Structural relaxation around substitutional Cr3+ in pyrope garnet

TL;DR

This study investigates the local structural environment of Cr3+ ions in pyrope garnet using advanced X-ray techniques and first-principles calculations, revealing detailed relaxation mechanisms around the impurity.

Contribution

It provides new insights into the atomic-scale relaxation processes of Cr3+ in pyrope garnet, combining experimental and computational approaches.

Findings

Cr-O distance similar to knorringite, indicating full relaxation of first neighbors.

Retention of local C3i symmetry during substitution.

Partial relaxation of second and third neighbor shells.

Abstract

The structural environment of substitutional Cr3+ ion in a natural pyrope Mg3Al2Si3O12 has been investigated by Cr K-edge Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) coupled with first-principles computations. The Cr-O distance is close to that in knorringite Mg3Cr2Si3O12, indicating a full relaxation of the first neighbors. The local C3i symmetry of the octahedral Y site is retained during Cr-Al substitution. The second and third shells of neighbors (Mg and Si) relax only partially. Site relaxation is accommodated by strain-induced bond buckling, with angular tilts of the Si-centered tetrahedra around the Cr-centered octahedron, and by a radial deformation of the Mg-centered dodecahedra.