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)O24, instead of the simple stoichiometric formula Yb3BO6. This compound is paramagnetic but its susceptibility deviated from the Curie–Weiss law at low temperature.

Graphical abstractThe 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.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch 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.