The Crystal Dynamics of Uranium Dioxide

Dolling, G.; Cowley, R. A.; Woods, A. D. B.

doi:10.1139/p65-135

Cited by 265 publications

(151 citation statements)

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“…In the entire pressure range considered in our study, C 11 is prominently larger than C 12 , indicating that the bonding strength along the [ [14]. Finally, for the Debye temperature, our calculated result of 398.1 K is in excellent agreement with experimental data [52,55].…”

Section: B Elasticity Of Fluorite Uo2supporting

confidence: 81%

Phonon spectrum, thermodynamic properties, and pressure-temperature phase diagram of uranium dioxide

et al. 2013

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We present a study of the structural phase transition and the mechanical and thermodynamic properties of UO2 by means of the local density approximation (LDA)+U approach. A phase transition pressure of 40 GPa is obtained from theory at 0 K, and agrees well with the experimental value of 42 GPa. The pressure-induced enhancements of elastic constants, elastic moduli, elastic wave velocities, and Debye temperature of the ground-state fluorite phase are predicted. The phonon spectra of both the ground state fluorite structure and high pressure cotunnite structure calculated by the supercell approach show that the cotunnite structure is dynamically unstable under ambient pressure. Based on the imaginary mode along the Γ−X direction and soft phonon mode along the Γ−Z direction, a transition path from cotunnite to fluorite has been identified. We calculate the lattice vibrational energy in the quasiharmonic approximation using both first-principles phonon density of state and the Debye model. The calculated temperature dependence of lattice parameter, entropy, and specific heat agrees well with experimental observations in the low temperature domain. The difference of the Gibbs free energy between the two phases of UO2 has predicted a boundary in the pressure-temperature phase diagram. The solid-liquid boundary is approximated by an empirical equation using our calculated elastic constants.

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Section: B Elasticity Of Fluorite Uo2supporting

confidence: 81%

Phonon spectrum, thermodynamic properties, and pressure-temperature phase diagram of uranium dioxide

et al. 2013

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“…The variables u(q, j) and ω(q, j) have been calculated in one of our earlier works ? and can be fitted to the entire Brillouin Zone using a simple rigid-ion model 48,49 . If we further assume that the quadrupoles only couple to t a 2g and t b 2g quadrupolar distortions of the O-cage around each U-ion, the coupling constants are assumed to have the form:…”

Section: Spin-lattice Couplingmentioning

confidence: 99%

Anisotropic multipolar exchange interactions in systems with strong spin-orbit coupling

Nanguneri

Savrasov

2014

Phys. Rev. B

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We introduce a theoretical framework for computaions of anisotropic multipolar exchange interactions found in many spin-orbit coupled magnetic systems and propose a method to extract these coupling constants using a density functional total energy calculation. This method is developed using a multipolar expansion of local density matrices for correlated orbitals that are responsible for magnetic degrees of freedom. Within the mean-field approximation, we show that each coupling constant can be recovered from a series of total energy calculations via what we call the "pair-flip" technique. This technique flips the relative phase of a pair of multipoles and computes corresponding total energy cost associated with the given exchange constant. To test it, we apply our method to Uranium Dioxide, which is a system known to have pseudospin J = 1 superexchange induced dipolar, and superexchange plus spin-lattice induced quadrupolar orderings. Our calculation reveals that the superexchange and spin-lattice contributions to the quadrupolar exchange interactions are about the same order with ferro-and antiferro-magnetic contributions, respectively. This highlights a competition rather than a cooperation between them. Our method could be a promising tool to explore magnetic properties of rare-earth compounds and hidden-order materials.

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“…Some weak inelastic intensity from the uranium dioxide surface layer was observed. However, since most of this intensity was at energies higher than the uranium phonon cutoff energy ͑ϳ15 meV͒, it was easily fit and subtracted from the elemental uranium scattering using a previously measured phonon DOS of uranium dioxide [19]. The scattering from the oxide accounted for about 10% of the total inelastic intensity in the energy range of the metallic uranium phonon DOS.…”

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confidence: 99%

Large Harmonic Softening of the Phonon Density of States of Uranium

et al. 2001

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Phonon density-of-states curves were obtained from inelastic neutron scattering spectra from the three crystalline phases of uranium at temperatures from 50 to 1213 K. The a-phase showed an unusually large thermal softening of phonon frequencies. Analysis of the vibrational power spectrum showed that this phonon softening originates with the softening of a harmonic solid, as opposed to vibrations in anharmonic potentials. It follows that thermal excitations of electronic states are more significant thermodynamically than are the classical volume effects. For the a-b and b-g phase transitions, vibrational and electronic entropies were comparable. DOI: 10.1103/PhysRevLett.86.3076 PACS numbers: 63.20.-e, 64.30.+t, 78.70.Nx Although first known for its unusual nuclear properties, uranium exhibits several unusual solid-state properties that may originate with electronic instabilities. The thermally induced softening of the phonon density of states (DOS) for most elements originates with anharmonicity [1,2]. For the actinides, however, a distinction between the normal anharmonic softening and harmonic softening arising from a temperature-dependent harmonic potential has been suggested [3]. In a detailed assessment of the thermodynamic data on the six crystalline phases of Pu, it was concluded that the anharmonic and electronic contributions to the equation of state could not be separated [4]. The origin of this phonon softening is a fundamental issue for the equation of state. In this Letter, we use the power spectrum of atom motions to show that the thermal softening of the phonon DOS in a-U originates with the weakening of force constants in a harmonic solid, as opposed to the typical softening in an anharmonic potential. Temperature alters the electronic structure sufficiently to change the lattice dynamics. This Letter also addresses the entropy of phonons, and by deduction the entropy of electrons, for the three low-pressure phases of crystalline uranium metal.Previous lattice dynamics studies on uranium have been performed at room temperature and below [5,6], motivated in part by the discovery of several charge density wave transitions at low temperatures [7,8]. Independently, there has been recent interest in the vibrational entropy contribution to the high temperature phase stability of metals and alloys [9][10][11], motivated by the discovery that vibrational entropy plays a larger role in phase stability than previously expected [12]. Other experimental and theoretical work has shown that electronic contributions to the entropies of high temperature phase transitions can also be significant [13][14][15].Diffraction measurements on a-U at ambient pressure have shown that the Debye temperature decreases dramatically with increasing temperature [3,16]. This softening is consistent with decreases in the elastic constants [17,18]. Specifically, the Debye temperature was expressed by u Х ͑306 2 0.158T ͒ K, where T is temperature [3]. The magnitude of this softening suggests that the Debye temperature decreases...

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The Crystal Dynamics of Uranium Dioxide

Cited by 265 publications

References 13 publications

Phonon spectrum, thermodynamic properties, and pressure-temperature phase diagram of uranium dioxide

Phonon spectrum, thermodynamic properties, and pressure-temperature phase diagram of uranium dioxide

Anisotropic multipolar exchange interactions in systems with strong spin-orbit coupling

Large Harmonic Softening of the Phonon Density of States of Uranium

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