Graduate Thesis Or Dissertation


Synthesis and investigation of layered oxides with honeycomb ordering Public Deposited

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  • Synthesis and discovery lies at the core of every field in chemistry. The investigation of the structure-property relationships in compounds known or new is the fundamental purpose of solid state chemistry. Mixing related solid state compounds to form solid solutions can lead to exciting new and/or unexpected properties. Layered compounds are used in everyday technology, especially batteries. A renewed interest in layered oxides with honeycomb ordering has arisen in the past two years with the discovery of many new compositions as well as the reinvestigation to clarify the structure and properties of previously reported compounds. In this work the discovery and characterization of 7 new compounds is presented and discussed. One family of solid solutions that was investigated have the compositions Na₃M[₂-x]M'[subscript x]SbO₆ where M, M' = Cu, Mg, Ni, Zn. All compositions crystallize in the C2/m space group and contain a honeycomb ordering within the M²⁺/Sb⁵⁺ edge-sharing octahedra. X-ray diffraction verified lattice parameter trends based on the ionic radii of the M²⁺ cations however, the Jahn-Teller active Cu²⁺ ion creates larger differences than predicted. The antiferromagnetic order in Na₃Ni₂SbO₆ and the spin gap magnetic behavior of Na₃Cu₂SbO₆ are suppressed upon substitution with the nonmagnetic Zn²⁺ and Mg²⁺ cations. Estimated band gaps for these compounds are determined from diffuse reflectance measurements. Two compositions that were discovered during this work were Na₃LiFeSbO₆ (space group C2/c) and the ion exchange delafossite Ag₃LiFeSbO₆ (space group P3112. The space group index of Na₃NaFeSbO₆ was also reinvestigated. These compounds are related to the previous materials with a lamellar structure with Na⁺ filling interlayer space between Li₁/₃Fe₁/₃Sb₁/₃O₂ slabs. A high degree of stacking faults was present in the X-ray diffraction patterns preventing full structural characterization. Magnetic susceptibility verified the presence of high spin Fe³⁺. In order to determine some information regarding the arrangement of cations within the slabs 57Fe Mössbauer spectroscopy was employed. The new layered compound Li₃Ni₂BiO₆ was discovered as well as the partially substituted phases Li₃NiM'BiO₆ (M' = Mg, Cu, Zn). These compounds crystallize in the monoclinic C2/m space group. These compounds have a lamellar structure with a honeycomb ordering between the Ni²⁺ and Bi⁵⁺ within the slabs which are separated by Li⁺ ions filling the interlayer space. Magnetic susceptibility measurements indicate paramagnetic behavior of all the compositions at high temperature and only the parent compound Li₃Ni₂BiO₆ contains an antiferromagnetic ordering at 5.5 K. Topotactic molten salt ion exchange was used to synthesize the new delafossite Ag₃Ni₂BiO₆. Compositions of Li₃Ni₂BiO₆ and the exchange compound Ag₃Ni₂BiO₆ were verified using inductively coupled plasma – atomic emission spectroscopy. Solid solution studies were also performed as part of this work where structural transitions and property modifications were observed for two families of compounds. In the family of P2-Na₂M₂TeO₆ (M = Co, Ni, Zn), three full solid solutions were prepared. These too contain a M²⁺/X⁶⁺ honeycomb ordering within slabs of edge-sharing octahedra which are separated by nonstoichiometric filling of Na⁺ in trigonal prismatic interlayer sites. All compounds crystallize in a hexagonal unit cell however, a different stacking sequence is observed for the nickel (space group P6₃/mcm) containing composition compared to the zinc and cobalt compositions (space group P6₃22) causing a structural transition to occur when nickel is substituted by zinc or cobalt which was monitored using X-ray diffraction. Magnetic susceptibility indicates high temperature paramagnetic behavior in all compositions with low temperature antiferromagnetic transitions in the compounds containing nickel or cobalt. This transition is suppressed upon zinc substitution. An interesting color transition from pink to light green to white was observed for M = Co, Ni, Zn, respectively, and was examined using diffuse reflectance spectroscopy.
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