Structural relaxations around Ti, Cr and Fe impurities in alpha-Al2O3 probed by x-ray absorption near edge structure combined with first-principles calculations

TL;DR

This study combines XANES experiments and first-principles calculations to analyze local structural relaxations around Ti, Cr, and Fe impurities in alpha-Al2O3, revealing high sensitivity of XANES to small atomic distance variations.

Contribution

It introduces a combined experimental and computational approach to accurately characterize impurity site geometries in corundum.

Findings

Structural relaxations are highly localized.

XANES detects small interatomic distance changes.

Linear dichroic signals are crucial for geometry characterization.

Abstract

We determine the structural relaxations around paramagnetic impurities (Ti, Cr, Fe) in corundum (alpha-Al2O3), by combining x-ray absorption near edge structure (XANES) experiments and ab initio calculations. The structural relaxations are found to be very local. We then show that XANES is sensitive to small variations in interatomic distances within the coordination shell of the absorbing atom. The experiments were carried out on single crystals of ruby and sapphires. Linear dichroic signals are essential to characterize the geometry of the impurity site. The calculations were performed within a self-consistent ``non muffin-tin'' framework, that uses pseudopotentials, plane-wave basis set, and the continued fraction for the absorption cross section.