  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.
