Equilibrium Order-Disorder in Spinels

The distribution of cations in different coordinations in the spinel structure as a function of equilibrium temperature was quantitatively determined for several phases. The spinels were synthesized and/or reacted at various temperatures and pressures. The relative X-ray intensities of various reflections of the quenched phases were measured and compared with intensities calculated for various models of cation distribution. In NiAl₂O₄ the cation arrangements at 600° and 1550°C are (Al_1.0)^IV(Ni_1.0 Al_1.0)^VIO₄ and (Ni_0.25Al_0.75)^IV(Ni_0.75Al_1.25)^VIO₄, respectively. In Ni₂GeO₄, between 610° and 1400°C, the concentration of Ni²⁺ ions in tetrahedral sites increases from 0 to 23% and in octahedral sites it decreases from 100 to 77% of total nickel present. All these transitions were reversible under equilibrium conditions and can be classified as second-order “reconstructive transition of disorder.” Many spinels cannot be classified as normal or inverse but have a cation distribution which is an equilibrium function of the temperature.