To accurately refine the positions and occupancies of light silicon and oxygen atoms, the sample was studied by means of TOF neutron diffraction. The diffraction patterns were fitted, and the structures refined using TOPAS Academic V7 , and the final solution was obtained using combined refinement of high resolution SPXRD and TOF neutron diffraction data. Selected Rietveld refinements performed against SXRD and neutron diffraction data are shown in Figure , while all profiles are shown in Figure . During the multi-pattern refinement, the unit cell parameters were fixed according to the highest resolution data (SPXRD), and other structural variables were refined using all datasets. The unit cell contains three yttrium, two barium, two silicon and seven oxygen sites. The occupancy of the Ba1 site refined to 50% within error based on the combined refinement, and the occupancy of the O7 site was fixed at 50% to charge balance the formula. A possibility to have a fully occupied oxygen site charge-balanced by hydrogen forming OH group was tested but the IR spectrum showed no signs of these groups (Figure ). Analysis of the difference Fourier maps revealed that the Ba1 and Y3 atoms are displaced along the c axis from the 4d and 2b positions respectively, therefore their occupancy parameters were reduced by half to respect site multiplicities. After obtaining a stable solution, the unit cell parameters were refined as a final step. Crystallographic and refinement details for Ba5Y13[SiO4]8O8.75 are given in Table . Refined atomic parameters are listed in Table , while the interatomic distances for the atoms can be found in Table .