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Because of their optical, photo-luminescence (PL), and afterglow properties, barium titanosilicates are compounds of great interest for functional materials and light-emitting devices. Among them, BaTiSi2O7 (BTS2) and BaTiSi4O11 (BTS4) are the most less known and most intriguing ones; they display peculiar properties (e.g. PL orange emission for the BTS2 compound) whose exhaustive explanation was hampered to date by the lack of structure models. In this work, BTS2 and BTS4 were synthesized through conventional solid-state reaction methods. Their structure solution and Rietveld structure refinement were attempted using synchrotron powder diffraction. BTS2 resulted to be an intergrowth of monoclinic and triclinic crystals. The monoclinic phase has space group P21/n and unit cell a = 7.98355(31) Å, b = 10.00843(37) Å, c = 7.47952(27) Å, and β = 100.321(3)° whereas the triclinic phase has space group P21/n and unit cell a = 7.99385(4) Å, b = 10.01017(5) Å, c = 7.47514(3) Å, α = 90.084(8)°, β = 100.368(8)°, and γ = 89.937(9)°. For BTS4, a preliminary structural model is composed of a framework of Si-Ba-Ti atoms but it must be confirmed by a robust Rietveld refinement. Notwithstanding, BTS4 has monoclinic symmetry with space group P21/c and unit cell a = 17.23804(33) Å, b = 10.04937(39) Å, c = 10.51313(37) Å, and β = 91.563(2)°. With the aim to clarify the existing theory for the explanation of PL in BTS2 and to obtain more information about the structure of both compounds, the coordination environment around Ti4+ was also investigated with X-ray absorption near edge structure spectroscopy and micro-Raman spectroscopy. The presence of VTi in TiO5 pyramidal units with one short Ti—O bond involving the apical oxygen was detected in both compounds. Interpretation of the vibrational signal from the silicate framework suggested that BaTiSi4O11 is a metasilicate containing building
units of SiO4 tetrahedra larger than those displayed by barium titanosilicates. These esults
confirmed the presence of 5-fold coordinated Ti-atoms in a distorted pyramidal configuration for BTS2 and BTS4. The proposed solution supports existing theories for the explanation of PL orange colour in BTS2.