Structural relaxation around substitutional Cr3+ in MgAl2O4

Amélie Juhin (1) , Georges Calas (1) , Delphine Cabaret (1) , Laurence Galoisy (1) , Jean-Louis Hazemann (2)

1 IMPMC - Institut de minéralogie et de physique des milieux condensés
2 NEEL - MRS - Matériaux, Rayonnements, Structure

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.

Mots clés: 61.72.Bb; 82.33.Pt; 78.70.Dm; 71.15.Mb

Domaines: Science des matériaux [cond-mat.mtrl-sci]; Minéralogie

Amélie Juhin, Georges Calas, Delphine Cabaret, Laurence Galoisy, Jean-Louis Hazemann. Structural relaxation around substitutional Cr3+ in MgAl2O4. Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2007, 76 (5), pp.054105. ⟨10.1103/PhysRevB.76.054105⟩. ⟨hal-00140475⟩