Synthesis of Rare Earth Polyborates Using Molten Boric Acid as a Flux

Hydrated rare earth hexaborates H 3 LnB 6 O 12 (Ln = Sm−Lu) have been synthesized by using molten boric acid as the reaction medium. H 3 LnB 6 O 12 decomposes at certain temperatures to form anhydrous pentaborates LnB 5 O 9 (Ln = Sm−Er) or orthoborates LnBO 3 (Ln = Tm−Lu). The crystal structures of the hexaborates and pentaborates have been determined by single-crystal and powder diffraction techniques. The hydrated hexaborates H 3 LnB 6 O 12 crystallize in a trigonal structure in the space group R 3 c, which contain a six-membered ring B 6 O 15 as a fundamental fragment. Anhydrous pentaborates LnB 5 O 9 crystallize in a tetragonal structure that is built up with B 4 O 9 and BO 3 . LnB 5 O 9 decomposes at higher temperature to metaborates (Ln = Sm−Tb) and orthoborates (Ln = Dy−Er). A phase diagram is presented that shows the stable range of both H 3 LnB 6 O 12 and LnB 5 O 9 . The luminescent property of the europium-doped GdB 5 O 9 was studied.