title: Structure and stability of BaTiSi₂O₇.
abstract: Due to 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) is certainly one of the most intriguing; it displays peculiar properties (e.g. PL orange emission) whose exhaustive explanation has been hampered to date by the lack of a structure model. In this work, BTS2 and the related compound BaTiSi4O11 (BTS4) were synthesized through conventional solid-state reaction methods. BTS2 invariably shows complex twinning patterns. Thus, its structure solution and Rietveld structure refinement were attempted using synchrotron powder diffraction. BTS2 was found to be an intergrowth of monoclinic and triclinic crystals. The monoclinic phase has the space group P21/n and unit cell a = 7.9836 (3), b = 10.0084 (4), c = 7.4795 (3) Å, and β = 100.321 (3)°, whereas the triclinic phase has the space group P\bar 1 and unit cell a = 7.99385 (4), b = 10.01017 (5), c = 7.47514 (3) Å, α = 90.084 (8), β = 100.368 (8) and γ = 89.937 (9)°. These lattices can be seen as a distortion of that of tetragonal synthetic β-BaVSi2O7 with Ti in place of V. The structure models obtained from this study confirm the presence of fivefold coordinated Ti atoms in a distorted pyramidal configuration. The proposed solution supports existing theories for the explanation of the PL orange colour in BTS2.
authors.affiliation: Institute of Theoretical and Applied Mechanics ASCR, Centre of Excellence Telč, Batelovská 485, Telč CZ-58856, Czech Republic.
authors.forename: Alberto
authors.initials: A
authors.lastname: Viani
authors.affiliation: Università di Modena e Reggio Emilia, Dipartimento di Scienze Chimiche e Geologiche, Via S. Eufemia 19, Modena I-41121, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Palermo
authors.affiliation: ENI s.p.a., Refining and Marketing Division, San Donato Milanese Research Center, Via F. Martiano 26, San Donato Milanese I-20097, Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Zanardi
authors.affiliation: Sincrotrone Trieste S.C.p.A., Strada Statale 14 - km 163,5, Basovizza I-34149, Italy.
authors.forename: Nicola
authors.initials: N
authors.lastname: Demitri
authors.affiliation: Department of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, Praha 6, 162 53, Czech Republic.
authors.forename: Václav
authors.initials: V
authors.lastname: Petrícek
authors.affiliation: Università di Modena e Reggio Emilia, Dipartimento di Scienze Chimiche e Geologiche, Via S. Eufemia 19, Modena I-41121, Italy.
authors.forename: Federico
authors.initials: F
authors.lastname: Varini
authors.affiliation: Dipartimento di Scienze della Terra, Università degli studi di Torino, Via Valperga Caluso 35, Torino I-10125, Italy.
authors.forename: Elena
authors.initials: E
authors.lastname: Belluso
authors.affiliation: Department of Chemistry, Technical University of Denmark, DTU:207, Lyngby DK-2800, Denmark.
authors.forename: Kenny
authors.initials: K
authors.lastname: Ståhl
authors.affiliation: Università di Modena e Reggio Emilia, Dipartimento di Scienze Chimiche e Geologiche, Via S. Eufemia 19, Modena I-41121, Italy.
authors.forename: Alessandro Francesco
authors.initials: AF
authors.lastname: Gualtieri
keywords: optical properties; photo-luminescence; solid-state reactions