Thermodynamics From First Principles: Prediction Of Phase Diagrams And Materials Properties Using Density Functional Theory

First principles calculations have become one of the main computational methods in condensed matter physics and physical chemistry due to their high degree of accuracy without the usage of any fitting parameters. Interest has been growing in the engineering disciplines to exploit these properties to predict new materials with desired material properties, greatly accelerating the prototyping of materials over experimental methods with a degree of accuracy not found in other computational methods. In this thesis, first principles calculations will be applied to understand material properties of four classes of chemical systems with promising mechanical or thermodynamic applications, but whose experimental characterizations are either incomplete or questionable: boron carbide, molybdenum-niobium-tantalum-tungsten, copper-palladium-sulfur, and various early-late transition metal alloys. For all classes, the phase stability will be examined, of particular interest B-C and Mo-Nb-Ta-W due to the controversy surrounding the phase diagram of the former and the interesting “high-entropy alloy” behavior of the later. In addition, for Cu-Pd-S, various thermodynamic quantities associated with resistance to sulfur poisoning will be calculated, and for the early-late transition metal alloys, the elasticity will be examined, with attention paid towards possible transferability to the field of amorphous materials. All four of these disparate systems show overall semi-quantitative agreement with known experimental results, highlighting the versatility of first principles calculation

Prediction of A2 to B2 Phase Transition in the High Entropy Alloy MoNbTaW

<p>In this article, we show that an effective Hamiltonian fit with first-principles calculations predicts that an order/disorder transition occurs in the high-entropy alloy Mo-Nb-Ta-W. Using the Alloy Theoretic Automated Toolkit, we find <em>T</em> = 0 K enthalpies of formation for all binaries containing Mo, Nb, Ta, and W, and in particular, we find the stable structures for binaries at equiatomic concentrations are close in energy to the associated B2 structure, suggesting that at intermediate temperatures, a B2 phase is stabilized in Mo-Nb-Ta-W. Our previously published hybrid Monte Carlo (MC)/molecular dynamics (MD) results for the Mo-Nb-Ta-W system are analyzed to identify certain preferred chemical bonding types. A mean field free energy model incorporating nearest-neighbor bonds is derived, allowing us to predict the mechanism of the order/disorder transition. We find the temperature evolution of the system is driven by strong Mo-Ta bonding. A comparison of the free energy model and our MC/MD results suggests the existence of additional low-temperature phase transitions in the system likely ending with phase segregation into binary phases.</p

Prediction of orientational phase transition in boron carbide

<p>The assessed binary phase diagram of boron–carbon exhibits a single intrinsically disordered alloy phase designated “B₄C” with rhombohedral symmetry occupying a broad composition range that falls just short of the nominal carbon content of 20%. As this composition range is nearly temperature independent, the phase diagram suggests a violation of the third law of thermodynamics, which typically requires compounds to achieve a definite stoichiometry at low temperatures. By means of first principles total energy calculations we predict the existence of two stoichiometric phases at <em>T</em> = 0 K: one of composition B₄C with monoclinic symmetry; the other of composition B₁₃C₂ with rhombohedral symmetry. Using statistical mechanics to extend to finite temperatures, we demonstrate that the monoclinic phase reverts to the observed disordered nonstoichiometric rhombohedral phase above <em>T</em> = 600 K, along with a slight reduction on carbon content.</p

First Principles Modeling of the Temperature Dependent Ternary Phase Diagram for the Cu-Pd-S System

<p>As an aid to the development of hydrogen separation membranes, we predict the temperature dependent phase diagrams using first principles calculations combined with thermodynamic principles. Our method models the phase diagram without empirical fitting parameters. By applying thermodynamic principles and solid solution models, temperature-dependent features of the Cu–Pd–S system can be explained, specifically solubility ranges for substitutions in select crystalline phases. Electronic densities of states calculations explain the relative favorability of certain chemical substitutions. In addition, we calculate sulfidization thresholds for the Pd–S₂ system and activities for the Cu–Pd binary in temperature regimes where the phase diagram contains multiple solid phases.</p

Hybrid Monte Carlo/Molecular Dynamics Simulation of a Refractory Metal High Entropy Alloy

<p>The high entropy alloy containing refractory metals Mo-Nb-Ta-W has a body-centered cubic structure, which is not surprising given the complete mutual solubility in BCC solid solutions of all pairs of the constituent elements. However, first principles total energy calculations for the binaries reveal a set of distinct energy minimizing structures implying the likelihood of chemically ordered low-temperature phases. We apply a hybrid Monte Carlo and molecular dynamics method to evaluate the temperature-dependent chemical order. Monte Carlo species swaps allow for equilibration of the structure that cannot be achieved by conventional molecular dynamics. At 300 K (27 °C), a cesium-chloride ordering emerges between mixed (Nb,Ta) sites and mixed (Mo,W) sites. This order is lost at elevated temperatures.</p

Thermodynamic Equilibria in Carbon Nitride Photocatalyst Materials and Conditions for the Existence of Graphitic Carbon Nitride g‑C₃N₄

We quantify the thermodynamic equilibrium conditions that govern the formation of crystalline heptazine-based carbon nitride materials, currently of enormous interest for photocatalytic applications including solar hydrogen evolution. Key phases studied include the monomeric phase melem, the 1D polymer melon, and the hypothetical hydrogen-free 2D graphitic carbon nitride phase “g-C₃N₄”. Our study is based on density functional theory including van der Waals dispersion terms with different experimental conditions represented by the chemical potential of NH₃. Graphitic carbon nitride is the subject of a vast number of studies, but its existence is still controversial. We show that typical conditions found in experiments pertain to the polymer melon (2D planes of 1D hydrogen-bonded polymer strands). In contrast, equilibrium synthesis of heptazine (h)-based g-h-C₃N₄ below its experimentally known decomposition temperature requires much less likely conditions, equivalent to low NH₃ partial pressures around 1 Pa at 500 °C and around 10³ Pa even at 700 °C. A recently reported synthesis of triazine (t)-based g-t-C₃N₄ in a salt melt is interpreted as a consequence of the altered local chemical environment of the C₃N₄ nanocrystallites

First Principles Calculation of Elastic Moduli of Early-Late Transition Metal Alloys

<p>Motivated by interest in the elastic properties of high-strength amorphous metals, we examine the elastic properties of select crystalline phases. Using first-principles methods, we calculate elastic moduli in various chemical systems containing transition metals, specifically early (Ta,W) and late (Co,Ni). Theoretically predicted alloy elastic properties are verified for Ni-Ta by comparison with experimental measurements using resonant ultrasound spectroscopy. Comparison of our computed elastic moduli with effective medium theories shows that alloying leads to enhancement of bulk moduli relative to averages of the pure elements and considerable deviation of predicted and computed shear moduli. Specifically, we find an enhancement of bulk modulus relative to effective medium theory and propose a candidate system for high-strength, ductile amorphous alloys. Trends in the elastic properties of chemical systems are analyzed using force constants, electronic densities of state, and crystal overlap Hamilton populations. We interpret our findings in terms of the electronic structure of the alloys.</p

The ELSI Infrastructure for Scalable Electronic Structure Theory

Lightning talk accompanying the poster describing the ELSI infrastructure in detail.<br