Electronic Structure and Core Spectroscopy of Scandium Fluoride Polymorphs

TL;DR

This study combines first-principles calculations and x-ray spectroscopy to analyze the stability, electronic structure, and spectroscopic features of six scandium fluoride polymorphs, revealing insights into their electronic properties and phase transitions.

Contribution

It provides a comprehensive theoretical and experimental analysis of scandium fluoride polymorphs, including their electronic structures and spectroscopic signatures, which was previously incomplete.

Findings

Identification of similarities among polymorphs based on formation energies and electronic properties

Confirmation that phase transitions involve mainly lattice rotations

Spectroscopic analysis reveals excitonic effects and matches experimental spectra

Abstract

The microscopic knowledge of the structural, energetic, and electronic properties of scandium fluoride is still incomplete, despite the relevance of this material as an intermediate for the manufacturing of Al-Sc alloys. In a work based on first-principles calculations and x-ray spectroscopy, we assess the stability and the electronic structure of six computationally predicted ScF$_3$ polymorphs, two of which correspond to experimentally resolved single-crystal phases. In the theoretical analysis based on density-functional theory (DFT), we identify similarities among the polymorphs based on their formation energies, charge-density distribution, and electronic properties (band gaps and density of states). We find striking analogies between the results obtained for the low- and high-temperature phases of the material, indirectly confirming that the transition occurring between them mainly consists of a rigid rotation of the lattice. With this knowledge, we examine the x-ray absorption spectra from the Sc and F K-edge contrasting first-principles results obtained from the solution of the Bethe-Salpeter equation on top of all-electron DFT with high-energy-resolution fluorescence detection measurements. The analysis of the computational results sheds light on the electronic origin of the absorption maxima and provides information on the prominent excitonic effects that characterize all spectra. Comparison with measurements confirms that the sample is mainly composed of the high- and low-temperature polymorphs of ScF$_3$. However, some fine details in the experimental results suggest that the probed powder sample may contain defects and/or residual traces of metastable polymorphs.