One of us (R.H.) would like to thank the hospitality shown to him during his stay at Rennes. We are grateful to Dr. M. Potel for his help in refining the crystal structure and to Dr. J. Rossat-Mignod and Dr. R. Toumier for useful and encouragingd iscussions.T his research was supportedb y the French Direction des Recherchese t Etudes Techniques under Contract 841 109;