Invited Paper AC+AS+MI+SA+SS-TuM3
Actinide Research with Hard Synchrotron Radiation

Tuesday, November 11, 2014, 8:40 am, Room 301

Advanced x-ray synchrotron radiation techniques, addressing spatial and temporal fluctuations of structural and electronic degrees of freedom, hold outstanding scientific promises for the future of actinide research [1]. Indeed, by probing hidden order parameters and elementary electronic excitations with high sensitivity and resolution, element- and edge-specific (resonant and non-resonant) x-ray scattering experiments provide the bricks for building the novel conceptual frameworks necessary to unravel the complexity of actinides. Here, I will present selected results from these experiments and discuss what they tell us.

Whereas resonant x-ray diffraction (RXD) with photon energies tuned to the M_4,5 absorption edges of actinide ions are used to study the order of electric quadrupole moments in oxides and intermetallics [2-4], resonant x-ray emission spectroscopy (RXES) and non-resonant inelastic X-ray scattering (NIXS) are applied to study the bulk electronic configuration in solids, liquids and gases [5,6]. In particular, the high penetration depth of hard X-rays employed in NIXS enables flexible containment concepts, facilitating investigations of radioactive materials in the liquid phase or under extreme conditions. Finally, inelastic x-ray scattering (IXS) can be used to map phonon dispersion branches with an energy resolution comparable to the one afforded by inelastic neutron scattering, but using crystal samples with sizes orders of magnitude smaller than those required by neutrons.

In the first example, I will show how a combination of diffraction, RXES, and absorption near-edge spectroscopy experiments at high-pressure can be used to study the correlation between polymorphism, mixing of different electronic configurations, and hybridization effects in elemental americium [7]. The potential of RXD in elucidating the nature of "hidden order" will be illustrated by the examples of the low-temperature phases in NpO₂ [8] and URu₂Si₂ [9]. Then, I will present NIXS results interrogating the O_4,5 absorption edges of uranium and plutonium materials, and results of IXS studies of the vibrational dynamics in PuCoGa₅ and NpO₂.

[1] R. Caciuffo, E. C. Buck, D. L. Clark, G. van der Laan, MRS Bulletin 35, 889 (2010)

[2] P. Santini et al., Rev. Mod. Phys. 81, 807 (2009)

[3] H. C. Walker et al., Phys. Rev. Lett. 97, 137203 (2011)

[4] Z. Bao et al., Phys. Rev. B 88, 134426 (2013)

[5] T. Vitova et al., Phys. Rev. B 82, 235118 (2010)

[6] R. Caciuffo et al., Phys. Rev. B 81, 195104 (2010)

[7] S. Heathman et al., Phys. Rev. B 82, 201103(R) (2010)

[8] N. Magnani et al., Phys. Rev. B 78, 104425 (2008)