CRYSTAL-STRUCTURE AND COMPLEX MAGNETIC-BEHAVIOR OF A NOVEL URANIUM OXYPHOSPHIDE UCUPO

The crystal structure of UCuPO was determined from single-crystal X-ra y data and refined to a residual of R = 0.033 for 192 structure factor s and 12 variables. It has a tetragonal ZrCuSiAs-type structure: P4/nm m, a = 379.3(1) pm, c = 823.3(2) pm, Z = 2. Magnetisation and magnetic susceptibility measurements of a powder sample in the range 5-300 K s how UCuPO to order antiferromagnetically at T-N = 220+/-2 K. Its chi(T ) dependence appears to be highly anomalous with a strong upturn at lo w temperatures. The close relationship between UCuPO and U2Cu2P3O with respect to their crystal structures and their magnetic behaviour is d iscussed.