1. a Institut für Kristallographie, Rhein-Westf. Technische Hochschule , 5100, Aachen , Germany
2. b Department of Pure and Applied Chemistry , University of Strathclyde , Glasgow , G1 1XL , UK
3. d ICI Colours and Fine Chemicals , Manchester , M9 3DA , UK
4. e Department of Physics , University of Assiut , Egypt
5. f Mineralogisches Institut, Universitat Bonn , W-5300, Bonn , Germany
6. c SERC Daresbury Laboratory , Warrington , WA4 4AD , UK