Transition Metal Based Zeotypes: Inorganic Materials at the Complex Oxide - Zeolite Border
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Transition Metal Based Zeotypes: Inorganic Materials at the Complex Oxide – Zeolite Border Paul F. Henry, Robert W. Hughes and Mark T. Weller University of Southampton, Highfield, Southampton, SO17 1BJ, UK. ABSTRACT The synthesis of compounds of the type CsAIIIMIVO4 and (Cs,Rb)AIIPO4 (AIII = Al, Ga, Fe; AII = Ni, Co, Cu; M = Si, Ge, Ti,), using gel decomposition techniques at high temperatures is described. Several new materials in this family have been characterised using powder X-ray diffraction and are shown to adopt the zeolite ABW framework topology. Evidence for the large monovalent cation templating the formation of the ABW framework is presented. Extension of the method to other AIII and MIV (A = Mn, Co, B, In; M = Sn, Zr) framework cations, which are known to adopt tetrahedral geometry in other structures, has proved unsuccessful. INTRODUCTION The existence of zeolites in the Li2O - Al2O3 - SiO2 - H2O system, including the ABW structure type, was first reported almost 50 years ago by Barrer [1]. However, the first structure determination of Li-ABW (LiAlSiO4.H2O), by Kerr, using powder X-ray diffraction techniques, was not carried out until the early seventies [2]. The structure was later confirmed by single crystal X-ray diffraction studies [3] and the hydrogen (deuterium) positions determined from powder neutron diffraction studies [4]. The structure can be described as being made up of 4-, 6and 8-membered rings, based on AlO4 and SiO4 tetrahedra arranged in accordance with Löwenstein’s rule, with the largest channels lying along a two fold screw axis. In Li-ABW this main channel is represented by the c axis and the Li+ counter cations occupy sites within these channels along with any zeolitic water. The zeolite ABW topology, Figure 1, is adopted by a wide range of materials, demonstrating a considerable range of chemical compositions, some of which have been tabulated by Stucky et al [5]. Examples of systems with monovalent (T+) / hexavalent (T6+) [6,7], divalent (T2+) / pentavalent (T5+) [8,9] and trivalent (T3+) / tetravalent (T4+) [10,11] tetrahedral centres have been characterised each with a monovalent counter cation (e.g. group I, Tl+ and NH4+ ions). Two fluoroberyllates have also been found to adopt this topology further expanding the compositional limits of the structure type [12,13]. The highest symmetry that is found for ABW type framework topology is Imma, however, to date only one example has been characterised, CsAlTiO4 [14]. In this case the framework cations Al and Ti are statistically disordered, probably a result of the similar ionic radii of tetrahedral Al3+ and Ti4+ (0.39 Å and 0.42 Å respectively) [15]. The majority of orthorhombic ABW materials adopt the space group Pna21, due to framework cation ordering, and monoclinic materials adopt the space group P21. Several other space groups have been found but only when the counter cation is caesium. In the cases of monoclinic symmetry the angle β deviates only slightly from orthogonal (examples range from 90 ° to 91.03 °). The non-centrosym
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