1. Catlow, C,R.A., Chadwick, A.V. and Corish, J., J. Solid State Chem. In press
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3. Bendall, P.J., D, Phil. Thesis, University of Oxford, 1980.
4. For an account of shear plane structures we refer to Bursill, L. and Hyde, B.G. Prog. in Solid State Chem, 7, 177 (1972).
5. Catlow, C.R,A, and James, R., Nature 272, 602.