1. Department of Chemistry, Faculty of Sciences, University of Douala, P.O. Box 24157, Douala, Cameroon
2. Department of Physical Sciences, Kaimosi Friends University, P.O. Box 385-50309, Kaimosi, Kenya
3. Institut für Mineralogie, Leibniz Universität Hannover, Callinstrasse 3, D-30167, Hannover, Germany
4. Department of Process Engineering, Saint Jerome Catholic University Institute, Av. Akwa Koumassi, Douala, Cameroon
5. Institut für Anorganische Chemie und Strukturchemie, Universität Düsseldorf, Universitätsstr. 1D-40225, Düsseldorf, Germany