Using in-depth structural and spectroscopic analysis, we unravel the nature of phenomena specific to the Fe3+/Fe4+ redox couple in P2-Na0.67−z[NiyMn0.5+yFe0.5−2y]O2.

Despite the widespread use of silicon in modern technology, its peculiar thermal expansion is not well understood. Adapting harmonic phonons to the specific volume at temperature, the quasiharmonic approximation, has become accepted for simulating the thermal expansion, but has given ambiguous interpretations for microscopic mechanisms. To test atomistic mechanisms, we performed inelastic neutron scattering experiments from 100 K to 1,500 K on a single crystal of silicon to measure the changes in phonon frequencies. Our state-of-the-art ab initio calculations, which fully account for phonon anharmonicity and nuclear quantum effects, reproduced the measured shifts of individual phonons with temperature, whereas quasiharmonic shifts were mostly of the wrong sign. Surprisingly, the accepted quasiharmonic model was found to predict the thermal expansion owing to a large cancellation of contributions from individual phonons.

No abstract

Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes were ∆εi/εi∆T = −0.07 giving a mean isobaric Grüneisen parameter of +6.95±0.67, significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory (DFT) or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes 0.15 kB/atom to the vibrational entropy, compared to the 0.03 kB/atom from quasiharmonicity. Excellent agreement was found between the entropy from phonon DOS measurements and the reference JANAF thermodynamic entropy from calorimetric measurements.2

No abstract

MCViNE (Monte-Carlo VIrtual Neutron Experiment)is an open-source Monte Carlo (MC) neutron ray-tracing software for performing computer modeling and simulations that mirror real neutron scattering experiments. We exploited the close similarity between how instrument components are designed and operated and how such components can be modeled in software. For example we used object oriented programming concepts for representing neutron scatterers and detector systems, and recursive algorithms for implementing multiple scattering. Combining these features together in MCViNE allows one to handle sophisticated neutron scattering problems in modern instruments, including, for example, neutron detection by complex detector systems, and single and multiple scattering events in a variety of samples and sample environments. In addition, MCViNE can use simulation components from linear-chain-based MC ray tracing packages which facilitates porting instrument models from those codes. Furthermore it allows for components written solely in Python, which expedites prototyping of new components. These developments have enabled detailed simulations of neutron scattering experiments, with non-trivial samples, for time-of-flight inelastic instruments at the Spallation Neutron Source. Examples of such simulations for powder and single-crystal samples with various scattering kernels, including kernels for phonon and magnon scattering, are presented. With simulations that closely reproduce experimental results, scattering mechanisms can be turned on and off to determine how they contribute to the measured scattering intensities, improving our understanding of the underlying physics.

A hydrated, surface-tethered polymer network capable of fivefold change in thickness over a 25-37 degrees C temperature range has been demonstrated via neutron reflectivity and fluorescence microscopy to be a novel support for single lipid bilayers in a liquid environment. As the polymer swells from 170 to 900 A, it promotes both in- and out-of-plane fluctuations of the supported membrane. The cushioned bilayer proved to be very robust, remaining structurally intact for 16 days and many temperature cycles. The promotion of membrane fluctuations offers far-reaching applications for this system as a surrogate biomembrane.

The November 1, 1952 thermonuclear explosion ("Mike") produced all of the uranium isotopes U 239 , U 240 , • • • U 255 through multiple neutron capture by U 238 . The long-lived products of successive 0~ decays from these isotopes were measured mass spectrometrically and radiometrically. The logarithms of the abundances decline smoothly with increasing mass number; the even-mass abundances slightly exceed the geometric mean of adjacent odd-mass abundances. Some nuclear properties of neutron-rich heavy nuclides, not subject to ordinary investigation are inferred.