Structural variations accompanied by thermal expansion of diaspore: in-situ single-crystal and powder X-ray diffraction study

In-situ single-crystal and powder X-ray diffraction (XRD) experiments were performed on diaspore at high temperatures. The powder XRD experiments showed that the dehydration reaction from diaspore to corundum occurs between 703 and 733 K. The in-situ single-crystal XRD measurements of diaspore could successfully determine the cell parameters, fractional atomic coordinates and anisotropic displacement parameters at high temperatures, i.e., from 295 to 698 K. Temperature variations in the cell parameters indicate that thermal expansion of the a-axis is a little higher than those of the b-axis and the c-axis. However, the axial thermal expansivity is not as anisotropic as was previously suggested. The results of structure refinements indicate that such lattice expansion behavior is the result of thermal expansion of the tunnels through O2–H···O1 hydrogen-bond separation in the diaspore structure. To the best of our knowledge, this is the first time that the thermal expansion of diaspore has been investigated at an atomic level by in-situ single-crystal XRD experiments at high temperatures.