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The reduction of UVI uranyl halides or amides with simple LnII or UIII salts forms highly symmetric, linear, oxo-bridged trinuclear UV /LnIII /UV , LnIII /UIV /LnIII , and UIV /UIV /UIV complexes or linear LnIII /UV polymers depending on the stoichiometry and solvent. The reactions can be tuned to give the products of one- or two-electron uranyl reduction. The reactivity and magnetism of these compounds are discussed in the context of using a series of strongly oxo-coupled homo- and heterometallic poly(f-block) chains to better understand fundamental electronic structure in the f-block.

The reduction of UVI uranyl halides or amides with simple LnII or UIII salts forms highly symmetric, linear, oxo-bridged trinuclear UV /LnIII /UV , LnIII /UIV /LnIII , and UIV /UIV /UIV complexes or linear LnIII /UV polymers depending on the stoichiometry and solvent. The reactions can be tuned to give the products of one- or two-electron uranyl reduction. The reactivity and magnetism of these compounds are discussed in the context of using a series of strongly oxo-coupled homo- and heterometallic poly(f-block) chains to better understand fundamental electronic structure in the f-block.

The physicochemical properties of the potassium neptunate K2NpO4 have been investigated in this work using X-ray diffraction, X-ray absorption near edge structure (XANES) spectroscopy at the Np-L3 edge, and low-temperature heat capacity measurements. A Rietveld refinement of the crystal structure is reported for the first time. The Np(VI) valence state has been confirmed by the XANES data, and the absorption edge threshold of the XANES spectrum has been correlated to the Mössbauer isomer shift value reported in the literature. The standard entropy and heat capacity of K2NpO4 have been derived at 298.15 K from the low-temperature heat capacity data. The latter suggest the existence of a magnetic ordering transition around 25.9 K, most probably of the ferromagnetic type.

The low-temperature heat capacity of lanthanide-doped (La, Nd) and americium-doped UO 2 samples was measured by calorimetry. The results showed a strong effect on the antiferromagnetic C p anomaly as a result of crystal lattice substitution, defects resulting from self-irradiation, and electron interaction. It was found that the substitution of trivalent ions ( La 3 + , Nd 3 + , Am 3 + ) leads to a split of the λ peak, for which we hypothesize that the clustering of the trivalent ions with the charge compensator pentavalent U 5 + can play a role. The overall behavior is confirmed by magnetization measurements using SQUID. The observed differences between La 3 + , Nd 3 + , and Am 3 + are attributed to the f – f exchange interaction. 241 Am decay causes in addition self-irradiation effects (point defects), and reduces Néel temperature, peak intensity, and magnetic entropy as a function of decay (accumulated α dose). The observed effects are similar to 238 Pu-doped UO 2 , the slight differences being attributed to a higher degree of disorder in the Am material. [ABSTRACT FROM AUTHOR]

The mineral cheralite [CaTh[(P[O.sub.4]).sub.2]] allows for the incorporation of tetravalent actinides in monazite-based 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[degrees] : (201.6 [+ or -] 2.6) J/(K x mol), which includes the configurational entropy of Ca and Th mixing, [DELTA][H.sup.ox.sub.f] = -(506.4 [+ or -] 9.5) M/mol, [DELTA][H.sup.el.sub.f] = -(3872.8 [+ or -] 10.2) kJ/mol, [DELTA][G.sup.ox.sub.f] = -(501.6 [+ or -] 9.6) M/mol, and [DELTA][G.sup.el.sub.f] = -(3635.5 [+ or -] 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. Keywords: Cheralite, actinide phosphates, thermodynamic properties, nuclear waste

Actinide +VI complexes (AnVI =UVI , NpVI and PuVI) with dipicolinic acid derivatives were synthesized and characterized by powder XRD, SQUID magnetometry and NMR spectroscopy. In addition, NpVI and PuVI complexes were described by first principles CAS based and two‐component spin‐restricted DFT methods. The analysis of the 1H paramagnetic NMR chemical shifts for all protons of the ligands according to the X‐rays structures shows that the Fermi contact contribution is negligible in agreement with spin density determined by unrestricted DFT. The magnetic susceptibility tensor is determined by combining SQUID, pNMR shifts and Evans' method. The SO‐RASPT2 results fit well the experimental magnetic susceptibility and pNMR chemical shifts. The role of the counterions in the solid phase is pointed out; their presence impacts the magnetic properties of the NpVI complex. The temperature dependence of the pNMR chemical shifts has a strong 1/T contribution, contrarily to Bleaney's theory for lanthanide complexes. The fitting of the temperature dependence of the pNMR chemical shifts and SQUID magnetic susceptibility by a two‐Kramers‐doublet model for the NpVI complex and a non‐Kramers‐doublet model for the PuVI complex allows for the experimental evaluation of energy gaps and magnetic moments of the paramagnetic center. [ABSTRACT FROM AUTHOR]

The isostructural compounds of the trivalent actinides uranium, neptunium, plutonium, americium, and curium with the hydridotris(1‐pyrazolyl)borato (Tp) ligand An[η3‐HB(N2C3H3)3]3 (AnTp3) have been obtained through several synthetic routes. Structural, spectroscopic (absorption, infrared, laser fluorescence) and magnetic characterisation of the compounds were performed in combination with crystal field, density functional theory (DFT) and relativistic multiconfigurational calculations. The covalent bonding interactions were analysed in terms of the natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) models. [ABSTRACT FROM AUTHOR]

In the present studies, the thermal behaviour of NaUO2BO3 has been investigated. This compound is a potential product of interaction between the coolant (Na), control rods (B4C), and the oxide fuel, which could form under accidental conditions in sodium-cooled fast reactors. The thermal expansion, the heat capacity, and thermal diffusivity of NaUO2BO3 have been measured. The thermal conductivity of the material is derived from these results and presented here for the first time. [ABSTRACT FROM AUTHOR]

A dramatic difference in the ability of the reducing AnIII center in AnCp3 (An=U, Np, Pu; Cp=C5H5) to oxo-bind and reduce the uranyl(VI) dication in the complex [(UO2)(THF)(H2L)] (L='Pacman' Schiff-base polypyrrolic macrocycle), is found and explained. These are the first selective functionalizations of the uranyl oxo by another actinide cation. At-first contradictory electronic structural data are explained by combining theory and experiment. Complete one-electron transfer from Cp3U forms the UIV-uranyl(V) compound that behaves as a UV-localized single molecule magnet below 4 K. The extent of reduction by the Cp3Np group upon oxo-coordination is much less, with a NpIII-uranyl(VI) dative bond assigned. Solution NMR and NIR spectroscopy suggest NpIVUV but single-crystal X-ray diffraction and SQUID magnetometry suggest a NpIII-UVI assignment. DFT-calculated Hirshfeld charge and spin density analyses suggest half an electron has transferred, and these explain the strongly shifted NMR spectra by spin density contributions at the hydrogen nuclei. The PuIII-UVI interaction is too weak to be observed in THF solvent, in agreement with calculated predictions. [ABSTRACT FROM AUTHOR]

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 anda= 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 temperatureTN= 68 K. PuB2C is a Pauli paramagnet most likely due to a strong hybridization of s(p,d) electrons with the Pu-5fstates. 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. [ABSTRACT FROM PUBLISHER]

Apart from its technological importance, plutonium (Pu) is also one of the most intriguing elements because of its non-conventional physical properties and fascinating chemistry. Those fundamental aspects are particularly interesting when dealing with the challenging study of plutonium-based nanomaterials. Here we show that ultra-small (3.2±0.9 nm) and highly crystalline plutonium oxide (PuO2) nanocrystals (NCs) can be synthesized by the thermal decomposition of plutonyl nitrate ([PuO2(NO3)2] ⋅3 H2O) in a highly coordinating organic medium. This is the first example reporting on the preparation of significant quantities (several tens of milligrams) of PuO2 NCs, in a controllable and reproducible manner. The structure and magnetic properties of PuO2 NCs have been characterized by a wide variety of techniques (powder X-ray diffraction (PXRD), X-ray absorption fine structure (XAFS), X-ray absorption near edge structure (XANES), TEM, IR, Raman, UV/Vis spectroscopies, and superconducting quantum interference device (SQUID) magnetometry). The current PuO2 NCs constitute an innovative material for the study of challenging problems as diverse as the transport behavior of plutonium in the environment or size and shape effects on the physics of transuranium elements. [ABSTRACT FROM AUTHOR]

Single-chain magnets (SCMs) are materials composed of magnetically isolated one-dimensional (1D) units exhibiting slow relaxation of magnetization. The occurrence of SCM behavior requires the fulfillment of stringent conditions for exchange and anisotropy interactions. Herein, we report the synthesis, the structure, and the magnetic characterization of the first actinide-containing SCM. The 5f-3d heterometallic 1D chains [{[UO2(salen)(py)][M(py)4](NO3)}] n, (M=Cd ( 1) and M=Mn ( 2); py=pyridine) are assembled trough cation-cation interaction from the reaction of the uranyl(V) complex [UO2(salen)py][Cp*2Co] (Cp*=pentamethylcyclopentadienyl) with Cd(NO3)2 or Mn(NO3)2 in pyridine. The infinite UMn chain displays a high relaxation barrier of 134±0.8 K (93±0.5 cm−1), probably as a result of strong intra-chain magnetic interactions combined with the high Ising anisotropy of the uranyl(V) dioxo group. It also exhibits an open magnetic hysteresis loop at T<6 K, with an impressive coercive field of 3.4 T at 2 K. [ABSTRACT FROM AUTHOR]

Discrete molecular compounds that exhibit both magnetization hysteresis and slow magnetic relaxation below a characteristic 'blocking' temperature are known as single-molecule magnets. These are promising for applications including memory devices and quantum computing, but require higher spin-inversion barriers and hysteresis temperatures than currently achieved. After twenty years of research confined to the d- block transition metals, scientists are moving to the f-block to generate these properties. We have now prepared, by cation-promoted self-assembly, a large 5f-3d U12Mn6 cluster that adopts a wheel topology and exhibits single-molecule magnet behaviour. This uranium-based molecular wheel shows an open magnetic hysteresis loop at low temperature, with a non-zero coercive field (below 4 K) and quantum tunnelling steps (below 2.5 K), which suggests that uranium might indeed provide a route to magnetic storage devices. This molecule also represents an interesting model for actinide nanoparticles occurring in the environment and in spent fuel separation cycles. [ABSTRACT FROM AUTHOR]

Actinide materials play a special role in condensed matter physics, spanning behaviours of itinerant d-electron and localized 4f-electron materials. This duality of the 5f electrons confer to actinide-based intermetallic compounds a broad variety of physical properties such as magnetic or multipolar ordering, heavy fermion behaviour, quantum criticality, unconventional superconductivity... Np Mössbauer spectroscopy is a unique microscopic tool for gaining information on the electronic and magnetic properties of Np systems. [ABSTRACT FROM AUTHOR]

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, (This character cannot be represented in ASCII code). 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. [ABSTRACT FROM AUTHOR]

In this study we reported the synthesis of three polycrystalline uranium borides UB1.78±0.02, UB3.61±0.041, and UB11.19±0.13 and their analyses using chemical analysis, X-ray diffraction, SQUID magnetometry, solid-state NMR, and Fourier transformed infrared spectroscopy. We discuss the effects of stoichiometry deviations on the lattice parameters and magnetic properties. We also provide their static and MAS-NMR spectra showing the effects of the 5f-electrons on the 11B shifts. Finally, the FTIR measurements showed the presence of a local disorder. [ABSTRACT FROM AUTHOR]

The low-temperature heat capacity of (U1−yThy)O2 and 238Pu-doped UO2 samples were determined using hybrid adiabatic relaxation calorimetry. Results of the investigated systems revealed the presence of the magnetic transition specific for UO2 in all three intermediate compositions of the uranium-thorium dioxide (y = 0.05, 0.09 and 0.12) and in the 238Pu-doped UO2 around 25 K. The magnetic behaviour of UO2 exposed to the high alpha dose from the 238Pu isotope was studied over time and it was found that 1.6% 238Pu affects the magnetic transition substantially, even after short period of time after annealing. In both systems the antiferromagnetic transition changes intensity, shape and Néel temperature with increasing Th-content and radiation dose, respectively, related to the increasing disorder on the crystal lattice resulting from substitution and defect creation. [ABSTRACT FROM AUTHOR]