Ba5Y13[SiO4]8O8.5 (Phase A) crystallizes in space group I4 � 2m, Pearson code tI142, Wyckoff sequence j 5 i 4 hf 2 ea that represents its own structure type which is not reported in ICSD ((Version 5.2.0, Data Release 2024.1) or Pearson's 27 databases. A phase with this space group and cell has been mentioned in conference proceedings by Wierbocka-Wieczorek et al. but no detailed crystallographic data or explanation was given in that publication. It has been recently followed by Yamane et al. in which a more Ba-rich phase (Ba10.22Y26Si16O81.22) with an incommensurate composite structure was reported to form at 1873 K. In contrast to the majority of known compounds in the Ba-Y-Si-O chemical space, phase A contains barium atoms coordinated via Si@O4 tetrahedra and condensed yttrium-centred polyhedra representing a mixed silicate-oxide (see Figure ). Ba1 and Ba2 atoms are coordinated by 8 and 9 oxygens, respectively, and all Ba-O distances (2.584-3.189 Å, Table ) are comparable to those reported for the rocksalt type barium (II) oxide (2.752 Å) and other Ba-Y-Si-O materials, e.g, Ba9Y2[SiO4]6 37 (Ba-O 2.535-3.321 Å). The disordered Ba1 atoms show rather short Ba1-Ba1 distances of 2.295 and 2.679 Å (the latter being half of the unit cell c dimension) between each pair of atoms; however, the Ba1-O1 distances (2.584-3.189 Å) lie within the ranges known for other barium oxide compounds. These sites sum to a total of 50% occupancy, therefore we can assume a statistical chance of locally observing only one barium atom in one of the four 8h sites per silica-coordinated column in the unit cell. Similar disordered models with proximal half-occupied barium sites have been used to describe structures of the mineral muirite (Ba10(Ca,Mn,Ti)4Si8O24(Cl,OH,O)12.4H2O, which has Ba3-Ba3 distances of 2.758 and 2.877 Å, as well as the synthetic titanosilicate (Ba,Sr)4Ti6Si4O24 xH2O (with Ba2-Ba2 distances of 2.866 Å). Both types of silicon atoms form nearly perfect tetrahedra; the Si1-O (1.617-1.647 Å) and Si2-O (1.629-1.632 Å ) distances are similar to those reported for α-quartz (1.603-1.610 Å). Slight asymmetry for the Si1@O4 tetrahedra can be attributed to the disordered Y3 and O7 sites which impose spatial strain on the neighbouring atoms hence causing slight deformation of their coordination polyhedra. There are two types of yttrium coordination in the structure: Y1 and Y2 atoms are situated inside distorted trigonal prisms with one additional coordinating oxygen atom (CN = 7) while disordered Y3 sites are surrounded by distorted octahedra (CN = 6). Y1-O and Y2-O distances of 2.212-2.521 Å have a slightly broader range compared to those observed in structures of other similar compounds, e.g, BaY4[Si2O7][Si3O10] (2.220-2.381 Å). Due to displacement from the high symmetry 00 1 2 position (Figure ), the coordination of the Y3 atom becomes distorted resulting in a variation of Y-O distances from 2.007 to 3.065 Å with the longest Y3-O7 distance exceeding the average bond value, which likely reflects weaker interaction with the half-occupied O7 site. Despite representing its own structure type, Ba5Y13[SiO4]8O8.5 contains structural fragments similar to those observed for known compounds. When viewed in the ab plane, the silicon-oxide tetrahedral network forms two types of distinct motifs arranged in a checkerboard-like pattern (A and B in Figure ). The first one (A) coordinates the half-occupied O7 sites at cell edges and Y3 atoms, displaced from the high symmetry 00