Unusual Defect Chemistry of Thorium Doping of PbS.

The unusual defect chemistry of thorium doping in the PbS system was investigated computationally to answer several open questions arising from the experimental observations. These include finding Th in a +4 oxidation state in contrast to Pb, attracting more than two oxygen atoms on average per thorium and affecting the growth morphology of PbS and its electronic properties. We find Th to be energetically stable at the lead lattice position in PbS and to attract 2-3 oxygens, including in the adjacent interstitial position, which binds strongly to Th.

Electrical and Optical Properties of γ-SnSe: A New Ultra-narrow Band Gap Material.

We describe the unusual properties of γ-SnSe, a new orthorhombic binary phase in the tin monoselenide system. This phase exhibits an ultranarrow band gap under standard pressure and temperature conditions, leading to high conductivity under ambient conditions. Density functional calculations identified the similarity and difference between the new γ-SnSe phase and the conventional α-SnSe based on the electron localization function.

Carbon nanowires under compression and their vibrational anomalies.

Anomalous pressure dependence of Raman frequencies of carbon nanowires encapsulated in carbon nanotubes has been recently reported. Two hypotheses have been proposed to explain this phenomenon in linear carbon chains: softening of a carbon bond with pressure or charge transfer to the chain. However, carbon chains bend easily under stress, although stable structures under these conditions have yet to be discovered.

A Novel Interstitial Site in Binary Rock-Salt Compounds.

The energetic and mechanical stability of interstitial point defects in binary rock-salt materials were studied using the first-principles method. A novel, stable, and energetically competitive interstitial site (base-interstitial) was identified for anion interstitials in rock-salts. The formation energies of base-interstitial defects were compared with well-explored tetrahedral (body-interstitial) and split interstitials and were found to be energetically highly competitive.

Dissociative Recombination of K and Rb in Diode-Pumped Alkali Lasers: Potential Energy Curves and Atomic Products.

Dissociative recombination (DR) of K and Rb ions is one of the most important processes in K and Rb diode-pumped alkali lasers (DPALs) strongly affecting their power. We report on the calculations of potential energy curves of the K and Rb molecular ions and of the diabatic Σ, Σ, Δ, Δ, Π, Π, Φ, and Φ valence states of K and Rb that provide the routes for DR of the ions. These curves are required for subsequent calculations of DR rate constants.

Machine learning-based modeling of high-pressure phase diagrams: Anomalous melting of Rb.

Modeling of phase diagrams and, in particular, the anomalous re-entrant melting curves of alkali metals is an open challenge for interatomic potentials. Machine learning-based interatomic potentials have shown promise in overcoming this challenge, unlike earlier embedded atom-based approaches. We introduce a relatively simple and inexpensive approach to develop, train, and validate a neural network-based, wide-ranging interatomic potential transferable across both temperature and pressure.

High-Pressure Phases of SnO and PbO: A Density Functional Theory Combined with an Evolutionary Algorithm Approach.

Tin monoxide, SnO, and its analog, lead monoxide, PbO, have the same tetragonal structure, shaped by nonbonding dispersion forces and lone pairs. The high-pressure phases of SnO and PbO have been explored in several experimental and theoretical studies, with conflicting results. In this study, the high-pressure structures of SnO and PbO are investigated using density functional theory calculations combined with an evolutionary algorithm to identify novel high-pressure phases.

Templated Assembly of Nanoparticles into Continuous Arrays.

The templated assembly of nanoparticles has been limited so far to yield only discontinuous nanoparticle clusters confined within lithographically patterned cavities. Here, we explored the templated assembly of nanoparticles into continuous 2D structures, using lithographically patterned templates with topographical features sized as the assembled nanoparticles. We found that these features act as nucleation centers, whose exact arrangement determines four possible assembly regimes (i) rotated, (ii) disordered, (iii) closely packed, and (iv) unpacked.

Phase Diagram of Binary Alloy Nanoparticles under High Pressure.

CALPHAD (CALculation of PHAse Diagram) is a useful tool to construct phase diagrams of various materials under different thermodynamic conditions. Researchers have extended the use of the CALPHAD method to nanophase diagrams and pressure phase diagrams. In this study, the phase diagram of an arbitrary A-B nanoparticle system under pressure was investigated.

Thermophysical Measurements in Liquid Alloys and Phase Diagram Studies.

Towards the construction of pressure-dependent phase diagrams of binary alloy systems, both thermophysical measurements and thermodynamic modeling are employed. High-accuracy measurements of sound velocity, density, and electrical resistivity were performed for selected metallic elements from columns III to V and their alloys in the liquid phase. Sound velocity measurements were made using ultrasonic techniques, density measurements using the gamma radiation attenuation method, and electrical resistivity measurements were performed using the four probe method.

Stability of cubic tin sulphide nanocrystals: role of ammonium chloride surfactant headgroups.

New semiconducting metastable cubic phases have been recently discovered in the tin monosulfide and monoselenide systems. Surface energy calculations and experimental studies indicate that this cubic π-phase is stabilized by specific ligand adsorption on the surface. In this work, it is shown experimentally that the synthesis carried out using mixtures of oleylamine and oleylammonium chloride (OACl) surfactants results in the cubic phase, transforming the growth from orthorhombic to cubic nanoparticles with increasing OACl concentration up to a limiting point.

π-Phase Tin and Germanium Monochalcogenide Semiconductors: An Emerging Materials System.

Cubic π-phase monochalcogenides (MX, M = Sn, Ge; X = S, Se) are an emerging new class of materials that has recently been discovered. Here, their thermodynamic stability, progress in synthetic routes, properties, and prospective applications are reviewed. The thermodynamic stability is demonstrated through density functional theory total energy and phonon spectra calculations, which show that the π-phase polytype is stable across the monochalcogenide family.

Novel experimental design for high pressure-high temperature electrical resistance measurements in a "Paris-Edinburgh" large volume press.

We present a novel experimental design for high sensitivity measurements of the electrical resistance of samples at high pressures (0-6 GPa) and high temperatures (300-1000 K) in a "Paris-Edinburgh" type large volume press. Uniquely, the electrical measurements are carried out directly on a small sample, thus greatly increasing the sensitivity of the measurement. The sensitivity to even minor changes in electrical resistance can be used to clearly identify phase transitions in material samples.

On the relation between the microscopic structure and the sound velocity anomaly in elemental melts of groups IV, V, and VI.

The sound velocity of some liquid elements of groups IV, V, and VI, as reported in the literature, displays anomalous features that set them apart from other liquid metals. In an effort to determine a possible common origin of these anomalies, extensive neutron diffraction measurements of liquid Bi and Sb were carried out over a wide temperature range. The structure factors of liquid Sb and Bi were determined as a function of temperature.