Measurement of the Phonon Density of States of PuO 2 (+2%Ga)
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Measurement of the Phonon Density of States of PuO2(+2%Ga) M. E. Manley1, J. R. Jeffries1, A. H. Said2, C. A. Marianetti3, H. Cynn1, B. M. Leu2, M. Wall1 1 Lawrence Livermore National Laboratory, Livermore, California 94550, USA, 2Argonne National Laboratory, Argonne, Illinois 60439, USA, 3Department of Applied Physics, Columbia University, New York, New York 10027, USA ABSTRACT Inelastic x-ray scattering measurements of the phonon density of states (DOS) of PuO2(+2%Ga) were made and compared to recent predictions from the literature made using three leading theoretical approaches; Density Functional Theory (DFT), DFT plus the Hubbard U (DFT+U), and Dynamical Mean-Field Theory (DMFT). The DFT prediction, which does not account for strong electronic correlations, underestimates the measured energies of most features. The DFT+U and DMFT predictions, which include approximations to strong correlation effects, more accurately reflect the low energy features but exaggerate splitting in the highest energy optic oxygen modes. The exaggeration of the splitting is worse for DFT+U than for DMFT. The transverse acoustic mode shows the least sensitivity to calculation type, and is well reproduced by all three theories. The longitudinal acoustic mode, which is thought to control the thermal conductivity, is more sensitive to calculation type, suggesting an important role for electronic correlations in making application-critical predictions. INTRODUCTION Plutonium dioxide (PuO2) is used in mixed oxide nuclear fuels and consequently there is a need for developing a predictive capability for its use and storage. However, because of difficulties handling PuO2, theoretical work has progressed with little to no experimental testing. Lattice vibrations, which are important to phase stability [1] and thermal transport [2], are known only through predictions made using theoretical approaches including Density Functional Theory (DFT) [3], DFT plus the Hubbard U (DFT+U) [4], and Dynamic Mean Field Theory (DMFT) [5]. However, DFT incorrectly predicts PuO2 to be a ferromagnetic conductor [6], when it is actually an insulator [7], and DFT+U obtains an insulator but incorrectly obtains an antiferromagnetic ground state [8, 9-11]. In addition, neither the DFT nor DFT+U calculations include spin-orbit coupling, which has been shown to be an important perturbation to the electronic structure [12], though it is unclear how important this is to the phonons. The DMFT calculation includes spin orbit coupling, but makes use of the Hubbard-I approximation and so is still not an exact calculation. To evaluate these predictions of the lattice dynamics and by extension the underlying approximations, we use high-resolution inelastic x-ray scattering (IXS) at a synchrotron source to measure the phonon density of states (DOS) of PuO2 (+2%Ga). We find that the limitations of these predictions also manifest as differences between theoretical and experimental phonons.
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EXPERIMENT To determine the phonon density of states of PuO2 we use the high-resolutio
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