Oxides of Tetravalent Elements

IR spectra clearly distinguish the three zirconia polymorphs [57]. The high-temperature cubic phase has only one IR active skeletal mode, found near 550 cm for powders, while tetragonal zirconia (the medium-temperature polymorph) has 

One fundamental mode is expected to be very weak near 130cm , and this could not be detected. 

Silicalite-1 is the totally siliceous form of the zeolite MFl (ZSM-5), another silica polymorph. It belongs to the P2iln = Clh monoclinic space group (n. 14) with Z = 96. It transforms into an orthorhombic structure, belonging to the Pnma = Dlt space group (n. 62) between 350 and 363 K. Substituted silicalites such as ZSM-5 zeolite (see below) and Ti-silicalite adopt the orthorhombic structure even at room temperature, the transition temperature being strongly shifted to lower temperatures. 

Factor group analysis [58] indicates that 429 IR active modes and 432 Raman active modes are expected for the monoclinic structure and 323 IR active modes and 432 Raman active modes are expected for the orthorhombic form. Only 16 components in the IR specrum and 11 components in the Raman spectrum are observed because of the superimposition of many of the expected fundamentals. However, a careful observation of the spectra, with the help of analysis of the perturbations arising from isomorphic substitution and with the aid of derivative spectra, showed the presence of a great number of very weak components (shoulders) in the spectra. 

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Ternary and polynary oxides of trivalent to hexavalent americium are obtained by solid-state reactions of Am20s and Am02 with the oxides of various elements. Compounds of pentavalent and hexavalent americium, which are iso-structural with the corresponding ternary oxides of Pa, U, Np, and Pu, are formed only with the oxides of alkali metals and alkaline earth metals. By reaction of Am02 with oxides of tetravalent elements—e.g., SiO, ZrO, or... 

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