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Synthesis and crystal structure of a new microporous silicate with a mixed octahedral-tetrahedral framework: Cs₃ScSi₈O₁₉

Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstraße 14, A-1090 Wien, Austria

^* E-mail: uwe.kolitsch{at}univie.ac.at

During investigations of the system Sc₂O₃-Al₂O₃-TiO₂-SiO₂, a new, unusual microporous compound, Cs₃ScSi₈O₁₉, was synthesized as colourless plates from a CsF-MoO₃ flux. The crystal structure was determined from single-crystal X-ray diffraction data (Mo-K radiation, CCD area detector). The compound is orthorhombic, space group Pnma, with a = 11.286(2), b = 7.033(1), c = 26.714(5) Å, and Z = 4 (R1(F) = 2.6% and wR2_all(F²) = 7.3%, using 3066 ‘observed’ reflections with F_o > 4(F_o)). The crystal structure of Cs₃ScSi₈O₁₉ represents a new microporous framework structure type (‘MCV-1’), and the compound is exceptional in being the first representative of a mixed octahedral-tetrahedral framework structure, in which the [TO₄]:[MO₆] ratio is >6:1. The structure is based on isolated, nearly regular ScO₆ octahedra [d_av(Sc–O) = 2.112 Å] sharing corners with SiO₄ tetrahedra to form an open framework with four-, six- and eight-membered rings; the latter are formed by SiO₄ tetrahedra only. Two fully occupied Cs positions are located in large framework voids close to the six-membered rings, whereas four partly occupied and disordered Cs positions are close to very large framework voids bordered by the puckered eight-membered rings. The cavities are linked into channels parallel to [100] and [010]. The structure is compared with that of Cs₂TiSi₆O₁₅ and related microporous scandium-, REE-, titano- and zirconosilicate minerals and compounds. Cs₃ScSi₈O₁₉ or derivatives may be important in the context of immobilization of radioactive ¹³⁷Cs waste, cationic conductivity or catalysis.

KEYWORDS: caesium scandium silicate (Cs₃ScSi₈O₁₉), crystal structure, microporous, eight-membered rings, disorder