Synthesis and structural characterization of CuAl2O4–CdAl2O4 spinel solid solutions
CuAl2O4 and cadmium-containing spinel solid solutions CdxCu1–xAl2O4 have been prepared by solid-state reaction of mixtures of CuO, CdO and Al2O3 at 1223 K. The spinels, which are cubic, have been studied by X-ray powder photography and diffractometry. The cation distributions and the oxygen parameter u have been determined using a statistical method to compare observed and calculated X-ray intensities. It is shown that Cd2+ ions enter tetrahedral sites exclusively, but that simultaneously there is displacement of some of the remaining Cu2+ from tetrahedral to octahedral sites. For CdxCu1–xAl2O4 at x= 0 the spinel is 35% inverse and inversion increases to 43% at x= 0.50. A solubility limit for Cd2+ in the spinel phase is reached at x= 0.58. The reaction of CdO with Al2O3 does not produce a spinel.
The results are discussed in terms of the balance developed between the covalency introduced by Cd2+ ions and the disturbance caused to the electrostatic energy by the enlargement of the tetrahedral sites. This enlargement is quantified by measured increases in the unit-cell parameter and the oxygen parameter.