| Summary: | With Tm<sub>3</sub>F[Si<sub>3</sub>O<sub>10</sub>], a new representative of the <i>Ln</i><sub>3</sub>F[Si<sub>3</sub>O<sub>10</sub>] series could be synthesized by the reaction of Tm<sub>2</sub>O<sub>3</sub>, TmF<sub>3</sub> and SiO<sub>2</sub> (molar ratio: 1:1:3), applying an excess of CsBr as a fluxing agent in gas-tightly sealed platinum crucibles for eight days at 750 °C, and designed to yield Tm<sub>3</sub>F<sub>3</sub>[Si<sub>3</sub>O<sub>9</sub>] or Cs<sub>2</sub>TmF[Si<sub>4</sub>O<sub>10</sub>]. Single crystals of Tm<sub>3</sub>F[Si<sub>3</sub>O<sub>10</sub>] (monoclinic, <i>P</i>2<sub>1</sub>/<i>n</i>; <i>a</i> = 725.04(6), <i>b</i> = 1102.43(9), <i>c</i> = 1032.57(8) pm, <i>β</i> = 97.185(7)°; <i>Z</i> = 4) appear as pale celadon, transparent, air- and water-resistant rhombic plates. According to its <i>thalenite</i>-type structure, Tm<sub>3</sub>F[Si<sub>3</sub>O<sub>10</sub>] contains <i>catena</i>-trisilicate anions [Si<sub>3</sub>O<sub>10</sub>]<sup>8−</sup> and triangular [FTm<sub>3</sub>]<sup>8+</sup> cations. The three crystallographically different Tm<sup>3+</sup> cations are coordinated by seven <i>plus</i> one (Tm1) or only seven anions (Tm2 and Tm3) exhibiting a single F<sup>−</sup> anion for each polyhedron, additional to the majority of O<sup>2−</sup> anions. Furthermore, the luminescence properties of the isotypic colorless compound Y<sub>3</sub>F[Si<sub>3</sub>O<sub>10</sub>] doped with Eu<sup>3+</sup> (red emission), Tb<sup>3+</sup> (green emission) and Er<sup>3+</sup> (yellow and infrared emission), respectively, are reported in presenting their different excitation and emission spectra.
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