Chalk River Laboratories

About: Chalk River Laboratories is a based out in . It is known for research contribution in the topics: Neutron diffraction & Neutron scattering. The organization has 2297 authors who have published 2700 publications receiving 73287 citations.

Phonon Lifetime Investigation of Anharmonicity and Thermal Conductivity of UO 2 by Neutron Scattering and Theory

Abstract: Inelastic neutron scattering measurements of individual phonon lifetimes and dispersion at 295 and 1200 K have been used to probe anharmonicity and thermal conductivity in UO2 They show that longitudinal optic phonon modes carry the largest amount of heat, in contrast to past simulations and that the total conductivity demonstrates a quantitative correspondence between microscopic and macroscopic phonon physics We have further performed first-principles simulations for UO2 showing semiquantitative agreement with phonon lifetimes at 295 K, but larger anharmonicity than measured at 1200 K

Geometric effects of deuteration on hydrogen-ordering phase transitions

Abstract: IT has long been known that the substitution of deuterium for hydrogen in hydrogen-bonded materials can lead to a change in the geometry of the hydrogen bonds—the so-called Ubbelohde effect1. There can also be significant accompanying changes in the physical properties of the material. Among the most striking examples is the increase in the transition temperature, Tc, of hydrogen-ordering systems of the KH2PO4 type by >100K on full deuteration. In all of these systems, Tc decreases under pressure, making it possible in principle to compare protonated and deuterated forms at the same Tc by applying pressure to the latter. Here we report the results of a high-pressure neutron diffraction study of the deuterated H(D)-ordering material PbDPO4. We find that much, and possibly all, of the increase in Tc on deuteration is attributable to the accompanying changes in the hydrogen-bond dimensions. This suggests that purely mass-dependent effects on the quantum-mechanical tunnelling between the equivalent H(D) potential minima—an explanation that has been favoured previously—have little or no direct influence on Tc. Substantial revisions to the theory of this type of ordering transition are apparently required.

From soft harmonic phonons to fast relaxational dynamics in CH 3 NH 3 PbBr 3

Abstract: The lead-halide perovskites, including ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbBr}}_{3}$, are components in cost effective, highly efficient photovoltaics, where the interactions of the molecular cations with the inorganic framework are suggested to influence the electronic and ferroelectric properties. ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbBr}}_{3}$ undergoes a series of structural transitions associated with orientational order of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}$ (methylammonium) molecular cation and tilting of the ${\mathrm{PbBr}}_{3}$ host framework. We apply high-resolution neutron scattering to study the soft harmonic phonons associated with these transitions, and find a strong coupling between the ${\mathrm{PbBr}}_{3}$ framework and the quasistatic ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}$ dynamics at low energy transfers. At higher energy transfers, we observe a ${\mathrm{PbBr}}_{6}$ octahedra soft mode driving a transition at 150 K from bound molecular excitations at low temperatures to relatively fast relaxational excitations that extend up to $\ensuremath{\sim}50\text{--}100$ meV. We suggest that these temporally overdamped dynamics enables possible indirect band gap processes in these materials that are related to the enhanced photovoltaic properties.

High temperature neutron diffraction study of the Al2O3-Y2O3 system

Abstract: The phase diagram of the Al2O3–Y2O3 system has been investigated with five different compositions by XRD and in situ high temperature neutron diffractometry. High purity YAG, YAP and YAM compounds have been produced successfully through a melt extraction technique. High temperature neutron diffraction has made it possible to follow, in real time, the reactions involved in this system, especially to determine the temperature range of each reaction, which would have been impossible to determine by other means. A good agreement between the experimental results and the phase diagram of the Al2O3–Y2O3 system has been observed.