New parameters for the electronegativity equalization model (EEM) and the split-charge equilibration (SQE) model are calibrated for silicate materials, based on an extensive training set of representative isolated systems. In total, four calibrations are carried out, two for each model, either using iterative Hirshfeld (HI) charges or ESP grid data computed with Density Functional Theory (DFT) as a reference. Both the static (ground state) reference quantities and their responses to uniform electric fields are included in the fitting procedure. The EEM model fails to describe the response data, while the SQE model quantitatively reproduces all the training data. For the ESP-based parameters, we found that the reference ESP data are only useful at those grid points where the electron density is lower than 10 −3 a.u. The density value correlates with a distance criterion used for selecting grid points in common ESP fitting schemes.All parameters are validated with DFT computations on an independent set of isolated systems (similar to the training set), and on a set of periodic systems including dense and microporous crystalline silica structures, zirconia, and zirconium silicate. Although the transferability of the parameters to new isolated systems poses no difficulties, the atomic hardness parameters in the HI-based models must be corrected to obtain accurate results for periodic systems. The SQE/ESP model permits the calculation of the ESP with similar accuracy in both isolated and periodic systems.

The mean bond length d between a central atom and its nearest neighbors can be estimated from the position of the first peak in the radial distribution function g(r). However, as we demonstrate here, this estimate does not allow one to deduce temperature-induced changes in d. Instead, skewness has to be included into the analysis, which can be achieved, for example, via the skew normal distribution (SND). Fits to the first peak using the SND give bond length in good agreement with direct measurements of nearest-neighbor distribution functions in crystals as well as with a Voronoi-tessellation based detection of nearest-neighbors in liquids. While the location of the first peak in g(r) may shift to smaller values with increasing temperature for three studied liquids-argon, copper, and the bulk-metallic-glass (BMG) forming alloy ZrCuAl-we find our improved estimates of d to systematically increase with temperature in all cases. Recent conclusions on temperature-induced bond contractions in simple metallic or BMG-forming liquids may therefore have arisen from the neglect of skewness effects.

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Using density-functional theory based simulations, we study how initially disconnected zinc phosphate molecules respond to different externally imposed deformations. Hybridization changes are observed in all cases, in which the coordination of zinc atoms changes irreversibly from tetrahedral to seesaw and square pyramidal, whereby the system stiffens substantially. The point at which stiff networks are formed does not only depend on the hydrostatic pressure. Stress anisotropy generally reduces the required hydrostatic network formation pressure. Moreover, networks obtained under isotropic deformations turn out stiffer, elastically more isotropic, and lower in energy after decompression than those produced under anisotropic stresses. We also find that the observed stress-memory effects are encoded to a significant degree in the arrangement of atoms in the second neighbor shell of the zinc atoms. These findings refine previously formulated conjectures of pressure-assisted cross-linking in zinc phosphate-based anti-wear films. Graphical Abstract

A mixed radial, angular three-body distribution function g3(rBC, θABC) is introduced, which allows the local atomic order to be more easily characterized in a single graph than with conventional correlation functions. It can be defined to be proportional to the probability of finding an atom C at a distance rBC from atom B while making an angle θABC with atoms A and B, under the condition that atom A is the nearest neighbor of B. As such, our correlation function contains, for example, the likelihood of angles formed between the nearest and the next-nearest-neighbor bonds. To demonstrate its use and usefulness, a visual library for many one-component crystals is produced first and then employed to characterize the local order in a diverse body of elemental condensed-matter systems. Case studies include the analysis of a grain boundary, several liquids (argon, copper, and antimony), and polyamorphism in crystalline and amorphous silicon including that obtained in a tribological interface.

Vibrational spectroscopy data were used to gain insight into the possible locations of extra oxygen ions introduced into La 8+x Sr 2−y (SiO 4 ) 6 O 2+δ compounds to raise their ionic conductivity. Perturbations observed in the Raman and infrared spectra of these compounds with increasing δ were explained by using the ab initio calculation results for the fully stoichiometric (x = y = δ = 0) lattice. This allowed the inference that the extra oxygen ions are incorporated into La-O tunnel-like fragments inherent in the studied structures.

In this work, we provide a theoretical analysis of quantized capacitance (also referred to as solvated Coulomb blockade) as a pseudocapacitive energy storage mechanism. In particular, we examine how redox species exhibiting quantized capacitance might be engineered to satisfy two basic criteria in the design of an “ideal” pseudocapacitive energy storage mechanism: (1) a near-rectangular voltammetric profile which mimics that of double-layer capacitance and (2) a linear rise in the pseudocapacitive current with respect to the voltammetric scan rate. It is demonstrated that nanoparticles exhibiting quantized capacitance may satisfy the first criterion by tailoring their charging and reorganization energies. It is also shown that the second criterion can be satisfied so long as the voltammetric scan rate does not exceed the electron-transfer rate. By formulating a comprehensive theoretical framework for understanding the electron-transfer properties of quantized capacitance, we arrive at a general phenomenological description of how pseudocapacitive properties might be practically engineered through this mechanism.