At the same time, there is always a possibility that disorder will resolve itself while moving to a different chemical space and/or different conditions leading to a potential functional material. For example, a Na[LnSiO4]-based phosphor forms as a disordered Pnma structure above 100 K while ordering with Imma symmetry below this point which also causes a change in luminescence performance. Full or partial substitution of atomic species in the structure of Ba3Y2[Si2O7]2 may force similar ordering and potentially improve synthetic conditions (i.e., transition to a chemical space without competing glass field). The individual coordination of the yttrium and barium atoms is hard to describe in terms of simple polyhedra due to coordination by disordered disilicate groups. The arising environments are realized as superpositions of polyhedra that could have been formed locally by the ordered species. This is most evident in comparison to the ordered Cs3CaBi[P2O7]2 aristotype. The yttrium atoms coordination forms polyhedra Y1@O13 and Y1@O14 (shaded in blue in Figure ) that in their overall shapes resemble a defect superposition of the pentagonal dipyramids surrounding the calcium atoms in the structure of Cs3CaBi[P2O7]2 (Figure ). They show a broad range of Y-O distances from 1.96 to 2.82 Å, which differs from those observed for other ordered Ba-Y silicates like BaY16Si4O33 (Y-O of 2.12 -2.62Å). The lower values from this region are comparable to those observed for e.g., YBa5Cu2O8.5+x (shortest Y-O of 2.01Å) or YBaFeO4 (shortest Y-O of 2.09Å). The structure of Ba3Y2[Si2O7]2 contains 4 crystallographically independent barium atoms that are coordinated by 11-14 half-occupied oxygens: Ba1@O11, Ba2@O12, Ba3@O14, and Ba4@O12 (shaded in light-green in Figure ). These sites are related to Cs3 and Cs2 in the structure of Cs3CaBi[P2O7]2 which are surrounded by already distorted 9-and 10-vertex polyhedra. The Ba-O distances lie within 2.41-3.19 Å and correlate well with the values reported for other oxides of barium [19]. The shortest bond of 2.41 Å for Ba4-O7a is slightly lower than those observed in the structure of phase A or reported for other Ba-Y silicates (see previous section). However, similar values have been reported in other systems such as the chloride silicate Ba5[SiO4]Cl6 with Ba2-O2 distances of 2.467 Å 62 or a Ba-Fe disilicate Ba2Fe[Si2O7] with Ba-O2 of 2.417 Å. The mixed occupied sites M1-M2 show a much broader range of Ba-O distances from 1.96 to 3.20 Å and are surrounded by irregularly shaped polyhedra (drawn in dark-green in Figure ). These sites show the most striking difference in coordination compared to the aristotype. The M2 and M4 sites arise after splitting of Cs1 site (see group-subgroup scheme in Figures ); they are surrounded by 13 partially occupied oxygen sites which is comparable to 11 for Cs1 atoms. At the same time, M1 and M3 sites that are equivalent to Bi sites are centred within much larger coordination: M1@O15 and M3@O14 versus Bi@O6. The distorted coordination of these two atomic sites arises both from superposition of the original distorted octahedra of bismuth atoms and inclusion of further oxygen atoms due to larger radii of the Ba/Y sites. At the same time, higher coordination numbers with respect to the aristotype allow for accommodation of both bigger and smaller coordination environments, as was emphasized in Figure .