Preparation, Crystal Structures and Properties of Two Modifications of UCr6P4

UCr6 P4 was prepared from a tin flux in two forms at low (a) and high (ß) temperatures (880 °C and 1000 °C), respectively. The crystal structures o f both modifications were deter­ mined from single-crystal data. a-U Cr6 P4: P6m2 (N o. 187), a = 698.5(3) pm, c = 350.8(1) pm, Z = 1, R = 0.052 for 18 variable parameters and 410 structure factors; /?-UCr6 P4: 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 o f both modifications are closely re­ lated, 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. H alf o f 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 at­ oms are located in trigonal prisms o f metal atoms with two or three additional metal atoms outside the rectangular faces o f the prisms. Both modifications o f UCr6 P4 show relatively high, almost temperature independent paramagnetism, as is frequently observerd for intermetallic phases o f uranium.