1. a Laboratoire de Cristallographie-CNRS , BP 166, 38042, Grenoble, France
2. b Department of Geology and Geophysics , Univ. of California , Berkeley, CA, 94720, USA
3. c GEC-Marconi Materials Technology Limited , Northamptonshire, NN12 8EQ, U. K
4. d Defence Research Agency , Malvern, Worcestershire, U. K
5. e Department of physics , Hong Kong Univ. of Science and Technology , Hong Kong
6. f School of Industrial and Manufacturing Science, Cranfield Univ. , Cranfield, Bedfordshire, MK430AL, U. K.