Title: Variations of Electronic Properties in High-Density Pu Systems
Citation: Plutonium Futures - the Science 2012
Publisher: Plutonium Futures 2012
Publication Year: 2012
JRC N°: JRC69396
URI: http://www.pufutures2012.co.uk
Type: Articles in periodicals and books
Abstract: Electronic properties of Pu intermetallics were traditionally understood as dominated by the Pu-Pu spacing. While very low spacing should lead to formation of a 5f band, its increase should drive the system to strong e-e correlation regime and eventually to a magnetic order. The large difference between the Sommerfeld coefficient of electronic specific heat , being 17 mJ/mol K2 for -Pu and 64 mJ/mol K2 for -Pu 1 (40 mJ/mol K2 given later) was understood in this framework. However, theoretical works over last years indicated that relatively small variations of 5f occupancy can have equally (or perhaps more) important effect on magnetic and other properties. A possibility to test whether the Pu-Pu spacing plays only a secondary role is to check properties of Pu systems with low Pu-Pu spacing for occurrence of species with high value of  and magnetic susceptibility. We investigated several U-Pu alloys, forming a high-density -phase, and showed that -values reach the range of -Pu. Magnetic susceptibility (T) even higher. Most recently we turned the attention to -Pu, which may actually be the most strongly correlated Pu phase. The monoclinic structure of -Pu cannot be stabilized to low T by doping, but we noticed that the crystal structure of Pu19Os (especially its high-T variety -Pu19Os) is almost identical as to volume and coordination. The reported room-temperature density 18.02 g/cm3 is only slightly lower than that of -Pu, if the 2 latter is corrected for the thermal expansion (18.20 g/cm3), and is very different from those of -Pu (19.82 g/cm3) or -Pu (15.92 g/cm3). This phase was found thermodynamically stable between 468 and 707 K. Below that range, the structure transforms into the other variety, -Pu19Os, with structure of slightly higher density (18.12 g/cm3). We succeeded to synthesize both phases. (T) weakly increases with decreasing T for both phases, from 8*10-9 m3/mol Pu at 300 K to 9*10-9 m3/mol Pu in the low-T limit (higher than for - and -Pu, both of 6-7*10-9 m3/mol). T- dependence of specific heat exhibits quite a non-Debye type of behaviour, similar to -Pu alloys.1,2 The lowest T achieved (<4 K) was nevertheless sufficient to extrapolate reliably to T  0. The obtained 's, (552) mJ/mol Pu K2 for -Pu19Os and (742) mJ/mol Pu K2 for -Pu19Os, are considerably higher than for -Pu. The slopes of the Cp/T vs. T2 plot, which determine the Debye temperatures, are even higher than for -Pu. It reveals D even lower than for -Pu, but the values D = 96 K for -Pu19Os and 101 K for Pu19Os in fact fall within the spread of -Pu stabilized by 6.1 % Ce (103 K) and by 8% Am (95 K). The results show that the Pu properties are not primarily controlled by the Pu-Pu spacing and the description cannot be based on the 5f-band approach only. ACKNOWLEDGMENT This work was supported by the Grant agency of the Academy of Sciences of the Czech Republic (IAA100100912), and the Grant Agency of the Czech Republic (P204/10/0330). Participation in the EC JRC-ITU Actinide User Laboratory program through the support of the EC-Transnational Access to Research Infrastructures Action of the “Structuring the European Research Area” specific program, contract RITA-CT-2006-026176, is acknowledged.
JRC Directorate:Nuclear Safety and Security

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