Study on the crystal structure of the rare earth oxyborate Yb₂₆B₁₂O₅₇ from powder X-ray and neutron diffraction

Yb₂₆B₁₂O₅₇ , a rare earth oxyborate previously assigned as Yb₃BO₆ , has been studied by various techniques including neutron and X-ray diffraction, ¹¹B NMR spectroscopy, electron diffraction and high angle angular dark field-scanning transmission electron microscopy (HADDF-STEM). It crystallizes in the space group C2/m with cell parameters of a = 24.5780(4) Å, b = 3.58372(5) Å, c = 14.3128(3) Å and β = 115.079(1)°. The structure consists of slabs of rare earth sesquioxide and borate groups. The sesquioxide part is identified from the structural refinement, and is observed from HADDF-STEM image, while elucidation of borate groups is not straightforward. An additional oxygen atom (O31), which links two B₂O₅ groups into a B₄O₁₁ polyanion, is identified from the analysis of neutron diffraction data. The occupancy of this oxygen site is only quarter, which results in a random distribution of B₄O₁₁ and B₂O₅ groups along the b-direction. The chemical formula of ytterbium oxyborate is Yb₂₆(BO₃)₄(B₂O₅)₂(B₄O₁₁)O_24, instead of the simple stoichiometric formula Yb₃BO₆ . This compound is paramagnetic but its susceptibility deviated from the Curie-Weiss law at low temperature.