High-temperature powder versus single-crystal X-ray diffraction studies: which method to choose? Thermal behavior of shumwayite, [(UO₂)(SO₄)(H₂O)₂]₂·H₂O

High-temperature powder X-ray diffraction is currently the most widely used method to trace changes in unit-cell dimensions and estimate the thermal behavior of crystal structures. Single-crystal X-ray diffraction (SCXRD) is used much less frequently for these purposes due to the technical difficulties associated with experiments. However, modern diffraction equipment allows researchers to make use of the undeniable advantages of direct determination of subtle crystal-chemical parameters, which are responsible for structural dynamics and transformations with temperature variations. The benefits of high-temperature SCXRD studies as well as the challenges that this technique brings are discussed in detail using a synthetic analog of the uranyl sulfate mineral shumwayite, [(UO 2 )(SO 4 )(H 2 O) 2 ] 2 ·H 2 O, as an example. In addition, the details of crystal lattice dynamics are discussed, including unit-cell parameters, bond lengths and bond-valence changes with temperature. Observed anomalies in bond-length and bond-valence behavior are discussed in the context of rigid-body motion and the independent atom model (IAM). To further illustrate the influence of the chosen refinement method, the structure of quartz was investigated using high-temperature SCXRD and refined using both IAM and Hirshfeld atom refinement.