Crystallization of ALn(BO) (A = Na, K; Ln = Lanthanide) from a Boric Acid Containing Hydroxide Melt: Synthesis and Investigation of Lanthanide Borates as Potential Nuclear Waste Forms.

A series of alkali metal rare-earth borates were prepared via high-temperature flux crystal growth, and their structures were characterized by single crystal X-ray diffraction (SXRD). NaLn(BO) (Ln = La-Lu) crystallize in the monoclinic space group 2/, the potassium series KLn(BO) (Ln = La-Tb) crystallize in the orthorhombic space group while the Ln = Dy, Ho, Tm, Yb analogues crystallize in the orthorhombic space group . To demonstrate the generality of this synthetic technique, high-entropy oxide (HEO) compositions KNdEuGdDyHo(BO) and KNdEuHoTmYb(BO) were obtained in single crystal form. Radiation damage investigations determined that these borates have a high radiation damage tolerance. To assess whether trivalent actinide analogues of NaLn(BO) and KLn(BO) would be stable, density functional theory was used to calculate their enthalpies of formation, which are favorable.