CRYSTAL-STRUCTURE AND MAGNETISM OF NOVEL COMPOUNDS U3(M', M'')5, M'=AL,GA, M''=SI,GE

Compounds with the formula U3M2'M3'' have been synthesized for M' = Al ,Ga and M '' = Si, Ge. The crystal structures of U3Al2Si3 and U3Al2Ge3 have been determined from single crystal X-ray counter data and were found to be isopointal with the Cr5B3 type. Atom order has been refine d for U3Al2Ge3 (a = 0.77579(13) nm, c = 1.10357(28) nm), revealing a r andom occupation for 8Ge + 8Al in the 16l sites and 4Ge in the 4c site s (space group I4/mcm-D4h18, No. 140, Z = 4). For 317 (276) reflection s (\F(O)\ > 3sigma) the obtained residual values, R(x) = SIGMA\DELTAF\ /SIGMA\F(O), were R. = 0.057 for U3(Al,Si)5 and R(x) = 0.074 for U3Al2 Ge3, respectively. Isotypism with the crystal structure of U3Al2Ge3 (C r5B3-type derivative) was confirmed from X-ray powder diffraction anal ysis for the novel compounds U3Ga2Si3 and U3Ga2Ge3. Whereas the compou nds U3Al2Ge3 and U3Ga2{Si, Ge}3 were obtained at the given stoichiomet ry, a small homogeneous range was observed for U3+y(Al1-x Si(x))5-y ra nging in as-cast alloys from x = 0.64 to x = 0.68 and 0 less-than-or-e qual-to y less-than-or-equal-to 0.04. U3(Al,Si)5 is a high-temperature compound only observed in as-cast alloys and transforms on annealing at temperatures below approximately 1000-degrees-C. For the homologous phases U3Al2Ge3 and U3Ga2(Si,Ge)3 no transformation has been observed in the range from 600-degrees-C to the melting. (C) 1994 Academic Pre ss, Inc.