Crystal field Superposition Model analysis of 3dN transition metal ions as a basis for application in mineralogy.

Research project P 13976 Superposition Model analysis for application in mineralogy Manfred WILDNER 11.10.1999 Crystal field theory (CFT) has met with widespread applications and successful interpretations in geosciences for a wide range of transition metal ion bearing minerals, explaining their anomalous geochemical behaviour and crystal chemical, thermodynamic, magnetic, and spectral properties. Generally, crystal field data are extracted from absorption measurements in the NIR, VIS, and UV spectral regions. In applying CFT, several approximations, especially concerning the calculation of the so called crystal field stabilisation energy (CFSE) of the transition metal in the respective structural site, were made. Mostly a higher pseudo-symmetry describing the spectra of transition metal ions is used. Compared to `real` coordination polyhedra in minerals the assumption of a regular coordination polyhedron is a crude oversimpification. Depending on the extent of distortion of the coordination polyhedra, subsequent quantitative conclusions on crystal chemistry based on the derived CFSE become doubtful. Thus, often only qualitative statements can be made and important crystal chemical information is lost. Consequently, a formalism is needed taking individual interatomic metal-ligand distances and bond angles into account. The Superposition Model (SM) provides such a formalism. A major problem for the SM calculations is the obvious lack of empirical or theoretical ab-initio data of the intrinsic parameters Bk and the power law exponents tk. Therefore, the necessary crystal field parameters of various transition metal ions have to be determined precisely using well characterised phases. Hence, our approach aimes at the determination of reliable SM parameters for several 3dN ions on the basis of the exact polyhedral geometry extracted with high accuracy from single crystal structure investigations, i.e. the structural information is obtained from endmember phases. The obtained parameters will then be transferred to and used as starting values for structurally less well defined, i.e. natural, systems. Single crystals of 3dN transition ion bearing phases will be studied. Both, natural samples close to endmember composition as well as synthetic material are subject of the investigations. Syntheses experiments on transition ion oxo-salt systems will be performed at hydrothermal conditions. The crystal structures of the respective samples will be determined using single crystal X-ray diffraction methods, resulting in polyhedral geometries of very high accuracy. These serve as input data for the calculation of the crystal field parameters Bkq in terms of the SM. Due to the small size of available single crystals in most crystal chemical systems, microscope-spectrometric techniques have to be employed. In order to obtain polarised single crystal spectra, oriented crystals slabs of appropriate thickness have to be prepared. The absorption bands will be assigned to the respective transitions, taking the local symmetry determined by the X-ray investigations as well as electronic selection rules into account. Using the crystal field computer program package HCFLDN2 (Y.Yeung) and some supplementary programs, the intrinsic crystal field parameters Bk, power law exponents tk, and interelectronic repulsion parameters Racah B and Racah C will be fitted to the absorption spectra. These data serve as a basis for calculating the CFSE of a given transition metal ion for any site symmetry and geometry. The extracted parameter sets will be applied to relevant systems in geosciences. The semi-empirical approach described above is currently the best available atomistic model. However, within the project proposed here, we plan to continuously compare our results with ab-initio calculations based on the Density Functional Theory (DFT). This will be done in a cooperation with B. Winkler and his co-workers at the `Computational Crystallography` group in Kiel/Germany.