A new class of mixed-valent europium halide ortho-oxoborates: Eu6X[BO3]4 (X = Cl and Br)

Attempts to obtain Eu5Br[BO3]3 by the reaction of europium sesquioxide (Eu2O3) and elemental boron (B) at 1323 K in the presence of sodium bromide (NaBr) acting as flux and bromine source for 72 h in a silica-jacketed niobium capsule yielded single crystals of Eu3[BO3]2 and the hitherto unknown bromide oxoborate Eu6Br[BO3]4 by serendipity. Optimizing the starting reactant ratios and variations of the fluxes gave access to isotypic Eu6X[BO3]4 representatives with X = Cl and Br. Their hexagonal crystal structures was determined by single-crystal X-ray diffraction and adopt the non-centrosymmetric space group P63mc with lattice parameters in the range of a = 1052–1061 p.m. and c = 681–685 p.m. at 298 K as well as a = 1050–1059 p.m. and c = 680–682 p.m. at 100 K for Z = 2. Eight- and nine-fold coordinated europium cations have planar [BO3]3– triangles and one of these also two halide anions as neighbors. The Raman and IR spectra of both compounds were acquired and compared to literature data to confirm the presence of isolated [BO3]3– anions. Additionally, Mosbauer spectra and magnetic susceptibility measurements confirmed the simultaneous presence of di- and trivalent europium cations according to (Eu2+)5(Eu3+)(X–)([BO3]3–)4 with X = Cl and Br.