X-ray Linear Dichroism in cubic compounds: the case of Cr3+ in MgAl2O4

TL;DR

This study investigates the angular dependence of X-ray linear dichroism in Cr3+-doped MgAl2O4 spinel using experimental spectroscopy and theoretical calculations, revealing how site distortion influences dichroic signals and enabling quantitative analysis of local symmetry.

Contribution

It introduces an efficient symmetry-based method to compute cubic crystal dichroism from a single site and compares DFT and LFM approaches for interpreting pre-edge features.

Findings

LFM reproduces the angular dependence better than DFT.

Site distortion and 3d-3d repulsion are key factors in dichroism.

Dichroic shape is sensitive to trigonal distortion, enabling site analysis.

Abstract

The angular dependence (x-ray linear dichroism) of the Cr K pre-edge in MgAl2O4:Cr3+ spinel is measured by means of x-ray absorption near edge structure spectroscopy (XANES) and compared to calculations based on density functional theory (DFT) and ligand field multiplet theory (LFM). We also present an efficient method, based on symmetry considerations, to compute the dichroism of the cubic crystal starting from the dichroism of a single substitutional site. DFT shows that the electric dipole transitions do not contribute to the features visible in the pre-edge and provides a clear vision of the assignment of the 1s-->3d transitions. However, DFT is unable to reproduce quantitatively the angular dependence of the pre-edge, which is, on the other side, well reproduced by LFM calculations. The most relevant factors determining the dichroism of Cr K pre-edge are identified as the site distortion and 3d-3d electronic repulsion. From this combined DFT, LFM approach is concluded that when the pre-edge features are more intense than 4 % of the edge jump, pure quadrupole transitions cannot explain alone the origin of the pre-edge. Finally, the shape of the dichroic signal is more sensitive than the isotropic spectrum to the trigonal distortion of the substitutional site. This suggests the possibility to obtain quantitative information on site distortion from the x-ray linear dichroism by performing angular dependent measurements on single crystals.