Decoding Anionic Organization of Hydroxide-Fluorides in a Diaspore-Type Network

The diaspore-type crystalline structure is historically well-known in mineralogy, but it has also been widely studied for various applications in the field of catalysis, electrocatalysis, and batteries. However, once two anions of similar ionic size but different electronegativity, such as F^– and O^2– or more precisely OH^–, are combined, the knowledge of the location of these two anions is of paramount importance to understand the chemical properties in relation with the generation of hydrogen bonds. Coprecipitation and hydrothermal routes were used to prepare hydroxide-fluorides that crystallize all in an orthorhombic structure with four formula units per cell. By coupling X-ray scattering techniques for both long- and short-range order (XRD and PDF) and by using multiple complementary spectroscopic probes (Raman, FTIR, and 1D/2D ¹⁹F and ¹H MAS NMR measurements), preferential anionic site occupancy by hydroxyl groups and fluoride ions is demonstrated. Moreover, in the Mg(OH)F diaspore network, 10% of Mg²⁺ is located in the tunnels, resulting in cationic vacancies in the main site. The preference for OH at the edges is clearly marked in the case of Mg(OH)F, whereas OH is preferred at the vertices in Zn(OH)F, regardless of the allotropic form. The dominant polar low-symmetry <i>Pna</i>2₁ form of Zn(OH)F has more OH groups at the vertices than the parent centrosymmetric <i>Pnma</i> variety, in agreement with its strongest hydrogen bonds. Given the remarkable flexibility of the diaspore-type network, the thermal stability of these hydroxide-fluorides is perfectly matched to the structural characteristics. The location of anionic groups within the diaspore framework should play a key role in understanding and optimizing physicochemical properties such as proton mobility for alkaline batteries and acid–base properties for heterogeneous catalysis.