Study on the crystal structure of the rare earth oxyborate Yb 26B 12O 57 from powder X-ray and neutron diffraction

Abstract

Yb 26B 12O 57, a rare earth oxyborate previously assigned as Yb 3BO 6, 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 B 2O 5 groups into a B 4O 11 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 B 4O 11 and B 2O 5 groups along the b-direction. The chemical formula of ytterbium oxyborate is Yb 26(BO 3) 4(B 2O 5) 2(B 4O 11)O 24, instead of the simple stoichiometric formula Yb 3BO 6. This compound is paramagnetic but its susceptibility deviated from the Curie–Weiss law at low temperature.