title: Novel silicate anion: Si(8)O(22)(12-). Hydrothermal synthesis and X-ray powder structure of three new niobium silicates.
abstract: A sodium niobium(V) tetrasilicate, Na(2)H(NbO)Si(4)O(11).1.25H(2)O (1), was synthesized hydrothermally at 190 degrees C from a mixture of silicic acid, NaOH, NbCl(5), and H(2)O(2) and added hydrochloric acid. The successive treatment of 1 with nitric acid yielded HNb(H(2)O)Si(4)O(11).H(2)O (2). Contact of 2 with cesium hydroxide solutions converted it to the partially exchanged Cs(+) phase Cs(0.66)H(0.33)Nb(H(2)O)Si(4)O(11) (3). The three structures were solved from X-ray powder diffraction data. All compounds crystallize in the monoclinic space group P2(1)/m with Z = 2, containing the Si(8)O(22)(12-) anion. This new silicate anion type is related to the Si(4)O(11)(6-) unit, which is present in the amphibole series. The difference between both anion types lies in the chain periodicity: two for amphiboles or four for the new niobium silicates. These niobium silicates have framework structures enclosing tunnels in which the alkali cations reside.
authors.affiliation: Departamento de Química Física y Analítica, Universidad de Oviedo, Spain.
authors.forename: M A
authors.initials: MA
authors.lastname: Salvadó
authors.forename: P
authors.initials: P
authors.lastname: Pertierra
authors.forename: S
authors.initials: S
authors.lastname: García-Granda
authors.forename: S A
authors.initials: SA
authors.lastname: Khainakov
authors.forename: J R
authors.initials: JR
authors.lastname: García
authors.forename: A I
authors.initials: AI
authors.lastname: Bortun
authors.forename: A
authors.initials: A
authors.lastname: Clearfield