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We report a first-principles study of lattice vibrations and thermal transport in Cs 2 SnI 6 , the vacancy-ordered double perovskite. Twofold rattlers of Cs atoms and SnI6 clusters in Cs 2 SnI 6 , being different from CsSnI 3 with only Cs atom rattlers, largely scatter heat-carrying acoustic phonons strongly coupled with low-lying optical phonons and lower phonon group velocity. Using renormalized phonon dispersions at finite temperatures, we reveal that anharmonicity and twofold rattling modes induce an ultralow thermal conductivity at room temperature.

The drive to develop maximal atom-efficient catalysts coupled to the continuous striving for more sustainable reactions has led to an ever-increasing interest in single-atom catalysis. Based on a periodic conceptual density functional theory (cDFT) approach, fundamental insights into the reactivity and adsorption of single late transition metal atoms supported on a fully hydroxylated amorphous silica surface have been acquired. In particular, this investigation revealed that the influence of van der Waals dispersion forces is especially significant for a silver (98 %) or gold (78 %) atom, whereas the oxophilicity of the Group 8-10 transition metals plays a major role in the interaction strength of these atoms on the irreducible SiO 2 support. The adsorption energies for the less-electronegative row 4 elements (Fe, Co, Ni) ranged from -1.40 to -1.92 eV, whereas for the heavier row 5 and 6 metals, with the exception of Pd, these values are between -2.20 and -2.92 eV. The deviating behavior of Pd can be attributed to a fully filled d-shell and, hence, the absence of the hybridization effects. Through a systematic analysis of cDFT descriptors determined by using three different theoretical schemes, the Fermi weighted density of states approach was identified as the most suitable for describing the reactivity of the studied systems. The main advantage of this scheme is the fact that it is not influenced by fictitious Coulomb interactions between successive, charged reciprocal cells. Moreover, the contribution of the energy levels to the reactivity is simultaneously scaled based on their position relative to the Fermi level. Finally, the obtained Fermi weighted density of states reactivity trends show a good agreement with the chemical characteristics of the investigated metal atoms as well as the experimental data., (© 2020 Wiley-VCH GmbH.)

Fission and fusion reactors can only play a role in the future energy landscape if they are inherently safe by design. For some reactor concepts, a major remaining issue is the undesired production of radiotoxic 210 Po. To filter out the volatile Po species, information on their molecular composition is needed. An experimental characterization is very challenging due to the large required amount of radioactive Po. An alternative quantum chemistry approach was taken to predict the temperature-dependent stability of relevant diatomic Po-containing molecules. Experimental data on lighter analogue molecules was used to establish a well-founded methodology. The relative occurrence of the Po species was estimated in the cover gas of (i) the lead-bismuth eutectic coolant in the accelerator-driven MYRRHA fission reactor and (ii) the Pb-Li eutectic tritium breeder in the DEMO fusion reactor. In both systems, Po is found to occur mainly as PbPo molecules and atomic Po.

Using first-principles total energy calculations we have evaluated the thermodynamics and the electronic properties of intrinsic vacancy defects in orthorhombic CaZrO 3 . Charge density calculations and the atoms-in-molecules concept are used to elucidate the changes in electronic properties of CaZrO 3 upon the introduction of vacancy defects. We explore the chemical stability and defect formation energies of charge-neutral as well as of charged intrinsic vacancies under various synthesis conditions and also present full and partial Schottky reaction energies. The calculated electronic properties indicate that hole-doped state can be achieved in charge neutral Ca vacancy containing CaZrO 3 under oxidation condition, while reduction condition allows to control the electrical conductivity of CaZrO 3 depending on the charge state and concentration of oxygen vacancies. The clustering of neutral oxygen vacancies in CaZrO 3 is examined as well. This provides useful information for tailoring the electronic properties of this material. We show that intentional incorporation of various forms of intrinsic vacancy defects in CaZrO 3 allows to considerably modify its electronic properties, making this material suitable for a wide range of applications.

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements., (Copyright © 2016, American Association for the Advancement of Science.)

Experimental nonstoichiometries of colloidal nanocrystals such as CdSe and PbS are accounted for by attributing to each constituent atom and capping ligand a formal charge equal to its most common oxidation state to obtain an overall neutral nanocrystal. In spite of its apparent simplicity, little theoretical support of this approach-called here the oxidation-number sum rule-is present in the current literature. Here, we introduce the ligand addition energy, which we define as the energy gained or expended upon the transfer of one ligand from a reference state to a metal-rich solid surface. For the combination of CdSe, ZnSe and InP with either chalcogen, halogen or hydrochalcogen ligands, we compute successive ligand addition energies using ab initio methods and determine the thermodynamically stable surface composition as that composition where ligand addition turns endothermic. We find that the oxidation-number sum rule is valid in many situations, although exceptions occur for each material studied, most notably when exposed to small oxidative ligands. In the case of InP, however, violations are more severe, extending toward the entire chalcogen ligand family. In addition, we find that electronegativity rather than chemical hardness is a reasonable predictor for ligand addition energies, with the most electronegative ligands yielding the most exothermic addition energies. Finally, we argue that the ligand addition energy will be a most useful quantity for future computational studies on the structure, stability and reactivity of nanocrystal surfaces.

We investigate the relation between the chain length of ligands used and the size of the nanocrystals formed in the hot injection synthesis. With two different CdSe nanocrystal syntheses, we consistently find that longer chain carboxylic acids result in smaller nanocrystals with improved size dispersions. By combining a more in-depth experimental investigation with kinetic reaction simulations, we come to the conclusion that this size tuning is due to a change in the diffusion coefficient and the solubility of the solute. The relation between size tuning by the ligand chain length and the coordination of the solute by the ligands is further explored by expanding the study to amines and phosphine oxides. In line with the weak coordination of CdSe nanocrystals by amines, no influence of the chain length on the nanocrystals is found, whereas the size tuning brought about by phosphine oxides can be attributed to a solubility change. We conclude that the ligand chain length provides a practical handle to optimize the outcome of a hot injection synthesis in terms of size and size dispersion and can be used to probe the interaction between ligands and the actual solute.