Articles on the spot

Spectral descriptors for bulk metallic glasses based on the thermodynamics of competing crystalline phases

Eric Perim, Dongwoo Lee, Yanhui Liu4, Cormac Toher, Pan Gong, Yanglin Li, W. Neal Simmons, Ohad Levy, Joost J. Vlassak, Jan Schroers, and Stefano Curtarolo

Abstract:
Metallic glasses attract considerable interest due to their unique combination of superb properties and processability. Predicting their formation from known alloy parameters remains the major hindrance to the discovery of new systems. Here, we propose a descriptor based on the heuristics that structural and energetic ‘confusion’ obstructs crystalline growth, and demonstrate its validity by experiments on two well-known glass-forming alloy systems. We then develop a robust model for predicting glass formation ability based on the geometrical and energetic features of crystalline phases calculated ab initio in the AFLOW framework. Our findings indicate that the formation of metallic glass phases could be much more common than currently thought, with more than 17% of binary alloy systems potential glass formers. Our approach pinpoints favourable compositions and demonstrates that smart descriptors, based solely on alloy properties available in online repositories, offer the sought-after key for accelerated discovery of metallic glasses.


Modeling Off-Stoichiometry Materials with a High-Throughput Ab-Initio Approach

Kesong Yang, CoreyOses, and Stefano Curtarolo

Abstract:
Predicting material properties of off-stoichiometry systems remains a long-standing and formidable challenge in rational materials design. A proper analysis of such systems by means of a supercell approach requires the exhaustive consideration of all possible superstructures, which can be a time-consuming process. On the contrary, the use of quasirandom approximants, although very computationally effective, implicitly bias the analysis toward disordered states with the lowest site correlations. Here, we propose a novel framework designed specifically to investigate stoichiometrically driven trends of disordered systems (i.e., having partial occupation and/or disorder in the atomic sites). At the heart of the approach is the identification and analysis of unique supercells of a virtually equivalent stoichiometry to the disordered material. We employ Boltzmann statistics to resolve system-wide properties at a high-throughput (HT) level. To maximize efficiency and accessibility, we integrated the method within the automatic HT computational framework AFLOW. As proof of concept, we apply our approach to three systems of interest, a zinc chalcogenide (ZnS1−xSex), a wide-gap oxide semiconductor (MgxZn1−xO), and an iron alloy (Fe1−xCux), at various stoichiometries. These systems exhibit properties that are highly tunable as a function of composition, characterized by optical bowing and linear ferromagnetic behavior. Not only are these qualities successfully predicted, but additional insight into underlying physical mechanisms is revealed.


Entropy Stabilized Oxides

C. M. Rost, E. Sachet, T. Borman, A. Moballegh, E. C. Dickey, D. Hou, J. L. Jones, S. Curtarolo, and J.-P. Maria

Abstract:
Configurational disorder can be compositionally engineered into mixed oxide by populating a single sublattice with many distinct cations. The formulations promote novel and entropy-stabilized forms of crystalline matter where metal cations are incorporated in new ways. Here, through rigorous experiments, a simple thermodynamic model, and a five-component oxide formulation, we demonstrate beyond reasonable doubt that entropy predominates the thermodynamic landscape, and drives a reversible solid-state transformation between a multiphase and single-phase state. In the latter, cation distributions are proven to be random and homogeneous. The findings validate the hypothesis that deliberate configurational disorder provides an orthogonal strategy to imagine and discover new phases of crystalline matter and untapped opportunities for property engineering.


Convergence of multi-valley bands as the electronic origin of high thermoelectric performance in CoSb3 skutterudites

Yinglu Tang, Zachary M. Gibbs, Luis A. Agapito, Guodong Li, Hyun-Sik Kim, Marco Buongiorno Nardelli, Stefano Curtarolo and G. Jeffrey Snyder

Abstract:
Filled skutterudites RixCo4Sb12 are excellent n-type thermoelectric materials owing to their high electronic mobility and higheffective mass, combined with low thermal conductivity associated with the addition of filler atoms into the void site. Thefavourable electronic band structure in n-type CoSb3 is typically attributed to threefold degeneracy at the conduction band minimum accompanied by linear band behaviour at higher carrier concentrations, which is thought to be related to the increase in effective mass as the doping level increases. Using combined experimental and computational studies, we show instead that a secondary conduction band with 12 conducting carrier pockets (which converges with the primary band at high temperatures) is responsible for the extraordinary thermoelectric performance of n-type CoSb3 skutterudites. A theoretical explanation is also provided as to why the linear (or Kane-type) band feature is not beneficial for thermoelectrics.


Dysprosium-doped cadmium oxide as a gateway material for mid-infrared plasmonics

Edward Sachet, Christopher T. Shelton, Joshua S. Harris, Benjamin E. Gaddy, Douglas L. Irving, Stefano Curtarolo, Brian F. Donovan, Patrick E. Hopkins, Peter A. Sharma, Ana Lima Sharma, Jon Ihlefeld, Stefan Franzen, and Jon-Paul Maria

Abstract:
The interest in plasmonic technologies surrounds many emergent optoelectronic applications, such as plasmon lasers, transistors, sensors and information storage. Although plasmonic materials for ultraviolet–visible and near-infrared wavelengths have been found, the mid-infrared range remains a challenge to address: few known systems can achieve subwavelength optical confinement with low loss in this range. With a combination of experiments and ab initio modelling, here we demonstrate an extreme peak of electron mobility in Dy-doped CdO that is achieved through accurate ‘defect equilibrium engineering’. In so doing, we create a tunable plasmon host that satisfies the criteria for mid-infrared spectrum plasmonics, and overcomes the losses seen in conventional plasmonic materials. In particular, extrinsic doping pins the CdO Fermi level above the conduction band minimum and it increases the formation energy of native oxygen vacancies, thus reducing their populations by several orders of magnitude. The substitutional lattice strain induced by Dy doping is sufficiently small, allowing mobility values around 500 cm2 V-1 s-1 for carrier densities above 1020 cm-3. Our work shows that CdO:Dy is a model system for intrinsic and extrinsic manipulation of defects affecting electrical, optical and thermal properties, that oxide conductors are ideal candidates for plasmonic devices and that the defect engineering approach for property optimization is generally applicable to other conducting metal oxides.


Reformulation of DFT+U as a Pseudohybrid Hubbard Density Functional for Accelerated Materials Discovery

Luis A. Agapito, Stefano Curtarolo, and Marco Buongiorno Nardelli

Abstract:
The accurate prediction of the electronic properties of materials at a low computational expense is a necessary condition for the development of effective high-throughput quantum-mechanics (HTQM) frameworks for accelerated materials discovery. HTQM infrastructures rely on the predictive capability of density functional theory (DFT), the method of choice for the first-principles study of materials properties. However, DFT suffers from approximations that result in a somewhat inaccurate description of the electronic band structure of semiconductors and insulators. In this article, we introduce ACBN0, a pseudohybrid Hubbard density functional that yields an improved prediction of the band structure of insulators such as transition-metal oxides, as shown for TiO2, MnO, NiO, and ZnO, with only a negligible increase in computational cost.


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