  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.
