The monoclinic model of Ba3Y2[Si2O7]2 contains four fully occupied barium and two yttrium atoms. Additionally, four sites are occupied by Ba/Y mixtures in different ratios close to 50:50. There were 8 initial silicon sites that were coordinated pairwise by oxygen, forming Si2O7 disilicate groups. This model did not however provide a good fit to the diffraction data and inspection of the difference Fourier maps showed multiple unassigned regions of electron and nuclear density. Given the uniformly disordered nature of the Ba/Y sites, it was decided to split atoms forming the disilicate groups in similar 50:50 ratios to model disorder occurring due to the substitution of bigger Ba for smaller Y atoms. This resulted in 4 equivalent pairs of disilicate groups (16 silicon and 56 oxygen sites), which were restrained to retain disilicate geometry. The occupancies of all atoms within each disilicate group were refined as one parameter, and each pair of disilicate groups was restrained to sum to 100% occupancy. Due to the high degree of disorder, the thermal parameters of all silicon and oxygen atoms were constrained to the same value per atomic type. This model yielded a good fit to both SPXRD and neutron diffraction data and provided necessary bonding for the heavy atoms. MEM analysis was applied to the observed synchrotron diffraction data to generate the electron density distribution which supports the split disilicate model (see Figure featuring "dumbbells" of electron density). The refined formula of Ba3.03(3)Y1.97(4)Si4O14 is charge balanced and close to the composition measured by EDX from particles selected for CRED measurement (average of Ba 3.03 ± 0.36 : Y 1.42 ± 0.38 : Si 4 ± 0.39, Figure ). Crystallographic and refinement details for Ba3Y2[Si2O7]2 are given in Table . Refined atomic parameters are listed in Table (atoms forming disilicate groups are listed together for clarity), while the representative interatomic distances for the atoms can be found in Table .