Flux Synthesis of Alkaline-Earth Lanthanide Borates as Potential Nuclear Waste Forms

Neodymium is typically considered the best surrogate for trivalent americium and can be used to identify Am3+ containing materials that are likely to form. We have explored the alkaline-earth lanthanide borate phase space using alkaline-earth halide/carbonate fluxes. This resulted in the synthesis of new compounds AE5Ln(BO3)4X (AE = Ca, Sr; Ln = Pr, Nd, Eu, Tb; X = Cl, Br) and AE3Ln2(BO3)4 (AE = Sr, Ba; Ln = Pr, Nd) as well as the synthesis of two compounds of Ba8Ln2(BO3)6(B2O5) (Ln = Eu, Tb) crystallizing in a new structure type. Ba8Ln2(BO3)6(B2O5) crystallizes in the space group P21/n with lattice parameters a = 8.6002(3) Å, b = 7.9245(3) Å, c = 17.6697(7) Å, and β = 91.3560(10)° for the Eu analogue, and the structure contains isolated LnO8 polyhedra connected into a framework by BO3 and B2O5 units. The fluorescence emission spectra of AE5Ln(BO3)4X (AE = Ca, Sr; Ln = Eu, Tb; X = Cl, Br) and Ba8Ln2(BO3)6(B2O5) (Ln = Eu, Tb) are reported.