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Calcium-aluminum-rich inclusions (CAIs) in chondritic meteorites are composed of refractory minerals thought to be the first solids to have formed in the solar nebula. Among them, hibonite, nominally CaAl12O19, holds particular interest because it can incorporate significant amounts of Ti into its crystal structure in both Ti3+ and Ti4+ oxidation states. The relative amounts of these cations that are incorporated reflect the redox conditions under which the grain formed or last equilibrated and their measurement can provide insight into the thermodynamic landscape of the early solar nebula. Here we develop a new method for the quantification of Ti oxidation states using electron energy-loss spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscope (STEM) to apply it to hibonite. Using a series of Ti-bearing oxides, we find that the onset intensity of the Ti L2,3 edge decreases with increasing Ti-oxidation state, which is corroborated by simulated Ti-oxide spectra using first-principles density-functional theory. We test the relationship on a set of synthetic hibonite grains with known Ti4+/ΣTi values and apply the developed method on a hibonite grain from a compact type A inclusion in the Northwest Africa (NWA) 5028 CR2 carbonaceous chondrite. The STEM-EELS data show that the chondritic hibonite grain is zoned with a Ti4+/ΣTi ratio ranging from 0.78 ± 0.04 to 0.93 ± 0.04 over a scale of 100 nm between the core and edge of the grain, respectively. The Ti substitution sites are characterized by experimental and calculated high-angle annular-dark-field (HAADF) images and atomic-level EEL spectrum imaging. Simulated HAADF images reveal that Ti is distributed between the M2 and M4 sites while Mg sits on the M3 site. Quantitative energy-dispersive X-ray spectroscopy shows that this grain is also zoned in Al and Ti. The Mg distribution is not well correlated with that of Ti and Ti4+/ΣTi at the nanoscale. The spatial decoupling of the element composition and Ti-oxidation states suggests a multistage evolution for this hibonite grain. We hypothesize that Ti and Mg were incorporated into the structure during condensation at high temperature through multiple reactions. Transient heating, presumably in the solar nebula, adds complexity to the crystal chemistry and potentially redistributed Ti and Mg. Concurrently, the formation of oxygen vacancies as a result of a reducing gas, led to the reduction of Ti4+ to Ti3+. The multiple defect reactions occurring in this single hibonite crystal preclude a simple relationship between the Ti4+/ΣTi and the fO2 of formation. However, moving forward, these measurements are fundamental inputs for modeling of the thermodynamic conditions under which hibonite formed in the early solar nebula.

We report a first-principles-based thermodynamic investigation of the interplay between cation inversion and twinning in MgAl2O4 spinel (MAS). We examine the atomic-scale structure of (111) twins and characterize the local octahedral and tetrahedral distortions. We observe that the asymmetric nature of polyhedral distortions about the (111) twin plane causes anisotropy in cation inversion energies near the planar fault. The predicted enthalpies and entropies of inversion reveal that in comparison to the Kagome layer, the anti-site occupancies of Al and Mg, i.e., cation inversion, on the mixed-cation-layer near the twin boundary are more favorable and stable in the entire range of temperature of twin stability. Structurally, such a stable inversion is necessitated by the minimization in the polyhedral distortions, especially by the octahedral distortion, which exhibits a reduction of four orders of magnitude relative to the polyhedra with no inversion. The fundamental understanding obtained on the thermodynamics of the twin-cation inversion interplay in conjunction with the kinetics of inversion was used as a basis for developing a thermochronometer for deducing the temperature of twinning in MAS. This work serves as an important steppingstone for experimental characterization of MAS structures within a host of Earth and planetary materials. In the case of the latter, our results enable the use of planar faults, such as twins, as important markers for deducing the physical and chemical landscape that MAS experienced in its evolution and transport within the solar protoplanetary disk.

The ability to polymerize solid-state C60 molecular crystals via intermolecular covalent bond formation provides controllable routes to obtaining structures with tunable mechanical and thermal properties. In this regard, using molecular dynamics simulations, fundamental insights into the interplay between degree of polymerization and the ensuing evolution in the thermophysical properties of C60 polymorphs are obtained for the first time. In particular, it is unambiguously shown that 2-D polymerized C60 polymorphs show a two order of magnitude enhancement in the thermal conductivity and one order of magnitude change in the elastic stiffness. The significant increase in the thermal conductivity is correlated to the presence of new THz thermal phonon modes, characterized by larger mean free paths. In addition, it is also seen that the Debye temperature of the C60 structures is strongly dependent on the extent of polymerization. The new understanding obtained in this work provides valuable guidelines for the design and development of new C60 based phononic metamaterials for applications as vibrational and thermal management systems.

New insight into the temperature dependent evolution of internal strain in the austenite phase during the martensitic phase transformation in NiTi shape memory alloys is provided via classical molecular dynamics simulations that employ well-established interatomic potentials for NiTi. It is shown, for the first time, that the developed strain tensor in the austenite phase is tetragonal in nature, with exponential temperature-dependence. Equally importantly, it is found that the developed internal strain (parallel to the habit plane) in the austenite varies linearly with the evolving martensite phase fraction. Interestingly, the Richard’s equation is found to describe the temperature dependence of the martensite phase fraction as well as the internal strain components parallel to the habit plane in the austenite phase. An analysis of the temperature dependent phonon dispersion of strained austenite revealed the competition between phonon softening of the TA2 branch and internal strain that leads to stabilization of the austenite phase in the two phase regime.

A phase diagram-assisted powder processing approach is shown to produce low-oxygen (0.06 wt%O) ZrB2 ceramics using minimal B4C additions (0.25 wt%) and spark plasma sintering. Scanning electron microscopy and scanning transmission electron microscopy with elemental spectroscopy are used to identify “trash collector” oxides. These “trash collector” oxides are composed of manufacturer metal powder impurities that form discreet oxide particles due to the absence of standard Zr–B oxides found in high oxygen samples. A preliminary Zr–B–C–O quaternary thermodynamic database developed as a part of this work was used to calculate the ZrO2–B4C pseudobinary phase diagram and ZrB2–ZrO2–B4C pseudoternary phase diagrams. We use the calculated equilibrium phase diagrams to characterize the oxide impurities and show the direct reaction path that allows for the formation of ZrB2 with an oxygen content of 0.06 wt%, fine grains (3.3 μm) and superior mechanical properties (flexural strength of 660 MPa).

The mechanical response of symmetric tilt grain boundaries (GBs) in silicon bicrystals under shear loading are characterized using molecular dynamics simulations. It is seen that under shear, high-angle GBs namely Σ5 and Σ13 having a rotation axis [0 0 1] demonstrate coupled GB motion, such that the displacement of grains parallel to the GB interface is accompanied by normal GB motion. An atomic-scale characterization revealed that concerted rotations of silicon tetrahedra within the GB are the primary mechanisms leading to the coupled GB motion. Interestingly, so far, this phenomenon has only been examined in detail for metallic systems. A distinguishing feature of the coupled GB motion observed for the silicon symmetric tilt bicrystals as compared to metallic bicrystals is the fact that in the absence of shear, spontaneous coupled motion is not observed at high temperatures.

The dynamics of SiC grain boundaries under shear are characterized using molecular dynamics simulations. At low-temperatures, low-angle grain boundaries exhibit stick-slip behavior due to athermal climb of edge dislocations along the grain boundary. With increasing temperature stick-slip becomes less pronounced due to dislocation glide, and at high-temperatures, structural disordering of the low-angle grain boundary inhibits stick-slip. In contrast, structural disordering of the high-angle grain boundary is induced under shear even at low temperatures, resulting in a significantly dampened stick-slip behavior.

Antiphase boundaries (APBs) play a crucial role in the anomalous yield behavior exhibited by Ni3Al with L12 structure. We investigated the changes in the vibrational properties associated with the formation of (0 0 1) and (1 1 1) APBs in Ni3Al by employing first-principles calculations. The phonon density of states of Ni3Al with and without (0 0 1) and (1 1 1) APBs revealed an interesting result: the (0 0 1) APB softens the phonons in its vicinity, while the (1 1 1) stiffens them. We also calculated the finite-temperature (0 0 1) and (1 1 1) APB Gibbs free energies from the first-principles quasi-harmonic approximation. The vibrational entropy of formation is positive (e.g. 0.053 mJ K−1 m−2 at 300 K) for the (0 0 1) APB and is negative (e.g. −0.0157 mJ K−1 m−2 at 300 K) for the (1 1 1) APB over the entire temperature range. We also find a significant change in the thermal electronic free energy due to the creation of the (0 0 1) or (1 1 1) APB. The anisotropy ratio of the APB energies, i.e. the ratio of the (1 1 1) APB free energy to the (0 0 1) APB free energy, changes from 2.9 at 300 K to 15.9 at 1000 K.

The NaCl–KCl–ZnCl2 ternary system is examined and modeled using the CALPHAD methodology in conjunction with molecular dynamics (MD) simulations. In particular, MD simulations are used for calculating liquid enthalpies of mixing as a function of composition for the ternary and its binary sub-systems. In addition, key structural features are obtained from MD that is then used for informing the employed two-sublattice ionic liquid model (Na+1, K+1: Cl−1, ZnCl2), which describes the ternary liquid phase. The structure of the simulated liquid systems show that Zn+2 cations primarily exhibit 4-fold coordination in addition to a smaller percentage of 5-fold followed by 3-fold coordination; in contrast, the coordination of both Na+ and K+ cations are distributed between 2- and 4-fold states. The optimized self-consistent thermodynamic model parameters show good agreement with MD data obtained in this work and available experimental literature data.

Thermodynamics of defects in the (LaxCa1-x)FeO3-δ perovskite is modeled by means of the CALPHAD approach. In this phase, the A-sites are occupied by La+3 and Ca+2, and Fe in the B-site is known to exist in +2, +3, and +4 oxidation states depending on the oxygen vacancy concentration. Therefore, the ionic sublattice model: (La+3, Ca+2)(Fe+2, Fe+3, Fe+4)(O−2, Va)3 is used to describe the phase, and the model parameters are evaluated from experimental oxygen nonstoichiometry and phase equilibria data. With the Fe+2 and Fe+4 treated as the major species in the B-site, the calculated phase diagrams are in good agreement with the experimentally reported phase equilibria data. The concentration of various defects in (LaxCa1-x)FeO3-δ as a function of oxygen partial pressure and temperature are calculated at different concentrations of Ca. At high oxygen partial pressures, Fe+4 is predicted to be dominant while Fe+2 is dominant at low oxygen partial pressures.

The energetics of defects in B 4+ x C boron carbide and β -boron are studied through first-principles calculations, the supercell phonon approach and the Debye–Gruneisen model. It is found that suitable sublattice models for β -boron and B 4+ x C are B 101 (B,C) 4 and B 11 (B,C) (B,C,Va) (B,Va) (B,C,Va), respectively. The thermodynamic properties of B 4+ x C, β -boron, liquid and graphite are modeled using the CALPHAD approach based on the thermochemical data from first-principles calculations and experimental phase equilibrium data in the literature. The concentrations of various defects are then predicted as a function of carbon composition and temperature.

Molten mixtures of network-forming covalently bonded ZnCl2 and network-modifying ionically bonded NaCl and KCl salts are investigated as high-temperature heat transfer fluids for concentrating solar power plants. Specifically, using molecular dynamics simulations, the interplay between the extent of the network structure, composition, and the transport properties (viscosity, thermal conductivity, and diffusion) of ZnCl2-NaCl-KCl molten salts is characterized. The Stokes-Einstein/Eyring relationship is found to break down in these network-forming liquids at high concentrations of ZnCl2 (>63 mol. %), while the Eyring relationship is seen with increasing KCl concentration. Further, the network modification due to the addition of K ions leads to formation of non-bridging terminal Cl ions, which in turn lead to a positive temperature dependence of thermal conductivity in these melts. This new understanding of transport in these ternary liquids enables the identification of appropriate concentrations of the network formers and network modifiers to design heat transfer fluids with desired transport properties for concentrating solar power plants.

Thermodynamic description of the Al–Pt binary system is modeled by combining first-principles calculations with the CALPHAD method. The four-sublattice and two-sublattice compound energy formalisms are used to model the ordered L1 2 and B2 phases, respectively. The modeling includes the solution phases and the stoichiometric Al 21 Pt 5 , Al 21 Pt 8 , Al 2 Pt, Al 3 Pt 2 , AlPt, Al 3 Pt 5 and AlPt 2 intermetallic phases. The enthalpy of formation for the stoichiometric compounds and end-members of ordered L1 2 and B2 phases are calculated from first-principles study. In addition the enthalpies of mixing for the disordered fcc and bcc phases and the ordered L1 2 and B2 phases are calculated from first-principles study of special quasirandom structures. The obtained phase equilibria and thermodynamic properties are in good agreement with the experimental data in the literature as well as the first-principles calculations from the present work.

Ab initio molecular dynamics simulations have been used to predict diffusion coefficients in Al–Si binary melts as a function of composition and temperature. The self-diffusion coefficients of Al and Si are obtained from the mean-squared displacements and velocity autocorrelation functions employing two different exchange correlation functionals. The Si-concentrations include 0, 7, 12 and 15 at% Si with temperature ranging from 900–1800 K. In these Al–Si melts, aluminum and silicon exhibit nearly identical diffusion coefficients. The first nearest neighbor-shell coordinations of Al and Si vary slightly with composition and temperature and exhibit no significant changes in the vicinity of the eutectic. The density of the binary liquids including the eutectic composition (Al-12 at% Si) showed linear dependence on temperature.

The effect of divalent metal-dopants, Mg, Ca, and Zn, on the stability of LiBH4 is studied by using the first-principles calculations and CALPHAD (CALculation of PHAse Diagram) modeling. The ground states of Mg1/2BH4, Ca1/2BH4, and Zn1/2BH4 are shown to be I 4 ¯ m 2 , F2dd, and I 4 ¯ m 2 , respectively, through first-principles calculations. Positive enthalpy of mixing between Li and the alloying element is predicted, indicating unfavorable solubility of alloying elements in LiBH4 and thus offering possibility to decrease the stability of LiBH4. The ionic sublattice model of (Li+, M2+, Va)1(BH4−)1 is adopted for the metal substituted LiBH4 phase. It is observed that the addition of Mg or Zn has limited effect as the decomposition temperature is between those of LiBH4 and M1/2BH4 for Mg and Zn substitutions. LiBH4 is destabilized with magnesium borides or LiZn4 formation but its decomposition temperature is higher than that of M1/2BH4. On the other hand, the addition of Ca significantly reduces the H2 releasing temperature due to the formation of highly stable CaB6.

Stress introduces anisotropy in the transport coefficients in materials, affecting diffusion. Using first-principles quantum-mechanical methods for activation barriers of atomic jumps, combined with the extended self-consistent mean-field theory to compute transport coefficients with strain-reduced symmetry, we predict significant stress-induced anisotropy for Si impurity diffusion in nickel. This causes complex spatial- and temperature-dependent fluxes; as an example, the heterogenous strain field of a dislocation creates unusual flow patterns that affect mechanical and segregation behavior.

A first-principles approach to calculating the elastic stiffness coefficients at finite temperatures was proposed. It is based on the assumption that the temperature dependence of elastic stiffness coefficients mainly results from volume change as a function of temperature; it combines the first-principles calculations of elastic constants at 0 K and the first-principles phonon theory of thermal expansion. Its applications to elastic constants of Al, Cu, Ni, Mo, Ta, NiAl, and Ni₃Al from 0 K up to their respective melting points show excellent agreement between the predicted values and existing experimental measurements.

The anisotropy of antiphase boundary (APB) energies in ordered L12–Ni3Al is studied, owing to its importance in understanding the anomalous flow behavior of this intermetallic compound. We report first-principle calculations for two types of APBs in the ordered compound: (001) and (111). The magnetic perturbations associated with these APBs are calculated as a function of supercell size to ensure that their periodic images are isolated both magnetically and elastically. The magnetic perturbation associated with the (111) APB is wider than that of the (001) APB. The fully relaxed, spin-polarized APB energies for (001) and (111) APBs are calculated to be 82 mJ/m2 and 177 mJ/m2, respectively. The resulting anisotropy ratio of σ(111)/σ(001)=2.16, which is significantly higher than previously reported values, suggests that cross-slip between (001) and (111) is highly favorable.