Crystal Structure and Thermochemical Properties of a First Scandium Borophosphate, Sc(H₂O)₂[BP₂O₈]·H₂O

Crystalline samples of scandium borophosphate, Sc(H₂O)[BP₂O₈]·H₂O, were synthesized hydrothermally from acidic suspensions of scandium sesquioxide and boric and phosphoric acid at 443 K. The chiral crystal structure (space group P6₅22 (No. 179), a = 957.52(3) pm, c = 1581.45(6) pm, Z = 6) was solved from single-crystal data and contains helical chain anions of alternating borate and phosphate tetrahedra interconnected by ScO₄(H₂O)₂ coordination octahedra. On thermal treatment, a reversible release of the hydrate water from the structural channels is observed, and rehydation appears as soon as the compound is exposed to air moisture for at least 1 day. X-ray powder diffractions show that the structure is retained during the de-/rehydration process. Both the crystal structure of the dehydrated phase, Sc(H₂O)₂[BP₂O₈] (a = 953.5(1) pm, c = 1576.8(2) pm), and that of the rehydrated phase, which is identical to the hydrothermal product, were refined from X-ray powder data with Rietveld methods. When the dehydrated phase is exposed to an ammonia atmosphere, NH₃ absorption is observed.