High-pressure Syntheses and Characterization of the Rare Earth Borates RE5(BO3)2F9 (RE=Dy, Ho)

The new rare earth fluoride borates RE5(BO3)2F9 (RE = Ho, Dy) were synthesized under highpressure/high-temperature conditions of 1.5 GPa, 1250 ◦C for Dy5(BO3)2F9 and 2.5 GPa, 1200 ◦C for Ho5(BO3)2F9 in a Walker-type multianvil apparatus from the corresponding rare earth oxides, rare earth fluorides, and boron oxide. The single-crystal structure determinations have revealed that both compounds are isotypic to the known rare earth fluoride borates RE5(BO3)2F9 (RE = Er, Tm, Yb). The new fluoride borates crystallize monoclinically in the space group C2/c (Z = 4) with the lattice parameters a = 2046.7(4), b = 615.9(2), c = 829.6(2) pm, β = 100.1(1)◦ for Dy5(BO3)2F9 and a = 2039.5(4), b = 612.7(2), c = 827.1(2) pm, β = 100.2(1)◦ for Ho5(BO3)2F9. Three crystallographically different nine-fold coordinated rare earth cations can be identified in the crystal structure. All boron atoms build up isolated trigonal-planar [BO3] groups. In addition to the Raman and IR spectroscopic investigations, DFT-calculations were performed to support the assignment of the vibrational bands.