PHASE RELATIONS, STRUCTURES AND BONDING FOR THE PA - H SYSTEM
The phase relations, structures and chemical properties of the Pa - H system were derived from pressure - temperature - composition data and X-ray diffraction results. Measurements were performed at temperatures in the range 373 - 795 K and at pressures from 10 mubar to 45 bar. A two-phase region exists between the hydrogen saturated metal and the composition with [H]/[Pa] = 1.3 +- 0.1. Three well defined hydride phases were identified at various temperatures. Above about 500 K a hydride with metal atoms in a complex f.c.c. arrangement (Fd3m (C15)) forms over a solid solution range corresponding to Pa-3H-4 - Pa-3H-5. This unique phase exhibits both metal - metal and metal - hydrogen bonding and has a paramagnetic moment mu-eff of 0.84 mu-B at 298 K. At higher hydrogen contents, a narrow two-phase region is followed by a cubic hydride isostructural with beta-UH-3 (the metal atoms are in the A15 arrangement) but with a wide composition range (2.0 <= [H]/[Pa] <= 3.0). The third cubic hydride, which forms rapidly below 500 K, is isostructural with a alpha-UH-3 (b.c.c. metal atoms) but exhibits an extremely wide composition range (1.3 <= [H]/[Pa] <= 3.0). The low temperature phase transforms to the high temperature modifications but the reverse processes are not observed, indicating either that the low temperature phase is metastable or that the transition from the high temperature phases to the low temperature phase is kinetically hindered. Possible temperature - composition diagrams derived from pressure - composition isotherms and diffraction results are presented. The properties of the C15 and A15 related hydrides are considered in relation to electronic structure and bonding, magnetism and the role of 5f electrons in early actinide compounds.
Bibliographic Reference: JOURNAL OF THE LESS-COMMON METALS, VOL. 100 (1984), PP. 195-214
Record Number: 1989123001500 / Last updated on: 1987-01-01
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