Structure and catalytic activity of CoχMg1–χAl2O4 spinel solid solutions. Part 1.—Cation distribution of Co2+ ions

Abstract

The cation distribution in CoAl₂O₄ and in several Co_χMg_1–χAl₂O₄ solid solutions with χ ranging from 0.05 to 0.5 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 cation positions.

The results show that all specimens have the cobalt ions in a predominantly tetrahedral distribution and that there is a change towards a random cation distribution when the preparation temperature is increased from 1073 to 1473 K. The fraction of Co²⁺ ions in tetrahedral sites, α, is equal to 0.82 and 0.77 for the CoAl₂O₄ spinel prepared at 1073 and 1473 K, respectively.

The X-ray results show, moreover, that α does not vary significantly with composition for χ varying from 1.0 to 0.1. For the most dilute cobalt solid solution (χ= 0.05), however, the value of α is lower; this trend is observed in both series of specimens prepared at different temperatures and may be explained in terms of anion–anion contact effects.