Emergence of a few distinct structures from a single formal structure type during high-throughput screening for stable compounds: The case of RbCuS and RbCuSe

Giancarlo Trimarchi, Xiuwen Zhang, Michael J. DeVries Vermeer, Jacqueline Cantwell, Kenneth R. Poeppelmeier, and Alex Zunger

Phys. Rev. B 92, 165103 – Published 2 October 2015

Abstract

Theoretical sorting of stable and synthesizable “missing compounds” from those that are unstable is a crucial step in the discovery of previously unknown functional materials. This active research area often involves high-throughput (HT) examination of the total energy of a given compound in a list of candidate formal structure types (FSTs), searching for those with the lowest energy within that list. While it is well appreciated that local relaxation methods based on a fixed list of structure types can lead to inaccurate geometries, this approach is widely used in HT studies because it produces answers faster than global optimization methods (that vary lattice vectors and atomic positions without local restrictions). We find, however, a different failure mode of the HT protocol: specific crystallographic classes of formal structure types each correspond to a series of chemically distinct “daughter structure types” (DSTs) that have the same space group but possess totally different local bonding configurations, including coordination types. Failure to include such DSTs in the fixed list of examined candidate structures used in contemporary high-throughput approaches can lead to qualitative misidentification of the stable bonding pattern, not just quantitative inaccuracies. In this work, we (i) clarify the understanding of the general DST-FST relationship, thus improving current discovery HT approaches, (ii) illustrate this failure mode for RbCuS and RbCuSe (the latter being a yet unreported compound and is predicted here) by developing a synthesis method and accelerated crystal-structure determination, and (iii) apply the genetic-algorithm-based global space-group optimization (GSGO) approach which is not vulnerable to the failure mode of HT searches of fixed lists, demonstrating a correct identification of the stable DST. The broad impact of items (i)–(iii) lies in the demonstrated predictive ability of a more comprehensive search strategy than what is currently used—use HT calculations as the preliminary broad screening followed by unbiased GSGO of the final candidates.

Received 25 June 2015

DOI:https://doi.org/10.1103/PhysRevB.92.165103

Authors & Affiliations

Giancarlo Trimarchi^1,*, Xiuwen Zhang², Michael J. DeVries Vermeer³, Jacqueline Cantwell³, Kenneth R. Poeppelmeier³, and Alex Zunger²

¹Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA
²Renewable and Sustainable Energy Institute, University of Colorado, Boulder, Colorado 80309, USA
³Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA

^*g-trimarchi@northwestern.edu

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Vol. 92, Iss. 16 — 15 October 2015

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Figure 1
(a) The I-I-VI $A B X$ compounds that are reported in the ICSD [16] are shown by check marks, while the question marks indicate the compounds in this family that are missing from the ICSD. Note that RbCuS is missing from the current release of the ICSD [23], but was initially reported in Refs. [26, 27]. (b) I-I-VI compounds predicted by generalized gradient approximation (GGA) formation energy calculations to be stable or unstable assuming only the $C 1_{b}$ structure (see the Supplemental Material [25] for details). The $+$ and $-$ signs indicate missing materials predicted to be, respectively, thermodynamically stable and unstable in the $C 1_{b}$ structure. (c) I-I-VI $A B X$ compounds predicted stable or unstable in Ref. [1] in which the lowest-energy structure for a missing compound was determined by screening 41 $A B X$ structure types. As discussed in Ref. [1], LiCuS [48] and KAgTe [49] were synthesized before but are not listed in the ICSD database [16], and therefore thermodynamics stability calculations were performed on them in Ref. [1] and they were found to be stable and, thus, included as plus signs in Fig. 4 in Ref. [1].
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Figure 2
Crystal-structure model of the formal structure types whose total energy was calculated by full local structure relaxation and shown in Fig. 3.
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Figure 3
Results of the high-throughput calculations on RbCuS and RbCuSe performed using a subset of the structure types that were used in Ref. [1], specifically we used the structure types predicted for the I-I-VI compounds. The cell-internal and lattice vector degrees of freedom were fully relaxed to the nearest local total-energy minimum. As a starting point for each local total-energy minimization, we took the atom coordinates of the associated prototype solid after which each structure type is named. The total energies refer to the experimentally observed structures, which for both RbCuS and RbCuSe correspond to the structure predicted by global space-group optimization (GSGO).
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Figure 4
Three-dimensional models depicting, for RbCuS, the (a) experimental structure, (b) local minimum structure found starting the structural relaxation from the coordinated atom of RbAuS, and (c) GSGO structure. The crystallographic parameters for these three structures are reported in Tables 2 and 3.
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Figure 5
Minimum-energy path connecting the experimental structure of RbCuS and the local minimum-energy structure found for this compound by local energy minimization starting from the atom positions in RbAuS and substituting Au with Cu. The zero of the energy is set equal to the total energy of the ab initio relaxed experimental structure of RbCuS.
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