Atomic thermal vibrations of the light actinide elements

Lawson, A. C.; Goldstone, J. A.; Cort, B.; Sheldon, R.I.; Foltyn, E.M.

doi:10.1016/0925-8388(94)90951-2

Cited by 28 publications

(21 citation statements)

References 2 publications

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“…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].…”

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confidence: 49%

“…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].…”

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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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Large Harmonic Softening of the Phonon Density of States of Uranium

et al. 2001

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“…Robert (Non-spin-polarized) 103 Robert (Spin-polarized AFM) 103 Kutepov (NM) 104 Kutepov (AFM) 104 Söderlind (SO) 38 Söderlind (SO+OP) 38 Kay (ultrasonic resonance) 87 our result (RUS) 20 (x-ray 12 , dilatometry [105][106][107][108] 169. 103 Robert (Spin-polarized AFM I) 103 Robert (Spin-polarized AFM II) 103 Söderlind (SO) 38 Söderlind (SO+OP) 38 Kay ( Robert (Non-spin-polarized) 103 Robert (Spin-polarized AFM) 103 Kutepov (NM) 104 Kutepov (AFM) 104 Shick (LSDA) 109 Shick (FLL LSDA+U) 109 Shick (AMF LSDA+U) 109 Söderlind (SO) 38 Söderlind (SO+OP) 38 Söderlind (EMTO) 38 Kay ( which summarizes the best previous thermal expansion and x-ray measurements of pure Pu.…”

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confidence: 99%

Temperature dependence of elastic moduli of polycrystalline β plutonium

et al. 2011

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The elastic moduli of pure polycrystalline beta plutonium were measured over its full range of existence (417 K to 491 K) using resonant ultrasound spectroscopy. The Debye temperature (138 K), Poisson's ratio (0.28), Grüneisen parameter (2.3), and the zerotemperature atomic volume (21.2 Å 3 ) were computed from the measurements. Both bulk and shear moduli decrease smoothly on warming with expected discontinuities at the phase boundaries. The shear modulus is surprisingly nearly the same for beta and gamma Pu. The temperature dependence of bulk moduli for beta Pu is, like gamma Pu, unusually small. Poisson's ratio shows very strong differences among alpha, beta, and gamma Pu indicating they are entirely different metals. The zero-temperature elastic moduli were computed for the three phases as well as for gallium-stabilized delta Pu (also measured by us) and compared to calculations.PACS numbers: 62.20. de, 63.70.+h, 65.40.G-, 62.20.dj

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“…The errors in the fitted Θ DW s are less than 1K for β-through ε-phases and about 10K for α. (See Lawson et al, 1994, for details of the fitting process.) There is nothing in the <u 2 > curve that correlates with the negative thermal expansion of the δ-phase.…”

Section: Electron Correlation and Plutonium Phase Diagramsmentioning

confidence: 99%