PREPARATION, CRYSTAL-STRUCTURES AND PROPERTIES OF 2 MODIFICATIONS OF UCR6P4

UCr6P4 was prepared from a tin flux in two forms at low (alpha) and high (beta) temperatures (880-degrees-C and 1000-degrees-C), respectively. The crystal structures of both modifications were determined from single-crystal data. Alpha-UCr6P4: P6m2BAR (No. 187), a = 698.5(3) pm, c = 350.8(1) pm, Z = 1, R = 0.052 for 18 variable parameters and 410 structure factors; beta-UCr6P4: Pmmn (No. 59), a = 698.6(1) pm, b = 350.85(4) pm, c = 1196.1(2) pm, Z = 2, R = 0.047 for 21 variables and 656 structure factors. Although the lattice constants of both modifications are closely related, the two forms can be transformed into each other only by a very sluggish, reconstructive phase transformation. Nevertheless, both structures have very similar coordination polyhedra. The U atoms have 6 P neighbours in trigonal prismatic arrangement. Half of the Cr atoms have tetrahedral, the other half square pyramidal P coordinations. As is typical for phosphides with high metal content, all metal atoms additionally have many metal neighbours. The P atoms are located in trigonal prisms of metal atoms with two or three additional metal atoms outside the rectangular faces of the prisms. Both modifications of UCr6P4 show relatively high, almost temperature independent paramagnetism, as is frequently observerd for intermetallic phases of uranium.