| AVS 64th International Symposium & Exhibition | |
| Actinides and Rare Earths Focus Topic | Monday Sessions |
| Session AC+MI+SA+SU-MoM |
| Session: | Magnetism, Complexity, and Superconductivity in the Actinides and Rare Earths |
| Presenter: | Roberto Caciuffo, European Commission, Joint Research Centre, Karlsruhe, Germany |
| Authors: | R. Caciuffo, European Commission, Joint Research Centre, Karlsruhe, Germany P. Maldonado, Uppsala University, Sweden L. Paolasini, European Synchrotron Radiation Facility, France P.M. Oppeneer, Uppsala University, Sweden T.R. Forrest, European Synchrotron Radiation Facility, France A. Prodi, Consiglio Nazionale delle Ricerche, Italy N. Magnani, European Commission, Joint Research Centre, Karlsruhe, Germany A. Bosak, European Synchrotron Radiation Facility, France G.H. Lander, European Commission, Joint Research Centre, Karlsruhe, Germany |
| Correspondent: | Click to Email |
The energy-wavevector dispersion relations for normal modes of vibration propagating along high-symmetry lines in NpO2 and UO2 have been determined by measuring the coherent one-phonon scattering of X-rays from single-crystal specimens with mass varying from a few μg to ~1 mg. The inelastic X-ray scattering (IXS) experiments were carried out using the ID28 beamline at ESRF with an incident energy E=17.794 keV. The results are compared against ab initio phonon dispersion simulations computed within the first-principles density functional theory in the generalized gradient approximation plus Hubbard U correlation (GGA+U) approach, taking into account third-order anharmonicity effects in the quasiharmonic approximation.
In the case of NpO2, the sample of dimension of 0.4x0.3x0.3mm3 was oriented with the specular direction along the (100) crystal axis and the (011) axis in the scattering plane. We have investigated different Brillouin zones in order to optimize the inelastic structure factor for the different optic branches. Optic phonons arise mainly from oxygen vibration modes and are very weak. Thermal expansion, heat capacity, thermal conductivity, phonon linewidth, and thermal phonon softening are calculated and compared with experimental data available for both NpO2 and UO2 [1]. We show that optical phonons contribute significantly to the heat transport due mainly to their large velocities and short lifetimes. Compared with UO2, the main differences in the phonon density of states of NpO2 are a softening of the optical modes and an increase of the peak centred around 55 meV, whereas the acoustic modes in NpO2 are shifted to higher frequencies. The calculated value at 0 K of the bulk modulus in NpO2 is in agreement with the experimental value and slightly smaller than the one determined by high-pressure X-ray diffraction for UO2. NpO2 has a smaller thermal conductivity than UO2, at least in the temperature range 600 to 1000 K for which experimental values are available.
The lattice dynamics of UO2 have been extensively investigated by neutron scattering both at low and high temperature. In this study we focussed our attention to the low temperature region, searching for vibronic contributions to the vibrational-magnetic-quadrupolar mixed modes that appear in UO2 because of magnetoelastic and multipolar superexchange interactions. The situation is cleaner than with neutrons, as the latter see both magnons and phonons – the IXS spectra just the phonons. This observation opens a new window on these “hidden excitations”, which drive much of the physics of UO2.
[1] P. Maldonado et al., Phys Rev B 93, 144301 (2016)