Phosphide und Phosphate des Cobalts - Kristallisation, Thermodynamik, Strukturen und Farben
Abstract
In this thesis systematic explorations into Phosphides and Phosphates of cobalt(II) have been made. Equilibrium investigations, chemical vapour transport, crystal structures of silico-phosphates and finally investigations on the electronic structure (colour) of Co(II)-phosphates were conducted.
The thermochemical characterization of the phosphides and phosphates is achieved by means of estimating thermodynamical data according to the results from equilibrium investigations of the systems Co/P/O.
The phosphides Co2P an CoP3 can be crystallized via chemical vapour transport reactions using iodine as transport agent. migration from higher to lower temperature is based on the following heterogeneous equillibria:
CoI2,l is observed as second condensed equillibrium phase besides Co2P in experiments carried out at lower temperatures (T<600°C) and with sufficiently high amounts of iodine. Depending on the transport temperature a change of the deposition sequence is observed in the chemical vapour transport of CoP3, when CoP occures as second condensed phase. Thermochemical model calculations reproduce the observed deposition rates as well as the compositions of the condensed equilibrium solids. These calculations allow a detailed description of the equllibrium gas phase in the system Co/P/I under various experimental conditions. The influence of traces of moisture on the transport behaviour will be discussed.
New silico-phosphates M2Si(P2O7)2 (M = Mn-Cu) have been synthesized and characterized (monoclinic, C2/c, isotypic to Cd2Si(P2O7)2).
The colour of anhydrous cobalt(II)-phosphates have been investigated by examination of their electronic absorption spectra. Within the parameterization scheme of the angular overlap model (AOM) understanding of the spectra is possible using chemically reasonable bonding-parameters (B, es,max). To reach a good fit between observed and calculated transition energies in model calculations (CAMMAG), consideration of an anisotropy in pi-bonding behaviour of O2- according to its coordination number is crucial ("second sphere ligand field effect"). The determined average interagtion energie es,av = 3780 cm-1 (d(Co-O) = 2,00 Å) allowes the prediction of d-electron energy levels of cobalt(II)oxo-compounds.
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