# Structural relaxation around substitutional Cr3+ in MgAl2O4

## TL;DR

This study investigates the local structural environment of Cr3+ ions in MgAl2O4 spinel using advanced spectroscopic techniques and first-principles calculations, revealing detailed relaxation mechanisms and deviations from Vegard's law.

## Contribution

It provides new insights into the local structural relaxation around Cr3+ in MgAl2O4, combining experimental spectroscopy with computational modeling to understand strain effects.

## Key findings

- Cr-O distance similar to MgCr2O4 indicating full relaxation
- Partial relaxation of the second shell of Al atoms
- Vegard's law is not obeyed in the solid solution

## Abstract

The structural environment of substitutional Cr3+ ion in MgAl2O4 spinel has been investigated by Cr K-edge Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopies. First-principles computations of the structural relaxation and of the XANES spectrum have been performed, with a good agreement to the experiment. The Cr-O distance is close to that in MgCr2O4, indicating a full relaxation of the first neighbors, and the second shell of Al atoms relaxes partially. These observations demonstrate that Vegard's law is not obeyed in the MgAl2O4-MgCr2O4 solid solution. Despite some angular site distortion, the local D3d symmetry of the B-site of the spinel structure is retained during the substitution of Cr for Al. Here, we show that the relaxation is accomodated by strain-induced bond buckling, with angular tilts of the Mg-centred tetrahedra around the Cr-centred octahedron. By contrast, there is no significant alteration of the angles between the edge-sharing octahedra, which build chains aligned along the three four-fold axes of the cubic structure.