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Authors: Tong, Jianwei
Title: Preparations, structures and properties of selected perovskite materials and molecular crystals
Other Titles: Darstellung, Strukturen und Eigenschaften ausgewählter Perowskit-Materialien und Molekülkristalle
Issue Date: 2010 Dissertation
Abstract: In my thesis preparative and structural studies of silver nitrides are discussed. Ag3+xN is found to adopt a perovskite structure with partially filled A site. For the preparation of such silver nitrides an ammonolysis reaction has been used, and this method has been applied for the synthesis of other noble metal nitrides, such as Pd, Au and Hg nitrides. Attempts for the preparation of ternary nitrides, e.g. silver copper nitride, have also been done, which requires AgCuF3 as a precursor. AgCuF3 and its isostructural analogue NaCuF3 crystallize in a distorted variant of the GdFeO3 type structure with symmetry. They exhibit interesting magnetic properties, and become typical 1D antiferromagnets at low temperature, correlated to cooperative Jahn-Teller effect. Cs2AgF4 crystallizes in the K2NiF4-type structure, and its magnetic and optical properties have been studied. It behaves as a 2D square-lattice Heisenberg ferromagnet, which is associated with orbital order. CsF has a strong tendency to form fluoride complexes, and includes easily molecules like H2O or Br2. We have studied the possibilities of the inclusion of tetrahedral molecules, e.g. OsO4 and P4, into the CsF lattice. From the reaction of CsF with OsO4 an oxyfluoride, Cs3OsO4F3, is formed, which contains [OsO4F2] octahedral units and [Cs2F] layers. In contrast, P4 is not included into the CsF lattice, but transformed into black amorphous phosphorus. In the last part of my thesis, structural studies have been performed on SiBr4 (m.p. 278 K), the only tetrahedral EX4 compound (E = C, Si, Ge, Sn, Pb; X = F, Cl, Br, I) for which no structural data has yet been reported. A full crystal-structure prediction of SiBr4 by lattice-energy minimizations was done in collaboration with A. Wolf from University Frankfurt am Main. The structures of two polymorphs of SiBr4, a high temperature phase (Pa-3) and a low temperature phase (P21/c) have been experimentally determined from the refinements of X-ray and synchrotron powder diffraction data as well as single crystal diffraction data. The transition temperature and halogen-halogen interactions of the experimental and calculated structures are discussed.
In dieser Dissertation werden präparative und strukturelle Untersuchungen von Silbernitriden diskutiert. Die triklinen ternären Fluoride, AgCuF3 und NaCuF3, enthalten stark Jahn-Teller verzerrte CuF6 Oktaeder und zeigen typische 1D antiferromagnetisches Verhalten bei niedrigen Temperaturen. Cs2AgF4, ein 2D Heisenberg-Ferromagnet mit quadratischem Gitter, kristallisiert in der K2NiF4-Struktur und seine magnetischen und optischen Eigenschaften werden diskutiert. Darüber hinaus haben wir die Möglichkeiten des Einschlusses von tetraedrischen Molekülen, z.B. OsO4 und P4, in das CsF-Gitter untersucht. Zum Schluß sind strukturelle Studien auf SiBr4 (m.p. 278 K) durchgeführt worden.
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