Ba3Y2[Si2O7]2 (phase B) crystalizes in space group P21 with a pseudo-tetragonal unit cell of data (a = 16.47640( ), b = 9.04150( ), c = 9.04114(7) Å, β = 90.0122(9)°), Pearson code mP172, Wyckoff sequence a 86 and is a direct disordered superstructure of the known structure type of the orthorhombic Cs3CaBi[P2O7]2 (P212121). The group-subgroup relationship is schematically described in Figure , a, while a more detailed description of the atomic site evolution is given in Figures . Despite the same stoichiometry, the heavy atoms in the structure of Ba3Y2[Si2O7]2 adopt a different colouring pattern then those in the structure of Cs3CaBi[P2O7]2 (Figure ). The caesium sites in the substructure are replaced with a mixture of barium and Ba/Y sites, the calcium sites are substituted by yttrium, while the bismuth positions are occupied by Ba/Y mixtures. Due to different valency of these heavy atoms as well as their radii, the equivalent sites in both structures have different coordination environments. Bonding to a different number of oxygen atoms at each site causes rotation of the disilicate groups in the structure of phase B compared to the diphosphates of Cs3CaBi[P2O7]2 and results in lowering of the symmetry of phase B.