The crystal structure of some plutonium borides

Abstract

448 diborides. Lattice parameters and interatomic distances of this series are listed in Table 1. A notable feature of the series is the unusually large metal-boron separation in both PuB~ and UB 2. For the transition-metal di-betides the experimental metal-boron distance exceeds the 'contact' distance (i.e. the sum of the close packed radii) by only 0.06-0.08 /1, whereas in UB 2 and PUB2, the measured metal-boron distances exceed the calculated distances by 0.34, and 0.21 /~ respectively. A tetraboride phase was formed at 1200 °C. when the atomic percentage of boron was greater than 70. The proportion of this phase formed increased rapidly with boron content up to 85 at.% boron, but thereafter decreased with increasing boron content. The PuB 4 phase is isomorphous with UB 4 and is tetragonal, with a =7.10 and c =4.014 A. The structure of UB 4 is given by Zalkin & Templeton (1950) who showed the space group to be P4/mbm and the atomic positions: Metal in ± (0.31, 0.81, 0; 0-21, 0.31, 0). 4 B I in ± (0, 0, 0.2; 0.5, 0.5, 0.2). 4B II ±(0.1, 0.6, 0.5; 0.4, 0.1, 0.5). 8 B III in ±(0.2,0.04,0.5; 0.7,0.46,0.5; 0.04,0.2,0.5; 0.54, 0.7, 0.5). Using the lattice parameters found for plutonium tetraboride, the metal-boron distances are PuB I 2.77, SHORT COMMUNICATIONS PuB II 2"84, PuB III 2"47 A i.e. the plutonium and the BIII boron atoms are in contact. Plutonium hexaboride was also formed at 1200 °C. when the amount of boron present exceeded 70 at.%. The proportion of the hexaboride formed increased steadily with increasing boron content. The structure is simple-cubic CaB6-type and is isomorphous with ThB6. With metal atoms at (0, 0, 0) and boron atoms at ± [(½, ½, 0.21), (½, 0.21,-lo), (0-21, ½, ½)], the plutonium-boron distance is 2.92 A, whereas the contact distance is 2.47 A. The boron atoms are in close contact since the boron-boron distance is 1.75 A. The lattice parameter varies from 4-115 ±0.001 to 4.140 ±0.001 A, suggesting that the hexaboride phase is not stoichiometric, and that excess boron is present in the structure. We should like to thank Mrs G. Tyson for her help with this work and the Managing