position along the c axis (Figure ). The O7 atoms lie within the plane of neighbouring Ba2 atoms, while disordered Y3 sites are surrounded by four Si1@O4 tetrahedra. Surprisingly, this first motif is remarkably similar to the structure of the mineral fresnoite, Ba2(TiO)Si2O7 50 (Figure , d), that features similar layers of barium atoms and silicon tetrahedra. In fresnoite, the silicon tetrahedra surround titanium atoms with rather unusual square pyramidal coordination which upon inclusion of an oxygen atom from the next layer resemble the distorted Y3@O6 octahedra observed in Phase A (emphasized by grey lines at Figure ). The main difference between these structural fragments is the orientation of the silicon tetrahedra: in fresnoite, their apexes are oriented in the same direction as the one of the titanium-centred pyramids, while they are alternately oriented in opposing directions in Phase A. Another motif (B) in the structure of Ba5Y13[SiO4]8O8.5 consists of disordered Ba1 sites in face-sharing square-antiprismatic coordination environments further coordinated by alternating pairs of Si2@O4 tetrahedra along the c axis (Figure ). This fragment can be compared with the structure of Na5Pr4[SiO4]4[OH] which features disordered Na2 sites coordinated by silicon tetrahedra (Figure , e). However, due to a different orientation of Si@O4, the sodium atoms are situated inside bicapped square prisms while fragment B contains square-antiprismatic coordination of Ba1 sites. Furthermore, the sodium sites in the structure of Na5Pr4[SiO4]4[OH] are separated with layers of praseodymium atoms while the fragment B in the structure of Ba5Y13[SiO4]8O8.5 has no clear separation due to more uniform distribution of 25% occupied Ba1 atoms along the columns. The distorted Y1-and Y2-centered polyhedra connect via common edges forming a peculiar condensed pattern, that occupies the voids within the silicon-oxygen network (see Figure ). Pairs of Y1@O7 "sandwich" pairs of Y2@O7 polyhedra (see Figure , a), and these layers are arranged on top of each other forming columns along the c axis. Surprisingly, such layered packing resembles those observed for known high-temperature and high-pressure modifications of yttria which adopt the Sm2O3 and Gd2S3 structure types (Figure ). The latter structures also feature broader ranges of Y-O distances of 2.188-2.699 Å and 2.187-2.789 Å for Sm2O3and Gd2S3-type yttria, respectively, which are comparable to the distances in the structure of Ba5Y13[SiO4]8O8.5. The sevenfold-coordinated atoms of yttrium have been also observed in the structure of another mixed oxide-silicate BaY16Si4O33, but in that case they are distributed within the structure alternating with the sixfold-coordinated sites. Condensed motifs of edge-sharing Y1@O7 / Y2@O7 / Y1@O7 polyhedra resemble the fragments of the structures of high-pressure and high-temperature modifications of yttria: (b) Gd2S3 structure type of Y1@O8 / Y2@O7 alternating layers and (c) Sm2O3 structure type with (Y2@O7 + Y3@O6) / Y1@O7 / (Y2@O7 + Y3@O6) layers. The diversity of coordination in the structure of Ba5Y13[SiO4]8O8.5 indicates the potential of this structure for function incorporation. The half-occupied O7 atoms signal the possibility of ionic conductivity induced by this disorder. The presence of rigidly-coordinated yttrium and barium sites indicates that this material has a potential as a phosphor host similarly to many other known silicates.