The effect of Cr content on the activation energy of order-disorder transition in Mg-Al spinel

Abstract

The order-disorder transition (ODT) in spinel is a critical physical characteristic that significantly affects its properties and applications. Although activation energy—a key thermodynamic parameter of ODT—is reported to be influenced by Cr content, the relationship between these two factors remains unclear. In this work, six natural Mg-Al spinel samples with varying Cr contents were studied. Each sample was cut into eight slices and heated to seven different temperatures (550–850 °C), creating a stepwise increase in the degree of disorder. Photoluminescence (PL) spectra were subsequently collected at liquid nitrogen temperature. The intensity of the N peaks in the zero-phonon line range and the N₄ peak increased with increasing Cr content. By integrating parameters extracted from these spectra into a thermodynamic model based on the Arrhenius relationship, we calculated the ODT activation energies for all the samples and established a quantitative relationship between activation energy and Cr content. As a result, we clarify the negative correlation between ODT activation energy and Cr content in Mg-Al spinel, which may be attributed to local structural distortion caused by the presence of Cr-pairs. This study provides a method to correct the influence of Cr content on ODT thermodynamic models, thereby enhancing the viability of spinel as geothermometers and deepening the scientific understanding of how elemental composition affects the thermal stability of spinel.