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

The new rare earth fluoride borates RE5(BO3)(2)F-9 (RE = Ho, Dy) were synthesized under high-pressure/high-temperature conditions of 1.5 GPa, 1250 degrees C for Dy-5(BO3)(2)F-9 and 2.5 GPa, 1200 degrees C for Ho-5(BO3)(2)F-9 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)(2)F-9 (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, beta = 100.1(1)degrees for Dy-5(BO3)(2)F-9 and a = 2039.5(4), b = 612.7(2), c = 827.1(2) pm, beta = 100.2(1)degrees for Ho-5(BO3)(2)F-9. 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](3-) groups. In addition to the Raman and IR spectroscopic investigations, DFT-calculations were performed to support the assignment of the vibrational bands.