arXiv subjects

Eva Zurek

Publications and source records attributed to Eva Zurek.

At least 37 records · Page 2Linked to original sources

arXiv2024

Interfacial defect properties of high-entropy carbides: Stacking faults, Shockley partial dislocations, and a new Evans-Polanyi-Semenov relation

Using first principles calculations, {111} intrinsic stacking fault (ISF) energies in Group IVB, VB, and VIB high-entropy transition metal carbides are shown to be predictable from an optimized rule of mixtures based on the properties of the single metal carbide constituents present near the stacking fault. A composition-independent linear relationship is demonstrated between the ISF energies and the unstable stacking fault (USF) energies along the <112>{111} gamma surface slip path. Treating the ISF and USF energies as analogous to the heat of reaction and transition state barrier in chemical reactions, this linear relationship represents a new application of the Evans-Polanyi-Semenov principle. Further, a full defect energy distribution can be obtained from the predicted ISF energies with only the composition as an input for the mixed early-transition metal carbides. Applying a model that balances the elastic repulsion between partial dislocations with the distribution of ISF energies, we show that Shockley partial edge dislocations should remain bound for all valence electron concentration values up to about 9.6, even when the average stacking fault energy is negative.

cond-mat.mtrl-sci↗

arXiv2024

Predicted High-Pressure Hot Superconductivity in Li$_2$CaH$_{16}$ and Li$_2$CaH$_{17}$ Phases that Resemble the Type-II Clathrate Structure

High-temperature high-pressure superconducting hydrides are typically characterized by cage-like hydrogenic lattices filled with electropositive metal atoms. Here, density functional theory based evolutionary crystal structure searches find two phases that possess these geometric features and are related to the Type-II clathrate structure. In these $Fd\overline{3}m$ Li$_2$CaH$_{16}$ and $R\overline{3}m$ Li$_2$CaH$_{17}$ phases the calcium atom occupies the larger cage and the lithium atom the smaller one. The highest superconducting critical temperatures predicted within the isotropic Eliashberg formalism, 330~K at 350~GPa for $Fd\overline{3}m$ Li$_2$CaH$_{16}$ and 370~K at 300~GPa for $R\overline{3}m$ Li$_2$CaH$_{17}$, suggest these structures fall in the class of high-energy-density quantum materials known as hot superconductors. As pressure is lowered the cage-like lattices distort with the emergence of quasimolecular hydrogenic motifs; nonetheless Li$_2$CaH$_{17}$ is predicted to be superconducting down to 160~GPa at 205~K.

cond-mat.supr-con↗

arXiv2024

Why Mg$_2$IrH$_6$ is predicted to be a high temperature superconductor, but Ca$_2$IrH$_6$ is not

The X$_2$MH$_6$ family, consisting of an electropositive cation X and a main group metal M octahedrally coordinated by hydrogen, has been predicted to hold promise for high-temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, Mg$_2$IrH$_6$ and Ca$_2$IrH$_6$, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg$_2$IrH$_6$ the vibrations of the IrH$_6^{4-}$ anions are key for the superconducting mechanism, and they induce coupling in the $e_g^*$ set, which are antibonding between the H 1$s$ and the Ir $d_{x^2-y^2}$ or $d_{z^2}$ orbitals. Because calcium possesses low-lying d-orbitals, $e_g^* \rightarrow$ Ca $d$ back-donation is preferred, quenching the superconductivity. Our analysis explains why high critical temperatures were only predicted for second or third row X metal atoms, and may hold implications for superconductivity in other systems where the antibonding anionic states are filled.

cond-mat.supr-con↗

arXiv2024

Powder X-Ray Diffraction Assisted Evolutionary Algorithm for Crystal Structure Prediction

Experimentally obtained X-ray diffraction (XRD) patterns can be difficult to solve, precluding the full characterization of materials, pharmaceuticals, and geological compounds. Herein, we propose a method based upon a multi-objective evolutionary search that uses both a structure's enthalpy and similarity to a reference XRD pattern (constituted by a list of peak positions and their intensities) to facilitate structure solution of inorganic systems. Because the similarity index is computed for locally optimized cells that are subsequently distorted to find the best match with the reference, this process transcends both computational (e.g. choice of theoretical method, and 0 K approximation) and experimental (e.g. external stimuli, and metastability) limitations. We illustrate how the proposed methodology can be employed to successfully uncover complex crystal structures by applying it to a range of test cases, including inorganic minerals, pure elements ramp-compressed to extreme conditions, and molecular crystals. The results demonstrate that our approach not only improves the accuracy of structure prediction but also significantly reduces the time required to achieve reliable solutions, thus providing a powerful tool for the advancement of materials science and related fields.

cond-mat.mtrl-sci↗

arXiv2024

Impact of Data Bias on Machine Learning for Crystal Compound Synthesizability Predictions

Machine learning models are susceptible to being misled by biases in training data that emphasize incidental correlations over the intended learning task. In this study, we demonstrate the impact of data bias on the performance of a machine learning model designed to predict the synthesizability likelihood of crystal compounds. The model performs a binary classification on labeled crystal samples. Despite using the same architecture for the machine learning model, we showcase how the model's learning and prediction behavior differs once trained on distinct data. We use two data sets for illustration: a mixed-source data set that integrates experimental and computational crystal samples and a single-source data set consisting of data exclusively from one computational database. We present simple procedures to detect data bias and to evaluate its effect on the model's performance and generalization. This study reveals how inconsistent, unbalanced data can propagate bias, undermining real-world applicability even for advanced machine learning techniques.

cond-mat.mtrl-sci↗

arXiv2024

Quantum Stabilization and Flat Hydrogen-based Bands of Nitrogen-doped Lutetium Hydride

We explore electronic and structural properties of Fm$\overline{3}$m Lu-H-N structures with specific N,H ordering as plausible candidates for near-ambient superconductivity possibly originating from their remarkably narrow hydrogen-based bands at the Fermi level. Although LuH$_{2.875}$N$_{0.125}$ exhibits an instability persisting up to 17 GPa, it is anharmonically stable near ambient pressure when accounting for quantum nuclear effects. The presence of flat bands near $E_\text{F}$ is understood to arise from destructive\ quantum interference between N-p and surrounding H-s orbitals, with certain types of defects leaving the flat bands unaffected. The results suggest there is an optimal pressure near ambient where the superconducting $T_{\text{c}}$ is maximized in this structure by anharmonically-stabilized low-frequency and non-adiabatically coupled high-frequency hydrogen modes. Despite the metastability of this structure, its electronic properties and dynamical stability when calculated beyond a classical harmonic approach can explain the reported near-ambient superconductivity in Lu-H-N.

cond-mat.supr-con↗

arXiv2024

XtalOpt Version 13: Multi-Objective Evolutionary Search for Novel Functional Materials

Version 13 of XtalOpt, an evolutionary algorithm for crystal structure prediction, is now available for download from the CPC program library or the XtalOpt website, https://xtalopt.github.io. In the new version of the XtalOpt code, a general platform for multi-objective global optimization is implemented. This functionality is designed to facilitate the search for (meta)stable phases of functional materials through minimization of the enthalpy of a crystalline system coupled with the simultaneous optimization of any desired properties that are specified by the user. The code is also able to perform a constrained search by filtering the parent pool of structures based on a user-specified feature, while optimizing multiple objectives. Here, we present the implementation and various technical details, and we provide a brief overview of additional improvements that have been introduced in the new version of XtalOpt.

physics.comp-ph↗

arXiv2024

Superconductivity in Dilute Hydrides of Ammonia under Pressure

In the last decade, there has been great progress in predicting and synthesizing polyhydrides that exhibit superconductivity when squeezed. Dopants allow these compounds to become metals at pressures lower than those required to metallize elemental hydrogen. Here, we show that by combining the fundamental planetary building blocks of molecular hydrogen and ammonia, conventional superconducting compounds can be formed at high pressure. Through extensive theoretical calculations we predict metallic metastable structures with NH$_n$ ($n=10,11,24$) stoichiometries that are based on NH$_4^+$ superalkali cations and complex hydrogenic lattices. The hydrogen atoms in the molecular cation contribute to the superconducting mechanism, and the estimated superconducting critical temperatures, $T_\text{c}$s, are comparable to the highest values computed for the alkali metal polyhydrides. The largest calculated (isotropic Eliashberg) $T_\text{c}$ is 179~K for $Pnma$-NH$_{10}$ at 300~GPa. Our results suggest that other molecular cations can be mixed with hydrogen under pressure yielding superconducting compounds.

cond-mat.mtrl-sci↗

arXiv2024

Machine Learned Interatomic Potentials for Ternary Carbides trained on the AFLOW Database

Large density functional theory (DFT) databases are a treasure trove of energies, forces and stresses that can be used to train machine learned interatomic potentials for atomistic modeling. Herein, we employ structural relaxations from the AFLOW database to train moment tensor potentials (MTPs) for four carbide systems: HfTaC, HfZrC, MoWC and TaTiC. The resulting MTPs are used to relax ~6300 random symmetric structures, and are subsequently improved via active learning to generate robust potentials (RP) that can relax a wide variety of structures, and accurate potentials (AP) designed for the relaxation of low-energy systems. This protocol is shown to yield convex hulls that are indistinguishable from those predicted by AFLOW for the HfTaC, HfZrC and TaTiC systems, and in the case of the MoWC system to predict thermodynamically stable structures that are not found within AFLOW, highlighting the potential of the employed protocol within crystal structure prediction. Relaxation of over three hundred Mo$_{1-x}$W$_x$C stoichiometry crystals first with the RP then with the AP yields formation enthalpies that are in excellent agreement with those obtained via DFT.

cond-mat.mtrl-sci↗

arXiv2024

Phase Boundaries, Isotope Effect and Superconductivity of Lithium Under Hydrostatic Conditions

We present theoretical and experimental studies of superconductivity and low temperature structural phase boundaries in lithium. We mapped the structural phase diagram of 6Li and 7Li under hydrostatic conditions between 5 top 55GPa and within the temperature range of 15 to 75K, observing the FCC-hR1-cI16 phase transitions. 6Li and 7Li show some differences at the structural boundaries, with a potential shift of the phase boundaries of 6Li to lower pressures. Density functional theory calculations and topological analysis of the electron density elucidates the superconducting properties and interatomic interactions within these phases of lithium.

cond-mat.supr-con↗

arXiv2023

Superconductivity in CH4 and BH4- Containing Compounds Derived from the High-Pressure Superhydrides

Inspired by the synthesis of the high-pressure Fm-3m LaH10 superconducting superhydride, systematic density functional theory (DFT) calculations are performed to study ternaries that could be derived from it by replacing two of the hydrogen atoms with boron or carbon and varying the identity of the electropositive element. Though many of the resulting alkali-metal and alkaline-earth MC2H8 phases are predicted to be dynamically stable at mild pressures, their superconducting critical temperatures (Tcs) are low because their metallicity results from the filling of an electride-like band. Substitution with a trivalent element leads to phases with substantial metal d-character at the Fermi level whose Tcs are typically above 40 K. Among the MB2H8 phases examined, KB2H8, RbB2H8 and CsB2H8 are predicted to be dynamically stable at very mild pressures, and their stability is rationalized by a DFT-Chemical Pressure analysis that elucidates the role of the M atom size. Quantum anharmonic effects strongly affect the properties of KB2H8, the highest predicted Tc compound, near 10 GPa, but molecular dynamics simulations reveal it would decompose below its Tc at this pressure. Nonetheless, at ca. 50 GPa KB2H8 is predicted to be thermally stable with a superconducting figure of merit surpassing that of the recently synthesized LaBeH8.

cond-mat.supr-con↗

arXiv2023

A priori procedure to establish spinodal decomposition in alloys

Spinodal decomposition can improve a number of essential properties in materials, especially hardness. Yet, the theoretical prediction of the onset of this phenomenon (e.g., temperature) and its microstructure (e.g., wavelength) often requires input parameters coming from costly and time-consuming experimental efforts, hindering rational materials optimization. Here, we present a procedure where such parameters are not derived from experiments. First, we calculate the spinodal temperature by modeling nucleation in the solid solution while approaching the spinode boundary. Then, we compute the spinodal wavelength self-consistently using a few reasonable approximations. Our results show remarkable agreement with experiments and, for NiRh, the calculated yield strength due to spinodal microstructures surpasses even those of Ni-based superalloys. We believe that this procedure will accelerate the exploration of the complex materials experiencing spinodal decomposition, critical for their macroscopic properties.

cond-mat.mtrl-sci↗

arXiv2023

On the Electride Nature of Na-hP4

Early quantum mechanical models suggested that pressure drives solids towards free-electron metal behavior where the ions are locked into simple close-packed structures. The prediction and subsequent discovery of high-pressure electrides (HPEs), compounds assuming open structures where the valence electrons are localized in interstitial voids, required a paradigm shift. Our quantum chemical calculations on the iconic insulating Na-hP4 HPE show that increasing density causes a 3s -> 3pd electronic transition due to Coulomb repulsion between the 1s2s and 3s states, and orthogonality of the 3pd states to the core. The large lobes of the resulting Na-pd hybrid orbitals point towards the center of an 11-membered penta-capped trigonal prism and overlap constructively, forming multi-centered bonds, which are responsible for the emergence of the interstitial charge localization in Na-hP4. These multi-centered bonds facilitate the increased density of this phase, which is key for its stabilization under pressure.

cond-mat.mtrl-sci↗

arXiv2023

Structurally Constrained Evolutionary Algorithm for the Discovery and Design of Metastable Phases

Metastable materials are abundant in nature and technology, showcasing remarkable properties that inspire innovative materials design. However, traditional crystal structure prediction methods, which rely solely on energetic factors to determine a structure's fitness, are not suitable for predicting the vast number of potentially synthesizable phases that represent a local minimum corresponding to a state in thermodynamic equilibrium. Here, we present a new approach for the prediction of metastable phases with specific structural features, and interface this method with the XtalOpt evolutionary algorithm. Our method relies on structural features that include the local crystalline order (e.g., the coordination number or chemical environment), and symmetry (e.g., Bravais lattice and space group) to filter the parent pool of an evolutionary crystal structure search. The effectiveness of this approach is benchmarked on three known metastable systems: XeN$_8$, with a two-dimensional polymeric nitrogen sublattice, brookite TiO$_2$, and a high pressure BaH$_4$ phase that was recently characterized. Additionally, a newly predicted metastable melaminate salt, $P$-1 WC$_{3}$N$_{6}$, was found to possess an energy that is lower than two phases proposed in a recent computational study. The method presented here could help in identifying the structures of compounds that have already been synthesized, and developing new synthesis targets with desired properties.

cond-mat.mtrl-sci↗

arXiv2023

Structure, Stability and Superconductivity of N-doped Lutetium Hydrides at kbar Pressures

The structure of the material responsible for the room temperature and near ambient pressure superconductivity reported in an N-doped lutetium hydride [Nature, 615, 244 (2023)] has not been conclusively determined. Herein, density functional theory calculations are performed in an attempt to uncover what it might be. Guided by a range of strategies including crystal structure prediction and modifications of existing structure types, we present an array of Lu-N-H phases that are dynamically stable at experimentally relevant pressures. Although none of the structures found are thermodynamically stable, and none are expected to remain superconducting above 17 K at 10 kbar, a number of metallic compounds with fcc Lu lattices -- as suggested by the experimental X-ray diffraction measurements of the majority phase -- are identified. The system whose calculated equation of states matches best with that measured for the majority phase is fluorite-type LuH2, whose 10 kbar superconducting critical temperature was estimated to be 0.09 K using the Allen-Dynes modified McMillan equation.

cond-mat.supr-con↗

arXiv2023

Intercalating Helium into A-site Vacant Perovskites

Evolutionary searches were employed to predict the most stable structures of perovskites with helium atoms on their A-sites up to pressures of 10 GPa. The thermodynamics associated with helium intercalation into [CaZr]F6, structure that [He]2[CaZr]F6 adopts under pressure, and the mechanical properties of the parent perovskite and helium-bearing phase were studied via density functional theory (DFT) calculations. The pressure-temperature conditions where the formation of HeAlF3, HeGaF3, HeInF3, HeScF3 and HeReO3 from elemental helium and the vacant A-site perovskites is favored were found. Our DFT calculations show that entropy can stabilize the helium-filled perovskites provided that the volume that the noble gas atom occupies within their pores is larger than within the elemental solid at that pressure. We find that helium incorporation will increase the bulk modulus of AlF3 from a value characteristic of tin to one characteristic of steel, and hinders rotations of its octahedra that occur under pressure.

cond-mat.mtrl-sci↗

arXiv2023

Crystal Chemistry at High Pressure

An overview of the behavior of materials at high pressure is presented, starting from the effects on single atoms driving electronic transitions and changes in periodic trends. A range of high-pressure-induced phenomena in the solid state are then discussed building on the atomic changes, including bizarre electronic structures, electrides, compounds of noble gases, changes in elemental miscibility, and strange structural and bonding configurations. In the final section, the field of high pressure superconductivity is discussed, as high pressure phases have generated immense study and excitement as some of their critical superconducting temperatures approach room temperature.

cond-mat.mtrl-sci↗

arXiv2022

Conventional high-temperature superconductivity in metallic, covalently bonded, binary-guest C-B clathrates

Inspired by the synthesis of XB3C3 (X= Sr, La) compounds in the bipartite sodalite clathrate structure, density functional theory (DFT) calculations are performed on members of this family containing up to two different metal atoms. A DFT-chemical pressure analysis on systems with X= Mg, Ca, Sr, Ba reveals that the size of the metal cation, which can be tuned to stabilize the B-C framework, is key for their ambient-pressure dynamic stability. High-throughput density functional theory calculations on 105 Pm-3 symmetry XYB6C6 binary-guest compounds (where X, Y are electropositive metal atoms) find 22 that are dynamically stable at 1 atmosphere, expanding the number of potentially synthesizable phases by 19 (18 metals and 1 insulator). The density of states at the Fermi level and superconducting critical temperature, Tc, can be tuned by changing the average oxidation state of the metal atoms, with Tc being highest for an average valence of +1.5. KPbB6C6, with an ambient-pressure Eliashberg Tc of 88 K, is predicted to possess the highest-Tc among the studied Pm-3n XB3C3 or Pm-3 XY B6C6 phases, and calculations suggest it may be synthesized using high-pressure high-temperature techniques then quenched to ambient conditions.

cond-mat.supr-con↗