Experiences and experiments with single crystals with focus on α-Al₂O₃

Work on single crystals at INE was initiated to better understand the interaction of radionuclides with mineral surfaces. Compared to particles, single crystals have the advantage that surface structure and site densities are known. First steps included elaborating a cleaning procedure for a range of α-Al₂O₃ faces and subsequent uptake and spectroscopic experiments with trivalent actinides [1]. Surprisingly, uptake was strongest on the face that was expected to be the least reactive, i.e., the (0001) face. On this face, the spectroscopy also showed different patterns than on the other faces studied. Subsequently, additional effort was made to gain more understanding of the (0001) face which exposes only doubly-co-ordinated oxygens. Literature [2] suggested an unexpectedly low isoelectric point, which was confirmed by own experiments [3]. The interpretation of a set of published data was possible by assuming a water layer on the surface that would turn the hydrophilic surface hydrophobic [4]. In a subsequent study, a systematic study on effects of, e.g. the origin of the crystals, the miscut, or the pretreatment was carried out using various methods [5]. Moreover, the adsorption of surfactants has become of interest [6], involving a multimethod approach [7]. The ultimate goal is to derive thermodynamic models, and first steps in this direction have been taken for isostructural hematite [8].