S. Cottenier, S. Demuynck, D. Kaczorowski, J.C. Spirlet, J. Meersschaut, M. Rots Journal of Physics: Condensed Matter 10 (37), 83811998A1Published while none of the authors were employed at the CMMDOI http://dx.doi.org/10.1088/0953-8984/10/37/022