Crystallization of A3Ln(BO3)2 (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). Na 3 Ln(BO 3 ) 2 (Ln = La-Lu) crystallize in the monoclinic space group P 2 1 / n, the potassium series K 3 Ln(BO 3 ) 2 (Ln = La-Tb) crystallize in the orthorhombic space group Pnma, while the Ln = Dy, Ho, Tm, Yb analogues crystallize in the orthorhombic space group Pnnm . To demonstrate the generality of this synthetic technique, high-entropy oxide (HEO) compositions K 3 Nd 0.15(1) Eu 0.20(1) Gd 0.20(1) Dy 0.22(1) Ho 0.23(1) (BO 3 ) 2 and K 3 Nd 0.26(1) Eu 0.29(1) Ho 0.22(1) Tm 0.14(1) Yb 0.10(1) (BO 3 ) 2 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 Na 3 Ln(BO 3 ) 2 and K 3 Ln(BO 3 ) 2 would be stable, density functional theory was used to calculate their enthalpies of formation, which are favorable.