Synthesis, structure, and properties of a novel family of layered transition nitride compounds.

摘要

The chemical properties of nitrogen present a number of difficulties in the synthesis of nitride compounds. Most of these compounds have small free energies of formation due in part to the strong triple bond of N2. Thus the standard solid state approach of synthesizing compounds from a mixture of their constituents at high reaction temperatures is generally precluded.;A general alternative to solid state synthesis is the molecularly mixed precursor route. The primary advantage of this approach is the control of composition. This concept has been demonstrated in this study through the use of a nonaqueous, complexed co-precipitation technique to synthesize a number of new ternary nitride compounds, including the a new nitride phase, CrWN2, and four series of substitutional variations of this compound, as wen as twelve additional new compounds crystallizing in the layered dinitride, eta-nitride, and pi-nitride crystal structures.;The layered nitride, CrWN2, is the focal point of this thesis. The details of its crystal and defect structure, synthesis, properties, and substitutional chemistry have been examined and discussed within. Briefly, this compound crystallizes in a trigonal structure and consists of alternating layers of chromium in octahedral coordination with nitrogen and tungsten in trigonal prismatic coordination with nitrogen. Microstructurally, the nitride frequently displays twinning and the formation of coherent rock salt structured intergrowth defects. As mentioned above, the layered nitride evolves from a co-precipitated precursor; a fine mixture of [Cr(NH3)6]Cl 2 and WOCl2·3NH3. Upon heat treatment in an ammonia atmosphere, the two transition metal complexes separately decompose into their non-complexed chloride state, i.e. CrCl2 and WOCl 2, before reacting with each other and the ambient ammonia atmosphere between 420°C and 570°C to form a glassy oxynitrochloride phase, CrWO 1.01N0.36Cl2.45. Around 600°C, this compound undergoes ammonolysis, forming a nanocrystalline, disordered rock salt structured phase, Cr0.5W0.5O0.32N0.68. If held isothermally at 600--775°C, beyond a given incubation time the disordered rock salt structure begins to transform to the layered dinitride phase. The kinetics of this transformation indicate that it is likely a relatively low energy two dimensional disorder-order nucleation and growth process. TEM studies confirm the order-disorder nature of the transformation.;The complexed co-precipitation technique has been found to be useful in preparing a number of substituted versions of the parent dinitride compound, CrWN2. This was done by preparing and examining four substituted variations: (1) the intersubstituted chromium or tungsten rich compounds, (2) two molybdenum-substituted series, (3) two manganese-substituted series, and (4) two lithium-substituted series. Conductivity and magnetic measurements on the parent phase, CrWN2, indicate that it is a metallic conductor and is Pauli paramagnetic over the temperature range of 10.5--300 K. (Abstract shortened by UMI.)