Material properties of $\alpha$-quartz that are relevant for its potential use in X-ray monochromators and analyzers

Abstract

Monochromators and analyzers for X-rays of energy $>$ 5 keV are overwhelmingly made from silicon crystals, of which large specimens of high purity and low defect density are readily available. Germanium and diamond crystals are also occasionally used. Bragg reflection of the X-rays determines their bandwidth, with backscattering reflections (with Bragg angles $\approx$ 90$^\circ$) providing the highest energy resolution along with high photon collection efficiency. The crystal lattices of all these materials share the same space group $Fd\overline{3}m$, which because of its high symmetry provides a low density of unique backscattering Bragg reflections per unit photon energy range. This is a disadvantage for solid-state studies using resonant inelastic X-ray scattering (RIXS) and nuclear resonant scattering (NRS), for which the X-ray energy must be tuned to a specific electronic or nuclear transition. As a result, crystals with lattices of lower symmetry are being considered. $\alpha$-quartz is one of the most promising. However, data on its material properties and fabrication techniques are widely scattered, and in the vital matter of the atomic positions, great inconsistencies exist in the literature. The material properties of $\alpha$-quartz that are relevant to X-ray studies are therefore summarized and clarified here as a guide to future work.