High-pressure synthesis, crystal structure, and structural relationship of the first ytterbium fluoride borate Yb5(BO3)2F9

Yb₅(BO₃)₂F₉ was synthesized under high-pressure/high-temperature conditions in a Walker-type multianvil apparatus at 7.5 GPa and 1100 °C, representing the first known ytterbium fluoride borate. The compound exhibits isolated BO₃-groups next to ytterbium cations and fluoride anions, showing a structure closely related to the other known rare-earth fluoride borates RE₃(BO₃)₂F₃ (RE=Sm, Eu, Gd) and Gd₂(BO₃)F₃. Monoclinic Yb₅(BO₃)₂F₉ crystallizes in space group C2/c with the lattice parameters a=2028.2(4) pm, b=602.5(2) pm, c=820.4(2) pm, and β=100.63(3)° (Z=4). Three different ytterbium cations can be identified in the crystal structure, each coordinated by nine fluoride and oxygen anions. None of the five crystallographically independent fluoride ions is coordinated by boron atoms, solely by trigonally-planar arranged ytterbium cations. In close proximity to the above mentioned compounds RE₃(BO₃)₂F₃ (RE=Sm, Eu, Gd) and Gd₂(BO₃)F₃, Yb₅(BO₃)₂F₉ can be described via alternating layers with the formal compositions “YbBO₃” and “YbF₃” in the bc-plane.