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

Graphical abstract: The combination of neutron and X-ray diffraction study reveals that ytterbium oxyborate has a complex composition Yb26B12O57 and a composite structure consisting of slabs of rare earth sesquioxide and borate groups BO3, B2O5 and B4O11. Display Omitted Research highlights: → Ytterbium oxyborate crystallizes in the Lu3BO6-type structure. → It has a formula of Yb26(BO3)4(B2O5)2(B4O11)O24 instead of the simple Yb3BO6. → An additional oxygen atom (O31) is identified by neutron diffraction data. → The 1/4 occupancy of O31 relates to the random distribution of B4O11 and B2O5 groups. → The structure consists of A-type rare earth oxide slabs and borate groups. - Abstract: 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) A, b = 3.58372(5) A, c = 14.3128(3) A and β = 115.079(1)o. 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)O24, instead of the simple stoichiometric formula Yb3BO6. This compound is paramagnetic but its susceptibility deviated from the Curie-Weiss law at low temperature.