions with x ranging from 0.05 to 0.90 has been studied by magnetic susceptibility, reflectance spectroscopy, lattice parameter variation and analysis of some X-ray reflections whose intensities are particularly sensitive to variation in the cation positions.

The results show that all specimens have a predominantly octahedral site distribution for their nickel ions and there is a small change towards a random distribution when the preparation temperature is increased from 1073 to 1473 K.

The X-ray and magnetic results show, moreover, that the fraction, γ, of Ni²⁺ ions in octahedral sites varies with nickel content for all the temperature series. For samples quenched from 1473 K, NiAl₂O₄ is 77% inverse, but between x= 1.0 and x= 0.20 the value of γ decreases from 0.77 to 0.64; the fraction of nickel in octahedral sites in the solid solutions is thus lower than in NiAl₂O₄. This effect is linked to a decrease in ionicity of the crystal going from NiAl₂O₄ to the Zn-rich aluminates, which causes a decrease of octahedral site-preference energy for Ni²⁺ ions. Below x= 0.20 there is clear evidence that γ increases and this may be explained in terms of anion–anion repulsion.