Franck Wastin

Plutonium-based superconductivity with a transition temperature above 18 K

Plutonium is a metal of both technological relevance and fundamental scientific interest. Nevertheless, the electronic structure of plutonium, which directly influences its metallurgical properties, is poorly understood. For example, plutoniums 5f electrons are poised on the border between localized and itinerant, and their theoretical treatment pushes the limits of current electronic structure calculations. Here we extend the range of complexity exhibited by plutonium with the discovery of superconductivity in PuCoGa5. We argue that the observed superconductivity results directly from plutoniums anomalous electronic properties and as such serves as a bridge between two classes of spin-fluctuation-mediated superconductors: the known heavy-fermion superconductors and the high-Tc copper oxides. We suggest that the mechanism of superconductivity is unconventional; seen in that context, the fact that the transition temperature, Tc ≈ 18.5u2009K, is an order of magnitude greater than the maximum seen in the U- and Ce-based heavy-fermion systems may be natural. The large critical current displayed by PuCoGa5, which comes from radiation-induced self damage that creates pinning centres, would be of technological importance for applied superconductivity if the hazardous material plutonium were not a constituent.

Low Temperature Heat Capacity of Nd2Zr2O7 Pyrochlore.

Abstract Heat capacity of neodymium zirconate (Nd 2 Zr 2 O 7 ) with pyrochlore structure was measured by adiabatic calorimetry and the hybrid adiabatic relaxation method in the temperature range (0.45 to 400)xa0K. Its excess component was obtained by comparison with the heat capacity of the lanthanum zirconate. A thermal anomaly was observed below T =7.2xa0K. From the heat capacity measurements, the thermodynamic functions of Nd 2 Zr 2 O 7 were determined.

Preparation and photoelectron spectroscopy study of UNx thin films

Abstract Thin films of UN and U 2 N 3 were prepared by reactive DC sputtering of U in N 2 -containing atmosphere. The composition of the films was modified by varying the partial pressure of N 2 . 4f-Core-level photoelectron spectra as well as valence-band spectra obtained with HeII and HeI photoexcitation confirm the itinerant character of the 5f-electronic states in UN, showing a high density of states at the Fermi energy. The 4f peaks in U 2 N 3 are shifted towards higher binding energy and are symmetric, indicating a low density of states at the Fermi level. Valence-band spectra show indeed a maximum of 5f emission at 0.8 eV below the Fermi level, but some 5f intensity at the Fermi level is preserved, contradicting full 5f localisation.

Thermodynamic properties of CaTh(PO4)2 synthetic cheralite

Abstract The mineral cheralite [CaTh(PO4)2] allows for the incorporation of tetravalent actinides in monazitebased crystalline phases. Experimental determination of its thermodynamic properties is crucial for defining its stability and subsequent long-term ability to immobilize radionuclides. Low-temperature heat capacity from 0.5 to 300 K, enthalpy increments from 485 to 1565 K, and the enthalpy of formation of cheralite from the oxides were measured and reported on for the first time. At 298.15 K, S° = (201.6 ± 2.6) J/(K·mol), which includes the configurational entropy of Ca and Th mixing, ΔHfox = -(506.4 ± 9.5) kJ/mol, ΔHfel = -(3872.8 ± 10.2) kJ/mol, ΔGfox = -(501.6 ± 9.6) kJ/mol, and ΔGfel = -(3635.5 ± 10.2) kJ/mol. In aqueous environments, cheralite is able to form from whitlockite or apatite and thorianite. Under anhydrous conditions, cheralite can form by solid-state reaction only if the resultant product includes very stable Ca salts instead of CaO.

Thermodynamic Properties of Uvarovite Garnet (Ca3Cr2Si3O12)

Abstract The low-temperature heat capacity of uvarovite (Ca3Cr2Si3O12) was measured between 2 and 400 K, and thermochemical functions were derived from the results. The measured heat-capacity curve shows a significant lambda-shaped anomaly peaking at around 9 K. The nature of this transition is unknown. From our data, we suggest a standard entropy for uvarovite at 298.15 K of 320.9 ± 0.6 J/(mol·K).

Electrical resistivity of AnT2Al3 (An = Np, Pu; T = Ni, Pd)

Structural data and resistivity studies of NpNi 2 Al 3 , Np x U 1−x Pd 2 Al 3 (x = 1, 0.5, 0,3, 0.1) and PuPd 2 Al 3 are reported. The resistivity curves display Kondo features and clear anomalies at the magnetic transition. Moreover, at least two kinds of magnetic structure exist in these compounds, one favouring the electrical conductivity and the other decreasing it

Structural chemistry of the neptunium–silicon binary system

Abstract The structural chemistry of the complete binary neptunium–silicon system has been investigated by means of X-ray diffraction. This system is characterised by the existence of six different compounds, whereas only three were previously reported in the literature. The formation and the crystal structure of the NpSi 3 compound (AuCu 3 type) and of the richest in neptunium binary Np 3 Si 2 compound (U 3 Si 2 type) were confirmed. The ThSi 2 type of compound was also confirmed but, as usually observed in the rare-earth or actinide systems, with a slight defect on the silicon site. The hexagonal AlB 2 type compound was isolated for the first time, showing, as expected, a higher silicon defect than in the ThSi 2 type of compounds. The last phase observed in this system was found to crystallise with the NpSi composition showing a polymorphic transformation. Depending on the annealing process a primitive orthorhombic FeB type, as observed with the rare-earth systems, or a C-centered orthorhombic new crystal structure type are obtained. These new results are described and compared with the recently revised Np–Ge and uranium homologue systems.

Self-irradiation effects in plutonium alloys stabilized in the δ-phase

Plutonium aging leads to the creation of decay products, such as americium, uranium, helium, but also to self-irradiation defects such as vacancies and interstitials. These self-irradiation defects can affect the properties of plutonium; as an example, swelling of PuGa or PuAl alloys has been observed at room temperature. This work is devoted to the study of the effect of aging on the magnetic susceptibility of pure and alloyed plutonium. An anomaly has been revealed at low temperatures (T < 75K) for magnetic fields lower than 4.5 T. This anomaly decreases after heat treatment. A tentative explanation, based on results from molecular dynamics calculation in δ-plutonium, is given to explain the origin of the anomaly.

Unusual behaviour of (Np,Pu)B2C

Two transuranium metal boron carbides, NpB2C and PuB2C have been synthesized by argon arc melting. The crystal structures of the {Np,Pu}B2C compounds were determined from single-crystal X-ray data to be isotypic with the ThB2C-type (space group , a = 0.6532(2) nm; c = 1.0769(3) nm for NpB2C and a = 0.6509(2) nm; c = 1.0818(3) nm for PuB2C; Z = 9). Physical properties have been derived from polycrystalline bulk material in the temperature range from 2 to 300 K and in magnetic fields up to 9 T. Magnetic susceptibility and heat capacity data indicate the occurrence of antiferromagnetic ordering for NpB2C with a Neel temperature TN = 68 K. PuB2C is a Pauli paramagnet most likely due to a strong hybridization of s(p,d) electrons with the Pu-5f states. A pseudo-gap, as concluded from the Sommerfeld value and the electronic transport, is thought to be a consequence of the hybridization. The magnetic behaviour of {Np,Pu}B2C is consistent with the criterion of Hill.

Ferro- and antiferro-magnetism in (Np, Pu)BC

Two new transuranium metal boron carbides, NpBC and PuBC, have been synthesized. Rietveld refinements of powder XRD patterns of {Np,Pu}BC confirmed in both cases isotypism with the structure type of UBC. Temperature dependent magnetic susceptibility data reveal antiferromagnetic ordering for PuBC below TN = 44 K, whereas ferromagnetic ordering was found for NpBC below TC = 61 K. Heat capacity measurements prove the bulk character of the observed magnetic transition for both compounds. The total energy electronic band structure calculations support formation of the ferromagnetic ground state for NpBC and the antiferromagnetic ground state for PuBC.