The Ba-and Ba/Y-centred polyhedra interconnect along the bc plane forming a checkerboardlike pattern. There are 3 such layers in the structure of Ba3Y2[Si2O7]2 (Figure , c): the one at the origin of the a axis is formed by Ba1-and M2-centred polyhedra (A), followed by a more diverse layer of Ba3@O14, Ba4@O12, M1@O15, and M3@O14 (B), and the third layer formed by Ba2-and M4-centred polyhedra in the middle of the unit cell (C). These fragments stack on top of each other along the a axis, populating the unit cell of Ba3Y2[Si2O7]2 (divided by dashed line in Figure , a). The similar layers are observed in the substructure of orthorhombic Cs3CaBi[P2O7]2; however, it features alternation of a uniform layer formed by Cs-centred polyhedra and a chessboard-like arrangement of Cs-centred polyhedra and Bi-centred octahedra (Figure ).