Article pubs.acs.org/cm
Combined Computational and in Situ Experimental Search for Phases in an Open Ternary system, Ba−Ru−S Ankita Bhutani,† Joshua A. Schiller,‡ Julia L. Zuo,† James N. Eckstein,§ Laura H. Greene,¶ Santanu Chaudhuri,∥ and Daniel P. Shoemaker*,† †
Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, ‡Department of Mechanical Science and Engineering, and §Department of Physics, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ∥ Illinois Applied Research Institute, University of Illinois at Urbana−Champaign, Champaign, Illinois 61820, United States ¶ National High Magnetic Field Laboratory, FSU-UF-LANL, and Department of Physics, Florida State University, Tallahassee, Florida 32317, United States ABSTRACT: Rapid materials discovery in inorganic chemistry should combine predictive computational tools with fast experimental syntheses. We apply such a tandem approach to explore the Ba−Ru−S phase space, where no ternary compounds are yet known to exist. Related ternary oxide ruthenates and ternary iron sulfides exhibit interesting electronic properties due to d-electron correlations, such as superconductivity, metamagnetism, and quantum phase transitions. We use a combination of evolutionary algorithms and density functional theory to inform traditional and in situ diffraction methods. In the course of our investigation, we find that convex hull constructions of the binary constituents inform interpretation of the ternary hull, which in this case has two compounds near thermodynamic stability. Our experimental study does not reveal formation of the candidates BaRu2S2 or BaRuS3, but it does provide the structure of a high-temperature polymorph of BaS2. This methodology can be exploited to study other ternary systems to screen for novel phases.
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INTRODUCTION Modern approaches to materials discovery can leverage advances in parallel computation to organize, predict, and compute the thermodynamic stability of hypothetical phases. For inorganic compounds, these efforts are often embodied by chemical substitution of ions in known structure types1,2 or by genetic algorithm approaches that build new structure types from their constituents.3−5 These techniques are not mutually exclusive, and known or hypothesized phases can be used to seed a genetic algorithm run for a given phase space. Synthesis experiments can likewise evolve. High-throughput synthesis has been performed by automation, for example, through fluidic approaches of framework materials6 or composition-spread film deposition of oxides and alloys.7−9 These techniques are well-suited to reactions where no specific compound is of interest, and their startup costs are high. When high-throughput computation leads to distinct candidates, the goal is to reduce the number of synthesis attempts.2,10,11 We find that encapsulated in situ XRD is a synthesis method that is chemically versatile and comparatively information-rich: it reveals the kinetic behavior of reagents, the course of their reaction, and the presence of any products regardless of their temperature existence window and whether or not they persist at the end of the reaction.12 Previous work © 2017 American Chemical Society
in our group has shown that it can be used to effectively screen materials predicted by ionic substitution algorithms alone13 (i.e., not with structure-building as performed here). Our interest in this particular ternary system arose from the likely correlated electronic properties of its constituents. The chemically related alkaline−ruthenium−oxides display a wealth of behavior: pseudogap formation in BaRuO3,14−17 unconventional superconductivity in Sr2RuO4,18,19 quantum phase transition in BaRu6O12,20 metamagnetic metallic behavior in Sr3Ru2O7,21,22 itinerant ferromagnetism in Sr2RuO4,23,24 and a metal−insulator transition in Ca3Ru2O7.25 Likewise, among the plethora of Ba−Fe−S phases26−30 are the infinitely adaptive series Ba1+xFe2S431 and BaFe2S3, which exhibit pressure-induced superconductivity32 and magnetoresistance.33 Known phases in the Ba−Ru−O and Ba−Fe−S systems are shown in the combined ternary phase diagram in Figure 1. Surprisingly, the intersection of these two groups of materials, the Ba−Ru−S system, has no known stable ternary phases. Numerous databases based on high-throughput computation and open source web based analysis tools now exist such as the Received: February 24, 2017 Revised: June 22, 2017 Published: June 27, 2017 5841
DOI: 10.1021/acs.chemmater.7b00809 Chem. Mater. 2017, 29, 5841−5849
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thus restrain, Z. Consequently, we chose to build structures out of unconstrained elemental building blocks rather than a fixed formula and restricted the number of atoms per unit cell to between 8 and 30. USPEX uses a combination of breeding parameters and smart mutations to determine the most stable structure for a given set of stoichiometric and geometric restrictions. For the binary Ba−S and Ru−S, these parameters were set to the autofrac function of USPEX, which automatically adjusts them over the course of the structure search. The initial population consisted of no less than 100 structures including seeds derived from the Materials Project structure predictor.37,76 Subsequent generations were restricted to 75 structures. Local relaxations for structures were conducted using the Vienna Abinitio Simulation Packaged (VASP)77 utilizing large core, projector augmented wave (PAW) pseudopotentials.78 The exchange-correlation functionals were treated within the generalized gradient approximation (GGA) as provided by Perdew−Burke−Ernzerhof (PBE).79 Relaxations were performed for both the geometry of the unit cell as well as the ionic positions. Final energies were calculated at a k-resolution of 2π × 0.08 Å−1 at an energy cutoff of 500 eV. For the full Ba−Ru−S search, final energies were calculated using an energy cutoff of 300 eV and a Monkhorst−Pack grid with a resolution of 2π × 0.08 Å−1. Unlike the binary searches, the ternary search represents a combination of different searches. The population size for these searches varied between 15 and 50 for all runs. Variation parameters were set such that the top 60% fittest structures were used to generate 50% of the structures in the next generation. The remainder were generated either randomly (20%), by soft mutations (20%), or by lattice mutations (10%). After the survey, specific stoichiometries were chosen to expand the ternary convex hull and avoid favoring binary compounds. We checked the mechanical stability of the predicted structures through phonon calculations using the phonopy code80 in conjunction with VASP. We used an 800 eV cutoff with Γ-centered k-point spacing of 0.15 Å−1 at VASP’s accurate precision setting. Energies were converged to within 10−8 eV.
Figure 1. Ternary phase diagram of Ba−Ru−S showing known phases in the Ba−Ru−O (×) and Ba−Fe−S (○ ) chemical spaces. Compounds predicted here, BaRu2S2 and BaRuS3, are labeled by their subscripts.
Materials Project,1 OQMD,34 and Aflowlib.35 Some reliance on data mining is typically used to seed an initial group of structures for substitution.3,36−38 Computational screening for thermodynamically stable phases, involving both small-scale and high-throughput searches, has been attempted in many areas of materials chemistry, including hydrogen storage,39,40 piezoelectrics,41 carbon capture materials,42 radiation detector materials,43 topological insulators,44 lithium ion batteries,45−48 thermoelectrics,2,49 binary alloys,50 superconductors,51 and clarification of crystal structure debates.52 A variety of methods, such as simulated annealing,53 Abinitio Random Structure Searching (AIRSS),54 particle swarm optimization,55,56 and evolutionary algorithms,57 have been used for crystal structure prediction, with some studies resulting in newly realized materials.58−60 We focused on the evolutionary algorithm approach in this investigation. Structure searches based on evolutionary algorithms have been performed most prolifically using the USPEX and XtalOpt programs, often applied to carbonates and silicates4,5,61−66 and metal polyhydrides under high pressures,67−73 where the PV free energy term becomes increasingly dominant. Their use in a wider compositional space is less common. Maddox’s challenge to the science community by calling crystal structure prediction “a scandal in physical sciences” still remains open.74 In this Article, we present the interaction between evolutionary-algorithm structure building, thermodynamic stability calculations, and experimental synthesis with fast in situ hightemperature X-ray diffraction. In addition to understanding the Ba−Ru−S space on its own merits, we hope that the detailed methods described here will serve as general guidelines to guide similar efforts in chemically and synthetically diverse systems.
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EXPERIMENTAL PROCEDURE
Samples were prepared from powdered BaS (99.7%, Alfa Aesar), Ru (99.95%, J&J Materials), and S (99.5%, Alfa Aesar), with manipulations carried out in an Ar-filled glovebox. Handling of materials was done in a glovebox under an inert argon atmosphere (0.1 eV/atom of the hull.13,97 Phonon-based methods of estimating the entropic term in the Gibbs free energy are also available,98,99 but cost of application to all candidate structures and the need for self-consistency along the hull should be considered. If the pure compounds are still not accessible, physical trends can often be investigated by approaching their stoichiometry through solid solutions, for example, S substitution into BaRu2As2100,101 or BaRuO3. Validating the structure search is helped immensely by an information-rich synthesis technique such as our in situ diffraction technique. Here, we can be sure that even refractory precursors such as BaS and Ru metal are reactive, and identify transient phases such as ht-BaS2, while simultaneously collecting sufficient diffraction information to solve their structures. Other in situ characterization tools should be used in concert with more varied synthesis methods.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ankita Bhutani: 0000-0001-8053-5432 Daniel P. Shoemaker: 0000-0003-3650-7551 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Applied Research Institutes’s 2014 Seed Funding Program. L.H.G. was supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Initial ex situ X-ray diffraction measurements were carried out in the Frederick Seitz Materials Research Laboratory Central Research Facilities, University of Illinois.
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