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

Hydrated rare earth hexaborates H3LnB6O12 (Ln = Sm−Lu) have been synthesized by
using molten boric acid as the reaction medium. H3LnB6O12 decomposes at certain
temperatures to form anhydrous pentaborates LnB5O9 (Ln = Sm−Er) or orthoborates LnBO3
(Ln = Tm−Lu). The crystal structures of the hexaborates and pentaborates have been
determined by single-crystal and powder diffraction techniques. The hydrated hexaborates
H3LnB6O12 crystallize in a trigonal structure in the space group R3c, which contain a six-membered ring B6O15 as a fundamental fragment. Anhydrous pentaborates LnB5O9 crystallize
in a tetragonal structure that is built up with B4O9 and BO3. LnB5O9 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 H3LnB6O12 and LnB5O9. The luminescent
property of the europium-doped GdB5O9 was studied.