An Introduction to Neutron Scattering

Investigation of the Dynamic Behaviour of H2 and D2 in a Kinetic Quantum Sieving System

Porous organic cages (POCs) are nanoporous materials composed of discrete molecular units that have uniformly distributed functional pores. The intrinsic porosity of these structures can be tuned accurately at the nanoscale by altering the size of the porous molecules, particularly to an optimal size of 3.6 Å, to harness the kinetic quantum sieving effect. Previous research on POCs for isotope separation has predominantly centered on differences in the quantities of adsorbed isotopes. However, nuclear quantum effects also contribute significantly to the dynamics of the sorption process, offering additional opportunities for separating H2 and D2 at practical operational temperatures. In this study, our investigations into H2 and D2 sorption on POC samples revealed a higher uptake of D2 compared to that of H2 under identical conditions. We employed quasi-elastic neutron scattering to study the diffusion processes of D2 and H2 in the POCs across various temperature and pressure ranges. Additionally, neutron Compton scattering was utilized to measure the values of the nuclear zero-point energy of individual isotopic species in D2 and H2. The results indicate that the diffusion coefficient of D2 is approximately one-sixth that of H2 in the POC due to the nuclear quantum effect. Furthermore, the results reveal that at 77 K, D2 has longer residence times compared to H2 when moving from pore to pore. Consequently, using the kinetic difference of H2 and D2 in a porous POC system enables hydrogen isotope separation using a temperature or pressure swing system at around liquid nitrogen temperatures.

Changes in the hydrogen nuclear kinetic energy across the several phases of methylammonium lead tribromide

We present an experimental investigation of methylammonium lead tribromide single crystals in the orthorhombic, tetragonal, and cubic phases based on inelastic and deep inelastic neutron scattering experiments. We show how the average hydrogen nuclear kinetic energy, mainly affected by zero-point vibrational energies, shows differences larger compared to the changes simply related to temperature effects when moving from one phase to another. In particular, the Gaussian contribution to the average nuclear kinetic energy is larger in the tetragonal phase compared to the cubic and orthorhombic ones. Moreover, we find that the vibrational densities of states of MAPbBr3 single crystals in the orthorhombic phase are compatible with previously reported results on powder samples, and that the only vibrational modes that show slightly different frequencies compared to MAPbI3 are those in the energy range between 100 and 300 cm-1, related to librational/rotational modes. As these shifts are of about 10 cm-1, and do not affect any higher-energy vibrational mode, we conclude that the zero-point energies and average nuclear kinetic energies in the two-hybrid organic/inorganic perovskites are expected to be approximately the same within a harmonic framework.

Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline α-MgAgSb

Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material, in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe, and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound's intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials.

Composition-Nanostructure Steered Performance Predictions in Steel Wires

Neutron scattering in combination with scanning electron and atomic force microscopy were employed to quantitatively resolve elemental composition, nano- through meso- to metallurgical structures and surface characteristics of two commercial stainless steel orthodontic archwires—G&H and Azdent. The obtained bulk composition confirmed that both samples are made of metastable austenitic stainless steel type AISI 304. The neutron technique’s higher detection sensitivity to alloying elements facilitated the quantitative determination of the composition factor (CF), and the pitting resistance equivalent number (PREN) for predicting austenite stability and pitting-corrosion resistance, respectively. Simultaneous neutron diffraction analyses revealed that both samples contained additional martensite phase due to strain-induced martensite transformation. The unexpectedly high martensite content (46.20 vol%) in G&H was caused by combination of lower austenite stability (CF = 17.37, p = .03), excessive cold working and inadequate thermal treatment during material processing. Together, those results assist in revealing alloying recipes and processing history, and relating these with corrosion resistance and mechanical properties. The present methodology has allowed access to unprecedented length-scale (μm to sub-nm) resolution, accessing nano- through meso-scopic properties. It is envisaged that such an approach can be extended to the study and design of other metallic (bio)materials used in medical sciences, dentistry and beyond.

Non-destructive quantitation of hydrogen via mass-resolved neutron spectroscopy

This work introduces the use of mass-selective neutron spectroscopy as an analytical tool for the quantitative and non-destructive detection of hydrogen in bulk media. To this end, systematic measurements have been performed on a series of polyethylene standards of known thickness and density, in order to establish optimal data-acquisition protocols as well as associated limits of detection and quantitation. From this analysis, we conclude that state-of-the-art epithermal-neutron instrumentation enables the detection of aeral molar densities of bulk hydrogen in the μmol cm^-2 range. We also discuss potential improvements on the horizon, with a view to broadening the scope of the technique across chemistry, materials science, and engineering.

Unexpected Cation Dynamics in the Low-Temperature Phase of Methylammonium Lead Iodide: The Need for Improved Models

High-resolution inelastic neutron scattering and extensive first-principles calculations have been used to explore the low-temperature phase of the hybrid solar-cell material methylammonium lead iodide up to the well-known phase transition to the tetragonal phase at ca. 160 K. Contrary to original expectation, we find that the Pnma structure for this phase can only provide a qualitative description of the geometry and underlying motions of the organic cation. A substantial lowering of the local symmetry inside the perovskite cage leads to an improved atomistic model that can account for all available spectroscopic and thermodynamic data, both at low temperatures and in the vicinity of the aforementioned phase transition. Further and detailed analysis of the first-principles calculations reveals that large-amplitude distortions of the inorganic framework are driven by both zero-point-energy fluctuations and thermally activated cation motions. These effects are significant down to liquid-helium temperatures. For this important class of technological materials, this work brings to the fore the pressing need to bridge the gap between the long-range order seen by crystallographic methods and the local environment around the organic cation probed by neutron spectroscopy.