[Show abstract][Hide abstract] ABSTRACT: The thermal conductivity of uranium dioxide has been studied for over half a century, as uranium dioxide is the fuel used in a majority of operating nuclear reactors and thermal conductivity controls the conversion of heat produced by fission events to electricity. Because uranium dioxide is a cubic compound and thermal conductivity is a second-rank tensor, it has always been assumed to be isotropic. We report thermal conductivity measurements on oriented uranium dioxide single crystals that show anisotropy from 4 K to above 300 K. Our results indicate that phonon-spin scattering is important for understanding the general thermal conductivity behaviour, and also explains the anisotropy by coupling to the applied temperature gradient and breaking cubic symmetry.
[Show abstract][Hide abstract] ABSTRACT: We probe the volume collapse transition (ΔV/Vo ∼ 15%) between the isostructural γ and α phases (T ∼ 100 K) of Ce0.9Th0.1 using the Hall effect, three-terminal capacitive dilatometry, and electrical resistivity measurements. Hall effect measurements confirm the itinerant ground state as the carrier concentration increases by a factor of 7 in the α phase, γ phase (nH = 5.28 × 10(26) m(-3)), and the α phase (nH = 3.76 × 10(27) m(-3)). We were able to detect a noise spectrum consisting of avalanches while slowly varying the temperature through the hysteretic region. We surmise that the avalanches originate from intergranular stresses at the interfaces between partially transformed high-volume and low-volume phases. The statistical distribution of avalanches obey power laws with energy exponent ϵ ≃ 1.5. Hall effect measurements, combined with universal critical exponents, point to short electron mean-free percolation pathways and carrier localization at phase interfaces. Carrier localization was predicted many years ago for elemental cerium by Johansson (1974 Phil. Mag. 30 469).
[Show abstract][Hide abstract] ABSTRACT: Valence band photoemission measurements have been made on crystalline and supercooled liquid gallium, and across the liquid and solid phases of bismuth and indium. Measurements are angle integrated and made using photon excitations of 21.21 and 40.81 eV. In all cases the Bloch states are destroyed upon melting and the free electron gas is constrained by a charge-neutral liquid. The spectra of indium show little change upon solidification, indicating a common electronic structure for crystalline and liquid phases. In contrast, the energy distribution curves for supercooled gallium and bismuth show large changes in the electronic structure from solid to liquid phases, giving rise to the formation of pseudogaps in the density of states at the Fermi energy, EF. Observations of this kind enable us to distinguish normal or anomalous melting from photoemission measurements.
[Show abstract][Hide abstract] ABSTRACT: Polyisoprene,
consists of isoprene (2--methyl--1,3--butadiene) molecules that are
linked end--to--end to form long chains. There are two different
isomers, cis and trans, with respect to configuration about the C=C
double bond. Cis--polyisoprene, the main component in natural rubber
derived originally from the Pará rubber tree, is a
non--crystalline elastomer at room temperature. In contrast,
trans--polyisoprene, derived originally from the gutta percha tree, is a
crystalline solid that is rigid and tough. More interestingly, the trans
isomer exhibits shape--memory properties, whereas the cis isomer does
not. As for shape--memory transformations in alloys, trans--polyisoprene
exhibits clear Af and Mf temperatures, with values
338 K and 300 K, respectively. Here we report thermodynamic and
structural measurements of the shape--memory effect in
trans--polyisoprene. We discuss our results in terms of the mechanism of
[Show abstract][Hide abstract] ABSTRACT: For decades UO2 has been the most widely studied actinide
oxide because of its technological importance as fuel material for
nuclear reactors. Therefore there is a large interest in understanding
its thermal, transport and thermodynamic properties. We present recent
experimental results for the thermal conductivity and thermal expansion
of high quality UO2 single crystal, obtained for different
crystallographic directions, and compare with results of molecular
dynamics simulations. We will discuss the implications of this study.
[Show abstract][Hide abstract] ABSTRACT: We present a new analysis of the heat capacity of δ-phase Pu-5 at.% Al with a fit using a single-ion Kondo term and a low-temperature Schottky anomaly in addition to the Debye and linear terms. The Kondo and Schottky terms together contribute 1.2 R to the entropy at 300 K. We show how the extra entropy could affect the alloy phase diagrams of δ-phase Pu.
[Show abstract][Hide abstract] ABSTRACT: Low-temperature specific heat and thermal expansion measurements are used to study the hydrogen-based ferroelectric lawsonite over the temperature range 1.8 K ≤ T ≤ 300 K. The second-order phase transition near 125 K is detected in the experiments, and the low-temperature phase is determined to be improper ferroelectric and co-elastic. In the ferroelectric phase T ≤ 125 K, the spontaneous polarization P(s) is proportional to (1) the volume strain e(s), and (2) the excess entropy ΔS(e). These proportionalities confirm the improper character of the ferroelectric phase transition. We develop a structural model that allows the off-centering of hydrogen positions to generate the spontaneous polarization. In the low-temperature limit we detect a Schottky anomaly (two-level system) with an energy gap of Δ ∼ 0.5 meV.
[Show abstract][Hide abstract] ABSTRACT: We address the error resulting from application of the harmonic Debye-Waller factor to anharmonic vibrations. The mean-square atomic displacement 〈u2〉 determined from the harmonic analysis is compared to values obtained from an exact anharmonic analysis. In the case of strong anharmonicity, we find that the harmonic approximation introduces at most a ∼25% error. The temperature dependence determined from the harmonic analysis follows that found from the exact anharmonic analysis. Errors introduced by the harmonic approximation are comparable in magnitude to the usual systematic errors associated with diffraction experiments and Rietveld refinements.
[Show abstract][Hide abstract] ABSTRACT: Despite large experimental and theoretical efforts the structure of the
superconducting gap and the origin of the pairing mechanism in
iron-based superconductors in still unresolved. Measurements of the low
temperature specific heat and its magnetic response inside the
superconducting state give important information about the symmetry of
the gap. Here, we present results of our studies of codoped
with a Tc of 32.5 K. The high quality of the material is
marked by a pronounced peak at Tc as well as by a low
residual specific heat γ0 = 2.4 mJ/mol K^2. We will
discuss the implications of the new specific heat results on the
symmetry of the order parameter in this system.
[Show abstract][Hide abstract] ABSTRACT: Quantized waves obeying Bose-Einstein statistics will contribute a T^3/2
term to the specific heat if the dispersion relation goes as q2. We
measure the magnetic and electric field dependence of the specific heat
on the ferroelectric material tri-glycine sulphate (TGS) over the
temperature range 0.05 K < T < 350 K. We detect a T^3/2 term in
the specific heat in the low-temperature limit, which is taken to be the
dielectric analog to magnetic spin wave. Near the Curie temperature
(TC = 320 K), the shape of the specific-heat anomaly is
thermally broadened. However, the anomaly changes to the characteristic
sharp lambda-shape expected for a continuous transition with the
application of either a magnetic field or electric field, giving the
expected entropy change at TC of Rln2. These results are
explained on the basis that the frequencies of optical dipole
oscillations are split by the magnetic field, and the resulting gas of
excitations are paramagnetic. Consequently they contribute to the
specific heat near TC, which increases with magnetic field.
[Show abstract][Hide abstract] ABSTRACT: The gamma to alpha isostructural transition in the
Ce(0.9-x)La(x)Th(0.10) system is measured as a function of La alloying
and external pressure up to 1 Mbar using magnetic susceptibility. We
probe a line of discontinuous transitions, as indicated by the change in
volume, decreasing exponentially from 118 K to close to 0 K with
increasing La doping, and the transition changes from being first order
to continuous at a critical concentration close to x = 0.14. At the
tricritical point, the magnetic susceptibility increase rapidly near the
critical concentration and approaches large values at x= 0.35 signifying
that a heavy Fermi-liquid state evolves at large doping near the
critical concentration. The Wilson ratio reaches a value above two for a
narrow range of concentrations where the specific heat and
susceptibility vary most rapidly with the doping concentration.
[Show abstract][Hide abstract] ABSTRACT: The Debye-Waller factor relates the intensities of the Bragg peaks to
the mean square displacements of the atoms. In the structural refinement
of diffraction data it is standard practice to use the harmonic
expression for Debye-Waller factor. For most materials and conditions
the phonons are only mildly anharmonic, thus the harmonic assumption is
a good one. For some materials and conditions, however, the phonons can
be strongly anharmonic, and thus the harmonic assumption is physically
unrealistic. As examples we cite the rattling atoms in clathrates and
skutterudites, and atoms participating in displacive phase transitions.
In the present study we investigate the error associated with using the
harmonic Debye--Waller factor to analyze anharmonic vibrations. We find
that even for strongly anharmonic potentials, such as a double well, the
mean square displacements deduced using the harmonic approximation are
at most 15% larger than those deduced using a full anharmonic analysis.
Furthermore, the quasi-harmonic and anharmonic values have nearly the
same temperature dependences. We conclude that the error introduced by
using the harmonic approximation is comparable to or smaller than the
usual errors associated with measurement and refinement of diffraction
[Show abstract][Hide abstract] ABSTRACT: We have used a suite of diffraction, thermodynamic, and transport measurements to study the localized rattling of Al guest atoms in VAl10.1. The mean-square displacement of the rattling atom shows a concave-down temperature dependence. This is characteristic of an anharmonic vibration, the frequency of which increases with amplitude, akin to a particle in a box. We find that the rattling is best described in terms of a sixth-order interatomic potential, with negligible contributions from harmonic and quartic terms. The rattler has a characteristic temperature of θRM=21 K and couples strongly to both the acoustic phonons and conduction electrons. The coupling to the phonons is evident from the large value of the Grüneisen parameter, which increases with decreasing temperature to Γ≈43 at 5 K. Below 6 K, the electrical resistivity varies as T3, which can be explained in terms of electron scattering from damped sixth-order vibrations. VAl10.1 is a superconductor below Tc=1.53 K, with an upper critical field of ∼1 kG.
[Show abstract][Hide abstract] ABSTRACT: We have measured the dose dependence of the degree of amorphization of titanite, CaTiSiO(5). Titanite is an often metamict mineral which has been considered as a matrix for the encapsulation of radiogenic waste, such as Pu. The amorphous fraction p of geologically irradiated samples (ages between 0.3 and 1 Ga) follows p = 1 - exp(-B(a)D) where D is the total dose and the characteristic amorphization mass is B(a) = 2.7(3) × 10(-19) g. Amorphization follows the direct impact mechanism where each α-decay leads to a recoil of the radiogenic atoms (mostly Th and U), which then, in turn, displaces some 5000 atoms of the titanite matrix. The amorphization behaviour is almost identical with that of zircon, ZrSiO(4), which has a similar molecular mass. While the recrystallization mechanism and elastic behaviour of the two minerals are very different, we do not find significant differences for the amorphization mechanism. Our samples have undergone little reheating over their geological history, since heating over 800 K would lead to rapid recrystallization for which we have found no evidence.
[Show abstract][Hide abstract] ABSTRACT: We report an experimental determination of the dispersion of the soft phonon mode along  in uranium as a function of pressure. The energies of these phonons increase rapidly, with conventional behavior found by 20 GPa, as predicted by recent theory. New calculations demonstrate the strong pressure (and momentum) dependence of the electron-phonon coupling, whereas the Fermi-surface nesting is surprisingly independent of pressure. This allows a full understanding of the complex phase diagram of uranium and the interplay between the charge-density wave and superconductivity.
[Show abstract][Hide abstract] ABSTRACT: In this paper we apply the Aptekar-Ponyatovsky (AP) regular solution thermodynamic model to the analysis of experimental data for the coefficient of thermal expansion (CTE) and determine the AP model parameters for unalloyed cerium metal, Ce-Th-La alloys, and Pu-Ga alloys. We find that the high temperature CTE of cerium metal follows the predictions of the AP model based on low temperature, high pressure data. For Ce-Th-La alloys we use the AP parameters to track the suppression of the first-order γ-α cerium transition. We show the AP model accounts for the negative CTE observed for Pu-Ga alloys and is equivalent to an earlier invar model. Finally, we apply the AP parameters obtained for Pu-Ga alloys to rationalize the observed δ-α transformation pressures of these alloys. We show that the anomalous values of the Grüneisen and Grüneisen-Anderson parameters are important features of the thermal properties of plutonium. A strong analogy between the properties of plutonium and cerium is confirmed.
[Show abstract][Hide abstract] ABSTRACT: We study some dynamic aspects of a Mott transition in a rare-earth alloy Ce0.90Th0.10 by resonant-ultrasound spectroscopy (RUS), electrical-transport, and thermal-expansion measurements. In the temperature range spanning the first-order transition, we observe a stiffening of the elastic response that is associated with a continuous front propagation (e.g. solitons). A defining characteristic of a mixed phase regime, slow scanning rates (0.01 K/min) show these solitons to be superimposed with jerks and avalanches in all three data sets: RUS, resistivity, and thermal expansion data. Analysis of the avalanche data give power law distributions with critical exponents P(E)=E^n for energy, in the case of thermal expansion data and length, in the case of electrical transport data.
[Show abstract][Hide abstract] ABSTRACT: Iron is a ubiquitous impurity in metamict (radiation-damaged and partially amorphized) materials such as titanite (CaSiTiO(5)). Using (57)Fe Mössbauer spectroscopy we find that iron in metamict titanite is partitioned between amorphous and crystalline regions based on valence. Trivalent iron exists in the crystalline titanite matrix whereas divalent iron exists almost exclusively in radiation-amorphized regions. We find that the relative abundances of the oxidation states correlate with the volume fraction of amorphous and crystalline regions. Our data also show that oxidation of iron proceeds along with the recrystallization of the amorphized regions. Recrystallization is confirmed to occur over the range 700 °C < T < 925 °C, and no further structural changes are observed at higher temperatures. It is surprising that our Mössbauer measurements show divalent iron to be surrounded by titanite with a high degree of short-range structural order in the amorphized regions. This observation is fundamentally different from other metamict materials such as zircon (ZrSiO(4)), where amorphized regions show no short-range order.
[Show abstract][Hide abstract] ABSTRACT: In this article we examine the low-temperature specific heat of slow-cooled Cu(0.2)Ag(2.8)SbSeTe(2) and the thermoelectric performance of quenched samples. We find that the low-temperature specific heat is dominated by two Einstein terms of approximate energies of 2.5 and 5 meV. The specific-heat behavior is consistent with the amorphous low-temperature thermal conductivity behavior and validates the glassy nature of the structure. We performed the synthesis of quenched samples in an attempt to eliminate the presence of micro-cracks, whose existence presumably enhances electronic scattering. We find that quenching eliminates the presence of micro-cracks but does not result in an improvement of the figure of merit. Specifically, the highest ZT obtained in the quenched samples (ZT = 1.5), though very competitive, is still significantly less that the ZT obtained in the slow-cooled samples (ZT = 1.75).