New layered quaternary BaCu6Sn2As4−x and BaCu6Sn2P4−x phases: Crystal growth and physical properties
Introduction
The Cu2Sb-type and related structures have been widely studied and are adopted by a huge number of transition metal pnictides with bonding properties ranging from typically ionic over covalent to metallic [1]. The transition metals in these compounds occupy two distinct crystallographic sites: M1 site where metals are tetrahedrally coordinated with pnictide atoms and form planar square nets, and M2 site where metals are coordinated with square pyramids formed by five pnictide atoms and capped with the square nets from M1. Depending on the Wyckoff position distortion/splitting and interstitial-site filling, various prototypic structures including Fe2As, UAs2 (ZrSiS), PbFCl, CeFeSi-type, and the extended HfCuSi2 and ZrCuSiAs-type structures could be derived [2], [3], [4], [5]. The compounds crystallized in these structures display not only fascinating chemistry features, but also a variety of physical properties, including superconductivity, Weyl semimetal, Dirac semimetal, antiferromagnetic order and anomalous Hall effect [6], [7], [8], [9], [10], [11], [12]. The paramount example among these phases is the unconventional superconductivity discovered in the iron pnictide superconductors [6], [7], [13].
The Cu-based pnictide family, on the other hand, is also rich in both structure diversity and interesting physical or functional applications. The binary Cu2Sb and Cu3P show promise as an anode material for Li-ion and Na-ion batteries [14], [15], [16]. The ternary antiferromagnetic CuMnAs is used as a multi-level memory cell [17]. Structurally, taking ternary Ba–Cu–As system as example, at least five different structures of BaCuAs, BaCu2As2, BaCu4As2, BaCu6As2, and BaCu8As4, ranging from two-dimensional layers to three-dimensional networks, are known with their analogous phosphides [18], [19], [20], [21], [22]. One interesting observation in ACu2Pn2 (A = Sr, Ba, and Eu, Pn = P, As and Sb) is that P- and As-based compounds all crystallize in the ThCr2Si2-type structure, while Sb-based analogs mainly adopt CaBe2Ge2-type structure or intergrowth of ThCr2Si2 and CaBe2Ge2-type structure [23], [24], [25], [26]. Recently, in the course of systematical investigation on the impact of flux for growing copper pnictide single crystals, we have discovered a new β-BaCu2As2 polymorphic phase with unique building sequence [27], which becomes superconducting under high pressure.
The abundant structural diversity in copper pnictides and opportunities for discovery of new materials through flux synthesis have motivated us to explore Cu-based pnictide system and discover new phases with potentially captivate interesting physical properties. Here, we report two new quaternary BaCu6Sn2As4−x and BaCu6Sn2P4−x phases with a new structure type, which are grown using Sn flux growth method with controlled starting Ba:Cu:Pn ratios. These two compounds adopt the same space group I4/mmm (#139), which are characterized by single crystal X-ray diffraction, SEM, TEM and STEM. The electrical transport measurements on these new compounds indicate overall metallic behaviors from room temperature to 2 K, with electron charge carriers and small positive magnetoresistance under magnetic field of 9 T.
Section snippets
Material synthesis and crystal growth
The starting materials Ba pieces (99.5%), Cu powder (99.99%), Sn shots (99.99%), As lumps (99.999%) and P lumps (99.999%) from Alfa Aesar were stored in Ar-flowed glovebox with a total O2 and moisture level less than 0.1 ppm. The BaCu6Sn2As4−x crystal is initially discovered as a by-product during crystal growth attempts of BaCu4As2 and BaCu8As4 phases using Sn flux. We observed that this phase also appeared to coexist with β-BaCu2As2 phase under certain specific stoichiometric ratios at low
Structural Description
The BaCu6Sn2As4−x and BaCu6Sn2P4−x are isostructural with each other, which crystallize in a new structure type (tI26) and tetragonal space group I4/mmm (#139). The refined lattice parameters are a = b = 4.164(1) Å, c = 24.088(3) Å for BaCu6Sn2As4−x and a = b = 4.053(2) Å, c = 24.08(1) Å, BaCu6Sn2P4−x, respectively, where no significant change of lattice c is observed from As-analog to P-analog. To the best of our knowledge, these two samples are the first quaternary compounds discovered in
Conclusion
In conclusion, quaternary BaCu6Sn2As4−x and BaCu6Sn2P4−x phases, as the only two quaternary compounds discovered in Ba–Cu–Sn–Pn (Pn = As, P) system so far, form a new class of compounds with characteristic slab structures. The synthesis, crystal structure, and basic physical properties of the two compounds have been described. The distinct pnictogen occupancy at the tetrahedral interstitial sites formed by neighboring metal atoms with Cu2Sb-type building motifs is rather intriguing, suggesting
CRediT authorship contribution statement
Hanlin Wu: Investigation, Formal analysis, Validation, Visualization, Writing – original draft, Preparation. Sheng Li: Formal analysis, Investigation, Writing – review & editing, Validation. Xiqu Wang: Investigation, Formal analysis. Sunah Kwon: Investigation, Formal analysis. Wenhao Liu: Investigation. Gareth A. Ofenstein: Investigation. Moon J. Kim: Resources, Investigation. Bing Lv: Conceptualization, Supervision, Resources, Writing – review & editing, Funding acquisition.
Accession Codes
CCDC 2072891, 2072892 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work at University of Texas at Dallas is supported by US Air Force Office of Scientific Research Grant No. FA9550-19-1-0037. This project is also partially funded by NSF- DMREF- 1921581, and the University of Texas at Dallas Office of Research through the Seed Program for Interdisciplinary Research (SPIRe) and Core Facility Voucher Program. M. J. K. was supported in part by the Louis Beecherl, Jr. Endowment Funds, and Global Research and Development Center Program (2018K1A4A3A01064272) and
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