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Phase-pure synthesis has been a major challenge for metal oxynitrides due to their sensitivity to synthesis conditions and the limited understanding of the underlying thermodynamics. The beta-phase tantalum oxynitride (beta-TaON), a promising material for applications in photocatalysis and energy storage, is particularly difficult to synthesize in a reproducible, phase-pure form. In this work, a computational thermodynamic model with experimental validation is presented to evaluate the phase-pure synthesis conditions for beta-TaON via ammonolysis reactions. The finite-temperature thermochemical properties of the reactant, product, and byproduct phases are predicted via first-principles calculations with the quasi-harmonic approach (QHA) as well as implemented from available thermodynamic databases. With the thermochemical properties, a thermodynamic model based on the CALculation of PHAse Diagrams (CALPAHD) approach is developed to assess the phase equilibria associated with the synthesis reactions and correspondingly predict the synthesis window for beta-TaON. A three-dimensional phase diagram is predicted as a function of gas composition and temperature, providing insights into optimal synthesis conditions. The computational predictions are further compared with available experimental data, offering a systematic framework for phase-pure beta-TaON synthesis., Comment: 12 pages, 7 figures

Lattice Parameter is an important material feature in High Entropy Alloy (HEA) Design. Vegards Law is typically used to estimate lattice parameters but is often inaccurate for metal alloys due to an inability to account for charge transfer which can affect atomic volumes. The present study used ab initio simulation to calculate bond lengths between atoms of dissimilar elements in B2 intermetallic compounds which was then combined with a bond counting model to produce a model to estimate the lattice parameters of Refractory BCC HEAS. The model was tested using a supercell method which modeled various random solid solution HEAs. The proposed model produced lattice parameters with superior accuracy to Vegards Law without the need for large DFT calculations or fitting parameters. The proposed model had a root mean squared error (RMSE) of 0.006 Angstroms which is half that of Vegards Law RMSE (0.012 Angstrom).

Refractory multi-principal element alloys (RMPEAs) are promising materials for high-temperature structural applications. Here, we investigate the role of chemical short-range ordering (CSRO) on dislocation glide in two model RMPEAs - TaNbTi and MoNbTi - using a multi-scale modeling approach. A highly accurate machine learning interatomic potential was developed for the Mo-Ta-Nb-Ti system and used to demonstrate that MoNbTi exhibits a much greater degree of SRO than TaNbTi and the local composition has a direct effect on the unstable stacking fault energies (USFE). From mesoscale phase-field dislocation dynamics simulations, we find that increasing SRO leads to higher mean USFEs, thereby increasing the stress required for dislocation glide. The gliding dislocations experience significant hardening due to pinning and depinning caused by random compositional fluctuations, with higher SRO decreasing the degree of USFE dispersion and hence, amount of hardening. Finally, we show how the morphology of an expanding dislocation loop is affected by the applied stress, with higher SRO requiring higher applied stresses to achieve smooth screw dislocation glide.

Body-centered cubic (bcc) refractory multicomponent alloys are of great interest due to their remarkable strength at high temperatures. Meanwhile, further optimizing the chemical compositions of these alloys to achieve a combination of high strength and room-temperature ductility remains challenging, which would require systematic predictions of the correlated alloy properties across a vast compositional space. In the present work, we performed first-principles calculations with the special quasi-random structure (SQS) method to predict the unstable stacking fault energy ($\gamma_{usf}$) of the $(1\bar10)[111]$ slip system and the $(1\bar10)$-plane surface energy ($\gamma_{surf}$) for 106 individual binary, ternary and quaternary bcc solid-solution alloys with constituent elements among Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re and Ru. Moreover, with the first-principles data and a set of physics-informed descriptors, we developed surrogate models based on statistical regression to accurately and efficiently predict $\gamma_{usf}$ and $\gamma_{surf}$ for refractory multicomponent alloys in the 10-element compositional space. Building upon binary and ternary data, the surrogate models show outstanding predictive ability in the high-order multicomponent systems. The ratio between $\gamma_{surf}$ and $\gamma_{usf}$ is a parameter to reflect the potency of intrinsic ductility of an alloy based on the Rice model of crack-tip deformation. Therefore, using the surrogate models, we performed a systematic screening of $\gamma_{usf}$, $\gamma_{surf}$ and their ratio over 112,378 alloy compositions to search for alloy candidates that may have enhanced strength-ductile synergies. Search results were also confirmed by additional first-principles calculations.

Chemical design of SiO2-based glasses with high elastic moduli and low weight is of great interest. However, it is difficult to find a universal expression to predict the elastic moduli according to the glass composition before synthesis since the elastic moduli are a complex function of interatomic bonds and their ordering at different length scales. Here we show that the densities and elastic moduli of SiO2-based glasses can be efficiently predicted by machine learning (ML) techniques across a complex compositional space with multiple (>10) types of additive oxides besides SiO2. Our machine learning approach relies on a training set generated by high-throughput molecular dynamic (MD) simulations, a set of elaborately constructed descriptors that bridges the empirical statistical modeling with the fundamental physics of interatomic bonding, and a statistical learning/predicting model developed by implementing least absolute shrinkage and selection operator with a gradient boost machine (GBM-LASSO). The predictions of the ML model are comprehensively compared and validated with a large amount of both simulation and experimental data. By just training with a dataset only composed of binary and ternary glass samples, our model shows very promising capabilities to predict the density and elastic moduli for k-nary SiO2-based glasses beyond the training set. As an example of its potential applications, our GBM-LASSO model was used to perform a rapid and low-cost screening of many (~105) compositions of a multicomponent glass system to construct a compositional-property database that allows for a fruitful overview on the glass density and elastic properties.

The interactions between solute atoms and crystalline defects such as vacancies, dislocations, and grain boundaries play an essential role in determining physical, chemical and mechanical properties of solid-solution alloys. Here we present a universal correlation between two electronic factors and the solute-defect interaction energies in binary alloys of body-centered-cubic (bcc) refractory metals (such as W and Ta) with transition-metal substitutional solutes. One electronic factor is the bimodality of the d-orbital local density of states for a matrix atom at the substitutional site, and the other is related to the hybridization strength between the valance sp- and d-bands for the same matrix atom. Remarkably, the correlation is independent of the types of defects and the locations of substitutional sites, following a linear relation for a particular pair of solute-matrix elements. Our findings provide a novel and quantitative guidance to engineer the solute-defect interactions in alloys based on electronic structures.

Theoretical descriptions of the Seebeck coefficient in terms of the differential electrical conductivity given by Cutler and Mott is the foundation of later works in terms of transmission function from the thermoelectric transport theory. On the other hand, recent studies in the literature have shown the relation between the Seebeck coefficient and chemical potential of electrons. In this work, this relation is rigorously derived from fundamental thermodynamics, and an formalism for the parameter-free calculation of the Seebeck coefficient based on the electronic density-of-states from first-principles calculations is presented. Numerical results are given using the n-type La3-xTe4 thermoelectric material as the prototype. With the rigid band approximation, the calculated temperature dependences of the Seebeck coefficients of La3-xTe4 as a function of carrier concentration show excellent agreements with experimental data.

Thermoelectric effects, measured by the Seebeck coefficients, refer to the phenomena in which a temperature difference or gradient imposed across a thermoelectric material induces an electrical potential difference or gradient, and vice versa, enabling the direct conversion of thermal and electric energies. All existing understanding and first-principles calculations of Seebeck coefficients have been based on the Boltzmann kinetic transport theory. Here we demonstrate that the Seebeck coefficient is a well-defined thermodynamic quantity that can be determined from the change in the chemical potential of electrons induced by the temperature change and thus can be efficiently computed solely based on the electronic density of states through first-principles calculations at different temperatures. The proposed approach is demonstrated using the prototype PbTe and SnSe thermoelectric materials. The proposed thermodynamic approach dramatically simplifies the calculations of Seebeck coefficients, making it possible to search for high performance thermoelectric materials using high-throughput first-principles calculations.