Crystal structures of K-and Cs-exchanged forms of zorite

The crystal structures of K-and Cs-exchanged forms of zorite were studied by X-ray diffraction and IR spectroscopy: K_4.75Na_1.82[Ti(Ti_0.79Nb_0.20)₄Si₁₂O₃₄(O,OH)_5.2] × 10.62 H₂O (sp. gr. Cmmm, R= 0.0481 for 516 independent reflections) and Cs_4.34Na_1.90[Ti(Ti_0.80Nb_0.18)₄Si₁₂O₃₄(O,OH)₅] × 5.37 H₂O (sp. gr. Cmmm, R = 0.0285 for 621 independent reflections). Both structures retain the mixed polyhedral framework of zorite: Na₆Ti(Ti,Nb)₄(Si₆O₁₇)₂(O,OH)₅ × nH₂O, where n ～ 11. It is shown that the positions of the atoms located in the cavities of the frameworks of these compounds differ from those in the structures of zorite and its synthetic analogs.     

Crystal structure of byelorussite-(Ce) NaMnBa<Subscript>2</Subscript>Ce<Subscript>2</Subscript>(TiO)<Subscript>2</Subscript>[Si<Subscript>4</Subscript>O<Subscript>12</Subscript>]<Subscript>2</Subscript>(F,OH) · H<Subscript>2</Subscript>O

The crystal structure of the mineral byelorussite-(Ce) NaMnBa₂Ce₂Ti₂Si₈O₂₆(F,OH) · H₂O belonging to the joaquinite group was solved and refined to R = 0.033 based on 4813 reflections with I > σ2(I). The parameters of the orthorhombic unit cell are a = 22.301(4) Å, b = 10.514(2) Å, c = 9.669(2) Å, V = 2267.1(8) ų, sp. gr. Ama2, and Z = 4. The structure is composed of three-layer sheets, which consist of dimers of edge-sharing Ti octahedra located between isolated four-membered [Si₄O₂] rings. The sheets are linked to each other by Mn 5-vertex polyhedra to form a heteropolyhedral framework. Large cavities in the framework are occupied by Na 6-vertex polyhedra, Ba 11-vertex polyhedra, and REE 9-vertex polyhedra.     

Variations on the theme of closest packing: The structural chemistry of barium titanate compounds

The crystal structures of most barium titanates can be described as hexagonal closest packings consisting of atoms of Ba and O and of vacancies between them. The pseudohexagonal cell constants of these compounds are close to the ideal hexagonal values. The most common stacking is the six-layer sequence (hcc)₂. The mean diameter of a closest packed particle in these packings is 2.85 Å, while the mean thickness of a layer is 2.33 Å. The recognition of this closest packing principle has been helpful in the solution of many of the crystal structures of this group. With the exception of BaTi₄O₉, all the barium titanates which are usually classified as tunnel structures or Wadsley-Andersson phases can be described as cubic closest packings. The deviations of the pseudocubic phases from the ideal cubic values are larger than in the pseudohexagonal cases. A few of the barium titanates are related to the so-called 3.0-Å phases based on the rutile-type octahedral chain. These structures are characterized by having two almost closest packed corrugated layers at right angles to each other. The structures of a number of compounds in which the Ti⁴⁺ atoms are replaced partly by Ti³⁺, Al, Pt⁴⁺, or Li are based on principles similar to those of the barium titanates proper. The mean TiO distances in the coordination octahedra of the barium titanates depend strongly on the distortions of the octahedra.     

Crystal structure of Pb-exchanged form of zorite

The crystal structure of a Pb-exchanged form of zorite is studied by X-ray diffraction: Pb_3.95(Ca_0.1Sr_0.05)[Ti(Ti_0.80Nb_0.20)₄Si₁₂O₃₈(OH)] · 9.52H₂O (sp. gr. Cmmm, R = 0.0530 for 680 independent reflections). The structure retains the mixed polyhedral framework of zorite, Na₆[Ti(Ti,Nb)₄(Si₆O₁₇)₂(O,OH)₅] · 11H₂O. This framework is composed of xonotlite-like [Si₆O₁₇] ribbons linked to each other by columns of vertex-sharing (Ti,Nb)O₆ octahedra and isolated TiO₅ half-octahedra. Lead atoms in the Pb-exchanged form occupy one site, unlike Cs cations in the Cs-exchanged form of zorite, which are strongly disordered and partially occupy eight positions. The position of Pb²⁺ cations corresponds to the Na(2) position in the zorite structure, the Sr position in the Sr-exchanged form of ETS-4, and the K position in the K-exchanged form and is similar to the position of the water molecule W(3) in the structure of the Cs-exchanged form of zorite.     

Nachweis und Bestimmung von Nitraten in der Milch

Ohne Zusammenfassung