No abstract

We report the results of resonant x-ray scattering experiments performed at the Np M(4,5) edges in NpO2. Below T(0)=25 K, the development of long-range order of Np electric quadrupoles is revealed by the growth of superlattice Bragg peaks. The polarization and azimuthal dependence of the intensity of the resonant peaks are well reproduced assuming anisotropic tensor susceptibility scattering from a triple-q(-->) longitudinal antiferroquadrupolar structure. Electric-quadrupole order in NpO2 could be driven by the ordering at T0 of magnetic octupoles of Gamma(5) symmetry, splitting the Np ground state quartet and leading to a singlet ground state with zero dipole-magnetic moment.

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 U(12)Mn(6) 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.

No abstract

No abstract

Inelastic neutron scattering has been applied to the study of the spin dynamics of Cr-based antiferromagnetic octanuclear rings where a finite total spin of the ground state is obtained by substituting one Cr 3+ ion (s = 3/2) with Zn (s = 0), Mn (s = 5/2) or Ni (s = 1) di-cations. Energy and intensity measurements for several intra-multiplet and inter-multiplet magnetic excitations allow us to determine the spin wavefunctions of the investigated clusters. Effects due to the mixing of different spin multiplets have been considered. Such effects proved to be important to correctly reproduce the energy and intensity of magnetic excitations in the neutron spectra. On the contrary to what is observed for the parent homonuclear Cr8 ring, the symmetry of the first excited spin states is such that anticrossing conditions with the ground state can be realized in the presence of an external magnetic field. Heterometallic Cr7M wheels are therefore good candidates for macroscopic observations of quantum effects.

Unlike their transition-metal analogues, the oxo groups of the uranyl dication, [UO 2 ] 2+ , which has a linear geometry and short, strong UÀO bonds are commonly considered inert. [1] Very little Lewis base character has been demonstrated for the uranyl oxo groups, [2,3] which makes them poor models for the heavier, highly radioactive transuranic actinyl cations such as neptunyl [NpO 2 ] n+ (n = 1, 2). [4,5] The heavier actinyls are important components in nuclear waste and demonstrate oxo basicity that can give rise to poorly understood cluster formation and problems in nuclear waste PUREX separation processes.[6] However, it has been shown recently that the more Lewis basic, pentavalent uranyl cation, [UO 2 ] + , can be stabilized indefinitely using suitable equatorial-binding ligands and anaerobic conditions. [7,8] Usually the [UO 2 ] + cation decomposes by disproportionation, which is also a poorly understood process, but is important in the precipitation of uranium salts out of aqueous environments. [9,10] The disproportionation is suggested, by analogy with the transuranic metal oxo Lewis base behavior, to involve the formation of cation-cation interactions (CCIs) [11,12] in which the oxo groups ligate to adjacent actinyl centers forming diamond (A) or T-shaped (B) dimers or clusters which can then allow the transfer of protons and electrons between metals, such as in C. [13] We reported that the use of rigid, Pacman-shaped macrocycles can allow the isolation of heterobimetallic uranyltransition metal complexes that form an oxo interaction with the transition metal in the solid state and solution, [14] and how the inclusion of more than one metal cation alongside the uranyl cation led to isolable, stable pentavalent uranyl complexes with a covalently functionalized oxo group. [15] More recently, pentavalent uranyl complexes with Group 1 cation oxo-coordination, [16] and a doubly silylated complex [17] have been isolated. Here, we report the first uranyl-4f metal interaction, prepared by either standard, or sterically induced reduction procedures, and demonstrate strong magnetic coupling between the 4f and 5f electrons.The reaction between the divalent samarium silylamide [Sm(THF) 2 {N(SiMe 3 ) 2 } 2 ] and the uranyl Pacman complex [UO 2 (py)(H 2 L)] (1) in pyridine resulted in the deposition of the new uranyl-samarium complex [UO 2 Sm(py) 2 (L)] 2 (2) as a very poorly soluble, thermally stable, red crystalline powder in good yield (Scheme 1), and containing crystals suitable for single-crystal X-ray diffraction studies (Figure 1).The 1 H NMR spectrum of 2 in [D 5 ]pyridine reveals the presence of paramagnetically shifted resonances between d = 12.4 and À21.5 ppm, the number and integrals of which are consistent with the retention of a wedged, Pacman structure in solution of C s symmetry. In the solid state, the molecular structure shows that 2 is dimeric (Figure 1) and the unit cell contains two similar molecules. Focusing on one molecule of 2, both the uranium and samarium centers are sevencoordinate w...

We report direct experimental evidence for long-range antiferro ordering of the electricquadrupole moments on the U ions. Resonant x-ray scattering experiments at the uranium M 4 absorption edge show a characteristic dependence in the integrated intensity upon rotation of the crystal around the scattering vector. Although quadrupolar order in uranium dioxide was advocated already in the 1960s no experimental evidence for this phenomenon was provided until now.We conclude with a possible model to explain the phase diagram of the solid solutions of UO 2 and NpO 2 . We suggest that in the region 0.30 < x < 0.75 neither the transverse nor the longitudinal quadrupole ordering can dominate, leading to frustration and only short-range ordering.