arXiv subjects

Chris Wolverton

Publications and source records attributed to Chris Wolverton.

At least 55 records · Page 3Linked to original sources

arXiv2019

Developing an improved Crystal Graph Convolutional Neural Network framework for accelerated materials discovery

The recently proposed crystal graph convolutional neural network (CGCNN) offers a highly versatile and accurate machine learning (ML) framework by learning material properties directly from graph-like representations of crystal structures ("crystal graphs"). Here, we develop an improved variant of the CGCNN model (iCGCNN) that outperforms the original by incorporating information of the Voronoi tessellated crystal structure, explicit 3-body correlations of neighboring constituent atoms, and an optimized chemical representation of interatomic bonds in the crystal graphs. We demonstrate the accuracy of the improved framework in two distinct illustrations: First, when trained/validated on 180,000/20,000 density functional theory (DFT) calculated thermodynamic stability entries taken from the Open Quantum Materials Database (OQMD) and evaluated on a separate test set of 230,000 entries, iCGCNN achieves a predictive accuracy that is significantly improved, i.e., 20% higher than that of the original CGCNN. Second, when used to assist high-throughput search for materials in the ThCr2Si2 structure-type, iCGCNN exhibited a success rate of 31% which is 310 times higher than an undirected high-throughput search and 2.4 times higher than that of the original CGCNN. Using both CGCNN and iCGCNN, we screened 132,600 compounds with elemental decorations of the ThCr2Si2 prototype crystal structure and identified a total of 97 new unique stable compounds by performing 757 DFT calculations, accelerating the computational time of the high-throughput search by a factor of 130. Our results suggest that the iCGCNN can be used to accelerate high-throughput discoveries of new materials by quickly and accurately identifying crystalline compounds with properties of interest.

physics.comp-ph↗

arXiv2019

Network analysis of synthesizable materials discovery

Assessing the synthesizability of inorganic materials is a grand challenge for accelerating their discovery using computations. Synthesis of a material is a complex process that depends not only on its thermodynamic stability with respect to others, but also on factors from kinetics, to advances in synthesis techniques, to the availability of precursors. This complexity makes the development of a general theory or first-principles approach to synthesizability currently impractical. Here we show how an alternative pathway to predicting synthesizability emerges from the dynamics of the materials stability network: a scale-free network constructed by combining the convex free-energy surface of inorganic materials computed by high-throughput density functional theory and their experimental discovery timelines extracted from citations. The time-evolution of the underlying network properties allows us to use machine-learning to predict the likelihood that hypothetical, computer-generated materials will be amenable to successful experimental synthesis.

cond-mat.mtrl-sci↗

arXiv2019

An Enormous Class of Double Half-Heusler Compounds with Low Thermal Conductivity

Since their discovery around a century ago, the structure and chemistry of the multi-functional half-Heusler semiconductors have been studied extensively as three component systems. The elemental groups constituting these ternary compounds with the nominal formula XYZ are well established. From the very same set of well-known elements we explore a phase space of quaternary double ($X'X''Y_2Z_2$, $X_2Y'Y''Z_2$, and $X_2Y_2Z'Z''$), triple ($X_2'X''Y_3Z_3$) and quadruple ($X_3'X''Y_4Z_4$) half-Heusler compositions which 10 times larger in size. Using a reliable, first-principles thermodynamics methodology on a selection of 347 novel compositions, we predict 127 new stable quaternary compounds, already more than the 89 reported almost exhaustively for ternary systems. Thermoelectric performance of the state-of-the-art ternary half-Heusler compounds are limited by their intrinsically high lattice thermal conductivity ($κ_{L}$). In comparison to ternary half-Heuslers, thermal transport in double half-Heuslers is dominated by low frequency phonon modes with smaller group velocities and limited by disorder scattering. The double half-Heusler composition Ti$_2$FeNiSb$_2$ was synthesized and confirmed to have a significantly lower lattice thermal conductivity (factor of 3 at room temperature) than TiCoSb, thereby providing a better starting point for thermoelectric efficiency optimization. We demonstrate a dependable strategy to assist the search for low thermal conductivity half-Heuslers and point towards a huge composition space for implementing it. Our findings can be extended for systematic discovery of other large families of multi-component intermetallic semiconductors.

cond-mat.mtrl-sci↗

arXiv2018

Ternary mixed-anion semiconductors with tunable band gaps from machine-learning and crystal structure prediction

We report the computational investigation of a series of ternary X$_4$Y$_2$Z and X$_5$Y$_2$Z$_2$ compounds with X={Mg, Ca, Sr, Ba}, Y={P, As, Sb, Bi}, and Z={S, Se, Te}. The compositions for these materials were predicted through a search guided by machine learning, while the structures were resolved using the minima hopping crystal structure prediction method. Based on $\textit{ab initio}$ calculations, we predict that many of these compounds are thermodynamically stable. In particular, 21 of the X$_4$Y$_2$Z compounds crystallize in a tetragonal structure with $\textit{I-42d}$ symmetry, and exhibit band gaps in the range of 0.3 and 1.8 eV, well suited for various energy applications. We show that several candidate compounds (in particular X$_4$Y$_2$Te and X$_4$Sb$_2$Se) exhibit good photo absorption in the visible range, while others (e.g., Ba$_4$Sb$_2$Se) show excellent thermoelectric performance due to a high power factor and extremely low lattice thermal conductivities.

cond-mat.mtrl-sci↗

arXiv2018

Remarkable thermoelectric performance in BaPdS$_2$ via pudding-mold band structure and ultralow lattice thermal conductivity

Efficient thermoelectric materials require a rare and contraindicated combination of materials properties: large electrical conductivity, large Seebeck coefficient, and low thermal conductivity. One strategy to achieve the first two properties is via low-energy electronic bands containing both flat and dispersive parts in different regions of crystal momentum space, known as a pudding-mold band structure. Here, we illustrate that BaPdS$_2$ successfully achieves the pudding-mold band structure, contributing to a large thermoelectric power factor, due to its anisotropic crystal structure containing zig-zag chains of edge-sharing square planar PdS$_4$ units. In addition, BaPdS$_2$ exhibits ultralow lattice thermal conductivity, and thus also achieves the third property, due to extremely soft and anharmonic interactions in its transverse acoustic phonon branch. We predict a remarkably large thermoelectric figure of merit, with peak values between 2 and 3 for two of the three crystallographic directions, suggesting BaPdS$_2$ warrants experimental investigation.

cond-mat.mtrl-sci↗

arXiv2018

A materials informatics approach to the identification of one-band correlated materials analogous to the cuprates

One important yet exceedingly rare property of the cuprate high-temperature superconductors is the presence of a single correlated $d$ band in the low-energy spectrum, leading to the one-band Hubbard model as the minimal description. In order to search for materials with interesting strong correlation physics as well as possible benchmark systems for the one-band Hubbard model, here we present a new approach to find one-band correlated materials analogous to the cuprates by leveraging the emerging area of materials informatics. Using the composition, structure, and formation energy of more than half a million real and hypothetical inorganic crystalline materials in the Open Quantum Materials Database, we search for synthesizable materials whose nominal transition metal $d$ electron count and crystal field are compatible with achieving an isolated half-filled $d$ band. Five Cu compounds, including bromide, oxide, selenate, and pyrophosphate chemistries, are shown to successfully achieve the one-band electronic structure based on density functional theory band structure calculations. Further calculations including magnetism and explicit on-site Coulomb interaction reveal significant evidence for strong correlation physics in the five candidates, including Mott insulating behavior and antiferromagnetism. The success of our data-driven approach to discovering new correlated materials opens up new avenues to design and discover materials with rare electronic properties.

cond-mat.str-el↗

arXiv2018

Thermoelectric Alchemy: Designing A Chemical Analog to PbTe with Intrinsic High Band Degeneracy and Low Lattice Thermal Conductivity

Improving the figure of merit $zT$ of thermoelectric materials requires simultaneously a high power factor and low thermal conductivity. An effective approach for increasing the power factor is to align the band extremum and achieve high band degeneracy ($\geq$ 12) near the Fermi level as realized in PbTe [\textcolor{blue}{Pei et. al. \textit{Nature} 473, 66 (2010)}], which usually relies on band structure engineering, e.g., chemical doping and strain. However, very few materials could achieve such a high band degeneracy without heavy doping or suffering impractical strain. By employing state-of-the-art first-principles methods with direct computation of phonon and carrier lifetime, we demonstrate that two new full-Heusler compounds Li$_2$TlBi and Li$_2$InBi, possessing a PbTe-like electronic structure, show exceptionally high power factors ($\sim$ 20 mWm$^{-1}$K$^{-2}$ at 300 K) and low lattice thermal conductivities (2.36 and 1.55 Wm$^{-1}$K$^{-1}$) at room temperature. The Tl$^{+}$Bi$^{3-}$ (In$^{+}$Bi$^{3-}$) sublattice forms a rock-salt structure, and the additional two valence electrons from Li atoms essentially make these compounds isovalent with Pb$^{2+}$Te$^{2-}$. The larger rock-salt sublattice of TlBi (InBi) shifts the valence band maximum from L point to the middle of the $Σ$ line, increasing the band degeneracy from fourfold to twelvefold. On the other hand, resonance bond in the PbTe-like sublattice and soft Tl-Bi (In-Bi) bonding interaction is responsible for intrinsic low lattice thermal conductivities. Our results present a novel strategy of designing high-performance thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2018

Performance of the strongly constrained and appropriately normed density functional for solid-state materials

Constructed to satisfy all known exact constraints and appropriate norms for a semilocal density functional, the strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation functional has shown early promise for accurately describing the electronic structure of molecules and solids. One open question is how well SCAN predicts the formation energy, a key quantity for describing the thermodynamic stability of solid-state compounds. To answer this question, we perform an extensive benchmark of SCAN by computing the formation energies for a diverse group of nearly one thousand crystalline compounds for which experimental values are known. Due to an enhanced exchange interaction in the covalent bonding regime, SCAN substantially decreases the formation energy errors for strongly-bound compounds, by approximately 50% to 110 meV/atom, as compared to the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE). However, for intermetallic compounds, SCAN performs moderately worse than PBE with an increase in formation energy error of approximately 20%, stemming from SCAN's distinct behavior in the weak bonding regime. The formation energy errors can be further reduced via elemental chemical potential fitting. We find that SCAN leads to significantly more accurate predicted crystal volumes, moderately enhanced magnetism, and mildly improved band gaps as compared to PBE. Overall, SCAN represents a significant improvement in accurately describing the thermodynamics of strongly-bound compounds.

cond-mat.mtrl-sci↗

arXiv2018

Exploring the high-pressure materials genome

A thorough in situ characterization of materials at extreme conditions is challenging, and computational tools such as crystal structural search methods in combination with ab initio calculations are widely used to guide experiments by predicting the composition, structure, and properties of high-pressure compounds. However, such techniques are usually computationally expensive and not suitable for large-scale combinatorial exploration. On the other hand, data-driven computational approaches using large materials databases are useful for the analysis of energetics and stability of hundreds of thousands of compounds, but their utility for materials discovery is largely limited to idealized conditions of zero temperature and pressure. Here, we present a novel framework combining the two computational approaches, using a simple linear approximation to the enthalpy of a compound in conjunction with ambient-conditions data currently available in high-throughput databases of calculated materials properties. We demonstrate its utility by explaining the occurrence of phases in nature that are not ground states at ambient conditions and estimating the pressures at which such ambient-metastable phases become thermodynamically accessible, as well as guiding the exploration of ambient-immiscible binary systems via sophisticated structural search methods to discover new stable high-pressure phases.

cond-mat.mtrl-sci↗

arXiv2018

Designing and discovering a new family of semiconducting quaternary Heusler compounds based on the 18-electron rule

Intermetallic compounds with sizable band gaps are attractive for their unusual properties but rare. Here, we present a new family of stable semiconducting quaternary Heusler compounds, designed and discovered by means of high-throughput \textit{ab initio} calculations based on the 18-electron rule. The 99 new semiconductors reported here adopt the ordered quaternary Heusler structure with the prototype of LiMgSnPd (F$\bar{\mathbf{4}}$3m, No.\,216) and contain 18 valence electrons per formula unit. They are realized by filling the void in the half Heusler structure with a small and electropositive atom, i.e., lithium. These new stable quaternary Heusler semiconductors possess a range of band gaps from 0.3 to 2.5\,eV, and exhibit some unusual properties different from conventional semiconductors, such as strong optical absorption, giant dielectric screening, and high Seebeck coefficient, which suggest these semiconductors have potential applications as photovoltaic and thermoelectric materials. While this study opens up avenues for further exploration of this novel class of semiconducting quaternary Heuslers, the design strategy used herein is broadly applicable across a potentially wide array of chemistries to discover new stable materials.

cond-mat.mtrl-sci↗

arXiv2017

Predicting the morphologies of γ' precipitates in cobalt-based superalloys

Cobalt-based alloys with γ/γ' microstructures have the potential to become the next generation of superalloys, but alloy compositions and processing steps must be optimized to improve coarsening, creep, and rafting behavior. While these behaviors are different than in nickel-based superalloys, alloy development can be accelerated by understanding the thermodynamic factors influencing microstructure evolution. In this work, we develop a phase field model informed by first-principles density functional theory and experimental data to predict the equilibrium shapes of Co-Al-W γ' precipitates. Three-dimensional simulations of single and multiple precipitates are performed to understand the effect of elastic and interfacial energy on coarsened and rafted microstructures; the elastic energy is dependent on the elastic stiffnesses, misfit strain, precipitate size, applied stress, and precipitate spatial distribution. We observe characteristic microstructures dependent on the type of applied stress that have the same γ' morphology and orientation seen in experiments, indicating that the elastic stresses arising from coherent γ/γ' interfaces are important for morphological evolution during creep. The results also indicate that the narrow γ channels between γ' precipitates are energetically favored, and provide an explanation for the experimentally observed directional coarsening that occurs without any applied stress.

cond-mat.mtrl-sci↗

arXiv2017

Dense superconducting phases of copper-bismuth at high pressure

Although copper and bismuth do not form any compounds at ambient conditions, two intermetallics, CuBi and Cu$_{11}$Bi$_7$, were recently synthesized at high pressures. Here we report on the discovery of additional copper-bismuth phases at elevated pressures with high-densities from ab initio calculations. In particular, a Cu$_2$Bi compound is found to be thermodynamically stable at pressures above 59 GPa, crystallizing in the cubic Laves structure. In strong contrast to Cu$_{11}$Bi$_7$ and CuBi, cubic Cu$_2$Bi does not exhibit any voids or channels. Since the bismuth lone pairs in cubic Cu$_2$Bi are stereochemically inactive, the constituent elements can be closely packed and a high density of 10.52 g/cm$^{3}$ at 0 GPa is achieved. The moderate electron-phonon coupling of $λ=0.68$ leads to a superconducting temperature of 2 K, which exceeds the values observed both in Cu$_{11}$Bi$_7$and CuBi, as well as in elemental Cu and Bi .

cond-mat.supr-con↗

arXiv2017

Cubine, a superconducting 2-dimensional copper-bismuth nano sheet

We report on the discovery of a 2-dimensional copper-bismuth nano sheet from \textit{ab initio} calculations, which we call cubine. According to our predictions, single layers of cubine can be isolated from the recently reported high-pressure CuBi bulk material (metastable at ambient conditions) at an energetic cost of merely $\approx 20$~meV/Å$^2$, comparable to separating single layers of graphene from graphite. Our calculations suggest that cubine has remarkable electronic and electrochemical properties: It is a superconductor with a moderate electron-phonon coupling $λ=0.5$, leading to a $T_c$ of $\approx1$~K, and can be readily intercalated with lithium with a high diffusibility, rendering it a promising candidate material as an anode in lithium-ion batteries.

cond-mat.mtrl-sci↗

arXiv2017

Nanoscale Voltage Enhancement at Cathode Interfaces in Li-ion Batteries

Interfaces are ubiquitous in Li-ion battery electrodes, occurring across compositional gradients, regions of multiphase intergrowths, and between electrodes and solid electrolyte interphases or protective coatings. However, the impact of these interfaces on Li energetics remains largely unknown. In this work, we calculated Li intercalation-site energetics across cathode interfaces and demonstrated the physics governing these energetics on both sides of the interface. We studied the olivine/olivine-structured LixFePO4/LixMPO4 (x=0 and 1, M=Co, Ti, Mn) and layered/layered-structured LiNiO2/TiO2 interfaces to explore different material structures and transition metal elements. We found that across an interface from a high- to low-voltage material the Li voltage remains constant in the high-voltage material and decays approximately linearly in the low-voltage region, approaching the Li voltage of the low-voltage material. This effect ranges from 0.5-9nm depending on the interfacial dipole screening. This effect provides a mechanism for a high-voltage material at an interface to significantly enhance the Li intercalation voltage in a low-voltage material over nanometer scale. We showed that this voltage enhancement is governed by a combination of electron transfer (from low- to high-voltage regions), strain and interfacial dipole screening. We explored the implications of this voltage enhancement for a novel heterostructured-cathode design and redox pseudocapacitors.

cond-mat.mtrl-sci↗

arXiv2016

Prediction of superconducting iron-bismuth intermetallic compounds at high pressure

The synthesis of materials in high-pressure experiments has recently attracted increasing attention, especially since the discovery of record breaking superconducting temperatures in the sulfur-hydrogen and other hydrogen-rich systems. Commonly, the initial precursor in a high pressure experiment contains constituent elements that are known to form compounds at ambient conditions, however the discovery of high-pressure phases in systems immiscible under ambient conditions poses an additional materials design challenge. We performed an extensive multi component $ab\,initio$ structural search in the immiscible Fe--Bi system at high pressure and report on the surprising discovery of two stable compounds at pressures above $\approx36$ GPa, FeBi$_2$ and FeBi$_3$. According to our predictions, FeBi$_2$ is a metal at the border of magnetism with a conventional electron-phonon mediated superconducting transition temperature of $T_{\rm c}=1.3$ K at 40 GPa. In analogy to other iron-based materials, FeBi$_2$ is possibly a non-conventional superconductor with a real $T_{\rm c}$ significantly exceeding the values obtained within Bardeen-Cooper-Schrieffer (BCS) theory.

cond-mat.mtrl-sci↗

arXiv2016

Lithium transport through Lithium-ion battery cathode coatings

The surface coating of cathodes using insulator films has proven to be a promising method for high-voltage cathode stabilization in Li-ion batteries. However, there is still substantial uncertainty about how these films function, specifically with regard to important coating design principles such as lithium solubility and transport through the films. This study uses Density Functional Theory to examine the diffusivity of interstitial lithium in crystalline α-$AlF_3$, α-$Al_2O_3$, m-$ZrO_2$, c-MgO, and α-quartz $SiO_2$, which provide benchmark cases for further understanding of insulator coatings in general. In addition, we propose an Ohmic electrolyte model to predict resistivities and overpotential contributions under battery operating conditions. For the crystalline materials considered we predict that Li+ diffuses quite slowly, with a migration barrier larger than 0.9 eV in all crystalline materials except α-quartz $SiO_2$, which is predicted to have a migration barrier of 0.276 eV along <001>. These results suggest that the stable crystalline forms of these insulator materials, except for oriented α-quartz $SiO_2$, are not practical for conformal cathode coatings. Amorphous $Al_2O_3$ and $AlF_3$ have higher Li+ diffusivities than their crystalline counterparts. Our predicted amorphous $Al_2O_3$ resistivity (1789 MΩm) is near the top of the range of fitted resistivities extracted from previous experiments on nominal $Al_2O_3$ coatings (7.8 to 913 MΩm) while our predicted amorphous $AlF_3$ resistivity (114 MΩm) is close to the middle of the range. These comparisons support our framework for modeling and understanding the impact on overpotential of conformal coatings in terms of their fundamental thermodynamic and kinetic properties, and support that these materials can provide practical conformal coatings in their amorphous form.

cond-mat.mtrl-sci↗

arXiv2016

Ultralow Thermal Conductivity in Full-Heusler Semiconductors

Semiconducting half- and, to a lesser extent, full-Heusler compounds are promising thermoelectric materials due to their compelling electronic properties with large power factors. However, intrinsically high thermal conductivity resulting in a limited thermoelectric efficiency has so far impeded their widespread use in practical applications. Here, we report the computational discovery of a class of hitherto unknown stable semiconducting full-Heusler compounds with ten valence electrons ($X_2YZ$, $X$=Ca, Sr, and Ba; $Y$= Au and Hg; $Z$=Sn, Pb, As, Sb, and Bi) through high-throughput $ab-initio$ screening. These new compounds exhibit ultralow lattice thermal conductivity $κ_{\text{L}}$ close to the theoretical minimum due to strong anharmonic rattling of the heavy noble metals, while preserving high power factors, thus resulting in excellent phonon-glass electron-crystal materials.

cond-mat.mtrl-sci↗

arXiv2015

Superconductivity in metastable phases of phosphorus-hydride compounds under high pressure

Hydrogen-rich compounds have been extensively studied both theoretically and experimentally in the quest for novel high-temperature superconductors. Reports on sulfur-hydride attaining metallicity under pressure and exhibiting superconductivity at temperatures as high as 200 K have spurred an intense search for room-temperature superconductors in hydride materials. Recently, compressed phosphine was reported to metallize at pressures above 45 GPa, reaching a superconducting transition temperature (T$_{c}$) of 100 K at 200 GPa. However, neither the exact composition nor the crystal structure of the superconducting phase have been conclusively determined. In this work the phase diagram of PH$_n$ ($n=1,2,3,4,5,6$) was extensively explored by means of {\it ab initio} crystal structure predictions using the Minima Hopping Method (MHM). The results do not support the existence of thermodynamically stable PH$_n$ compounds, which exhibit a tendency for elemental decomposition at high pressure even when vibrational contributions to the free energies are taken into account. Although the lowest energy phases of PH$_{1,2,3}$ display T$_{c}$'s comparable to experiments, it remains uncertain if the measured values of T$_{c}$ can be fully attributed to a phase-pure compound of PH$_n$.

cond-mat.supr-con↗