Synthesis of boehmite-type GaOOH: a new polymorph of Ga oxyhydroxide and geochemical implications

Gallium (Ga) and aluminum (Al) belong to group ⅢA elements in the periodic table. 20 They show a coupled geochemical behavior in most natural systems, and are considered as 21 “geochemical partners”. However, compared with the principal oxyhydroxides of Al in nature, 22 gibbsite (Al(OH) 3 ), boehmite (γ-AlOOH), and diaspore (α-AlOOH), only the analogs 23 söhngeite (Ga(OH) 3 ) and tsumgallite (α-GaOOH) were reported. In this work, boehmite-type 24 GaOOH (γ-GaOOH), a new polymorph of GaOOH, was synthesized for the first time using 25 boehmite (γ-AlOOH) as a template. The synthesized γ-GaOOH was characterized by a series 26 of techniques including X-ray diffraction (XRD), high angle annular dark-field scanning 27 transmission electron microscopy (HAADF-STEM) and selected area electron diffraction 28 (SAED). Furthermore, a model based on the boehmite structure was successfully applied to 29 define the γ-GaOOH structure by the Rietveld method. Results from sample characterization 30 and structural refinement support the successful synthesis of boehmite-type GaOOH, and thus 31 it is referred to as γ-GaOOH. The synthesis of γ-GaOOH in laboratory is valuable to 32 understanding the Ga geochemistry and its enrichment process in Ga-rich boehmite in coal 33 and bauxite.