Synthesis and crystal structure of a new microporous silicate with a mixed octahedral-tetrahedral framework: Cs 3 ScSi 8 O 19
2004; Cambridge University Press; Volume: 68; Issue: 4 Linguagem: Inglês
10.1180/0026461046840212
ISSN1471-8022
Autores Tópico(s)Zeolite Catalysis and Synthesis
ResumoAbstract During investigations of the system Sc 2 O 3 -Al 2 O 3 -TiO 2 -SiO 2 , a new, unusual microporous compound, Cs 3 ScSi 8 O 19 , was synthesized as colourless plates from a CsF-MoO 3 flux. The crystal structure was determined from single-crystal X-ray diffraction data (Mo- K α 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 ( R 1(F) = 2.6% and w R 2 all ( F 2 ) = 7.3%, using 3066 ‘observed’ reflections with F o > 4σ( F o )). The crystal structure of Cs 3 ScSi 8 O 19 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 [ T O 4 ]:[ M O 6 ] ratio is >6:1. The structure is based on isolated, nearly regular ScO 6 octahedra [ d av (Sc—O) = 2.112 Å] sharing corners with SiO 4 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 Cs 2 TiSi 6 O 15 and related microporous scandium-, REE -, titano- and zirconosilicate minerals and compounds. Cs 3 ScSi 8 O 19 or derivatives may be important in the context of immobilization of radioactive 137 Cs waste, cationic conductivity or catalysis.
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