Publications

Since ASU

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Prior Publications

• Evaluating NaREMgWO₆ (RE = La, Gd, Y) Doubly Ordered Double Perovskites as Eu³⁺ Phosphor Hosts

  Andrew R. Sharits, Jason F. Khoury, and Patrick M. Woodward

  Inorganic Chemistry 2016, 55 (23), 12383-12390. DOI: 10.1021/acs.inorgchem.6b02295

  Abstract

  Three doubly ordered double perovskites NaREMgWO₆ (RE = La, Gd, Y) have been synthesized via traditional solid-state methods, doped with Eu³⁺, and characterized to evaluate their promise as Eu³⁺ phosphor hosts. NaYMgWO₆, a new member of the family, was found to crystallize in the P21 space group and is isostructural with NaREMgWO₆. Emissions characteristic of Eu³⁺ ions (⁵D₀ → ⁷F_4,3,2,1,0) were observed, with the most intense transition being the ⁵D₀ → ⁷F₂ transition near 615 nm. Substitution of Eu³⁺ onto a more compressed RE site in the NaY1–xEu_xMgWO₆ and NaGd_1–xEu_xMgWO₆ hosts results in a blue shift of the charge-transfer excitation band and an increase in the intensity of the ⁵D₀ → ⁷F₂ transition compared to NaLa_1–xEu_xMgWO₆. All of the hosts can incorporate high concentrations of Eu³⁺ before concentration quenching is observed. When the rare-earth ion is either Gd³⁺ or Y³⁺, good energetic overlap between the Eu³⁺ charge-transfer band and the absorption of the host lattice results in sensitization and energy transfer from the perovskite host lattice to the Eu³⁺ activator sites. These hosts display comparable if not better luminescence than Y₂O₃:Eu³⁺, a commonly used commercial standard, demonstrating their promise as red phosphors.

  

• Quaternary Pavonites A_1+xSn_2-xBi_5+xS₁₀ (A+ = Li⁺, Na⁺): Site Occupancy Disorder Defines Electronic Structure

  Jason F. Khoury, Shiqiang Hao, Constantinos C. Stoumpos, Zhenpeng Yao, Christos D. Malliakas, Umut Aydemir, Tyler J. Slade, G. Jeffrey Snyder, Chris Wolverton, and Mercouri G. Kanatzidis

  Inorganic Chemistry 2018, 57 (4), 2260-2268. DOI: 10.1021/acs.inorgchem.7b03091

  Abstract

  The field of mineralogy represents an area of untapped potential for the synthetic chemist, as there are numerous structure types that can be utilized to form analogues of mineral structures with useful optoelectronic properties. In this work, we describe the synthesis and characterization of two novel quaternary sulfides A_1+xSn_2-xBi_5+xS₁₀ (A+ = Li⁺, Na⁺). Though not natural minerals themselves, both compounds adopt the pavonite structure, which crystallizes in the C2/m space group and consists of two connected, alternating defect rock-salt slabs of varying thicknesses to create a three-dimensional lattice. Despite their commonalities in structure, their crystallography is noticeably different, as both structures have a heavy degree of site occupancy disorder that affects the actual positions of the atoms. The differences in site occupancy alter their electronic structures, with band gap values of 0.31(2) eV and 0.07(2) eV for the lithium and sodium analogues, respectively. LiSn₂Bi₅S₁₀ exhibits ultralow thermal conductivity of 0.62 W m^–1 K^–1 at 723 K, and this result is corroborated by phonon dispersion calculations. This structure type is a promising host candidate for future thermoelectric materials investigation, as these materials have narrow band gaps and intrinsically low thermal conductivities.

  

• A New Three-Dimensional Subsulfide Ir₂In₈S with Dirac Semimetal Behavior

  Jason F. Khoury, Alexander J. E. Rettie, Mojammel A. Khan, Nirmal J. Ghimire, Iñigo Robredo, Jonathan E. Pfluger, Koushik Pal, Chris Wolverton, Aitor Bergara, Jidong Samuel Jiang, Leslie M. Schoop, Maia G. Vergniory, J. F. Mitchell, Duck Young Chung, Mercouri G. Kanatzidis

  Journal of the American Chemical Society 2019, 141 (48), 19130-19137. DOI: 10.1021/jacs.9b10147

  Abstract

  Dirac and Weyl semimetals host exotic quasiparticles with unconventional transport properties, such as high magnetoresistance and carrier mobility. Recent years have witnessed a huge number of newly predicted topological semimetals from existing databases; however, experimental verification often lags behind such predictions. Common reasons are synthetic difficulties or the stability of predicted phases. Here, we report the synthesis of the type-II Dirac semimetal Ir₂In₈S, an air-stable compound with a new structure type. This material has two Dirac crossings in its electronic structure along the Γ–Z direction of the Brillouin zone. We further show that Ir₂In₈S has a high electron carrier mobility of ∼10 000 cm²/(V s) at 1.8 K and a large, nonsaturating transverse magnetoresistance of ∼6000% at 3.34 K in a 14 T applied field. Shubnikov de-Haas oscillations reveal several small Fermi pockets and the possibility of a nontrivial Berry phase. With its facile crystal growth, novel structure type, and striking electronic structure, Ir₂In₈S introduces a new material system to study topological semimetals and enable advances in the field of topological materials.

  

• Ir₆In₃₂S₂₁, a Polar, Metal-Rich Semiconducting Subchalcogenide

  Jason F. Khoury, Jiangang He, Jonathan E. Pfluger, Ido Hadar, Mahalingam Balasubramanian, Constantinos C. Stoumpos, Rui Zu, Venkatraman Gopalan, Chris Wolverton, Mercouri G. Kanatzidis

  Chemical Science 2020, 11 (3), 870-878. DOI: 10.1039/C9SC05609B

  Abstract

  Subchalcogenides are uncommon, and their chemical bonding results from an interplay between metal–metal and metal–chalcogenide interactions. Herein, we present Ir₆In₃₂S₂₁, a novel semiconducting subchalcogenide compound that crystallizes in a new structure type in the polar P31m space group, with unit cell parameters a = 13.9378(12) Å, c = 8.2316(8) Å, α = β = 90°, γ = 120°. The compound has a large band gap of 1.48(2) eV, and photoemission and Kelvin probe measurements corroborate this semiconducting behavior with a valence band maximum (VBM) of −4.95(5) eV, conduction band minimum of −3.47(5) eV, and a photoresponse shift of the Fermi level by ∼0.2 eV in the presence of white light. X-ray absorption spectroscopy shows absorption edges for In and Ir do not indicate clear oxidation states, suggesting that the numerous coordination environments of Ir₆In₃₂S₂₁ make such assignments ambiguous. Electronic structure calculations confirm the semiconducting character with a nearly direct band gap, and electron localization function (ELF) analysis suggests that the origin of the gap is the result of electron transfer from the In atoms to the S 3p and Ir 5d orbitals. DFT calculations indicate that the average hole effective masses near the VBM (1.19m_e) are substantially smaller than the average electron masses near the CBM (2.51m_e), an unusual feature for most semiconductors. The crystal and electronic structure of Ir₆In₃₂S₂₁, along with spectroscopic data, suggest that it is neither a true intermetallic nor a classical semiconductor, but somewhere in between those two extremes.

  

• The Subchalcogenides Ir₂In₈Q (Q = S, Se, Te): Dirac Semimetal Candidates with Re-entrant Structural Modulation

  Jason F. Khoury, Alexander J. E. Rettie, Iñigo Robredo, Matthew J. Krogstad, Christos D. Malliakas, Aitor Bergara, Maia G. Vergniory, Raymond Osborn, Stephan Rosenkranz, Duck Young Chung, Mercouri G. Kanatzidis

  Journal of the American Chemical Society 2020, 142 (13), 6312-6323. DOI: 10.1021/jacs.0c00809

  Abstract

  Subchalcogenides are uncommon compounds where the metal atoms are in unusually low formal oxidation states. They bridge the gap between intermetallics and semiconductors and can have unexpected structures and properties because of the exotic nature of their chemical bonding as they contain both metal–metal and metal–main group (e.g., halide, chalcogenide) interactions. Finding new members of this class of materials presents synthetic challenges as attempts to make them often result in phase separation into binary compounds. We overcome this difficulty by utilizing indium as a metal flux to synthesize large (millimeter scale) single crystals of novel subchalcogenide materials. Herein, we report two new compounds Ir₂In₈Q (Q = Se, Te) and compare their structural and electrical properties to the previously reported Ir₂In₈S analogue. Ir₂In₈Se and Ir₂In₈Te crystallize in the P4₂/mnm space group and are isostructural to Ir₂In₈S, but also have commensurately modulated (with q vectors q = 1/6a* + 1/6b* and q = 1/10a* + 1/10b* for Ir₂In₈Se and Ir₂In₈Te, respectively) low-temperature phase transitions, where the chalcogenide anions in the channels experience a distortion in the form of In–Q bond alternation along the ab plane. Both compounds display re-entrant structural behavior, where the supercells appear on cooling but revert to the original subcell below 100 K, suggesting competing structural and electronic interactions dictate the overall structure. Notably, these materials are topological semimetal candidates with symmetry-protected Dirac crossings near the Fermi level and exhibit high electron mobilities (∼1500 cm² V^–1 s^–1 at 1.8 K) and moderate carrier concentrations (∼10²⁰ cm^–3) from charge transport measurements. This work highlights metal flux as a synthetic route to high quality single crystals of novel intermetallic subchalcogenides with Dirac semimetal behavior.

  

• Contrasting SnTe–NaSbTe₂ and SnTe–NaBiTe₂ Thermoelectric Alloys: High Performance Facilitated by Increased Cation Vacancies and Lattice Softening

  Tyler J. Slade, Koushik Pal, Jann A. Grovogui, Trevor P. Bailey, James Male, Jason F. Khoury, Xiuquan Zhou, Duck Young Chung, G. Jeffrey Snyder, Ctirad Uher, Vinayak P. Dravid, Chris Wolverton, Mercouri G. Kanatzidis

  Journal of the American Chemical Society 2020, 142 (28), 12524-12535. DOI: 10.1021/jacs.0c05650

  Abstract

  Defect chemistry is critical to designing high performance thermoelectric materials. In SnTe, the naturally large density of cation vacancies results in excessive hole doping and frustrates the ability to control the thermoelectric properties. Yet, recent work also associates the vacancies with suppressed sound velocities and low lattice thermal conductivity, underscoring the need to understand the interplay between alloying, vacancies, and the transport properties of SnTe. Here, we report solid solutions of SnTe with NaSbTe₂ and NaBiTe₂ (NaSn_mSbTe_m+2 and NaSn_mBiTe_m+2, respectively) and focus on the impact of the ternary alloys on the cation vacancies and thermoelectric properties. We find introduction of NaSbTe₂, but not NaBiTe₂, into SnTe nearly doubles the natural concentration of Sn vacancies. Furthermore, DFT calculations suggest that both NaSbTe₂ and NaBiTe₂ facilitate valence band convergence and simultaneously narrow the band gap. These effects improve the power factors but also make the alloys more prone to detrimental bipolar diffusion. Indeed, the performance of NaSn_mBiTe_m+2 is limited by strong bipolar transport and only exhibits modest maximum ZTs ≈ 0.85 at 900 K. In NaSn_mSbTe_m+2 however, the doubled vacancy concentration raises the charge carrier density and suppresses bipolar diffusion, resulting in superior power factors than those of the Bi-containing analogues. Lastly, NaSbTe₂ incorporation lowers the sound velocity of SnTe to give glasslike lattice thermal conductivities. Facilitated by the favorable impacts of band convergence, vacancy-augmented hole concentration, and lattice softening, NaSn_mSbTe_m+2 reaches high ZT ≈ 1.2 at 800–900 K and a competitive average ZT_avg of 0.7 over 300–873 K. The difference in ZT between two chemically similar compounds underscores the importance of intrinsic defects in engineering high-performance thermoelectrics.

  

• Chemical Bonds in Topological Materials

  Jason F. Khoury, Leslie M. Schoop

  Trends in Chemistry 2021, 3, 9, 700-715. DOI: 10.1016/j.trechm.2021.04.011

  Abstract

  Topological materials are of great significance in the chemistry, physics, and engineering communities due to their implications for fundamental and applied science. They are solid-state materials where electrons behave atypically, acting as analogs to particles in high-energy physics. In addition, topological materials are impactful in spintronics, catalysis, and quantum information science. However, we do not yet have a holistic understanding of the nature of chemical bonding in these materials, despite growing evidence that it plays a vital role. In this review, we explore how delocalized bonding can lead to topological electronic structures in one-, two-, and three-dimensional systems. We highlight the successes of chemical intuition in polymeric and square-net systems and the potential for one- and three-dimensional structures to follow.

  

• Evolving Devil’s Staircase Magnetization from Tunable Charge Density Waves in Nonsymmorphic Dirac Semimetals

  Ratnadwip Singha, Tyger H. Salters, Samuel M. L. Teicher, Shiming Lei, Jason F. Khoury, N. Phuan Ong, Leslie M. Schoop

  Advanced Materials 2021, 33, 41, 2103476. DOI: 10.1002/adma.202103476

  Abstract

  While several magnetic topological semimetals have been discovered in recent years, their band structures are far from ideal, often obscured by trivial bands at the Fermi energy. Square-net materials with clean, linearly dispersing bands show potential to circumvent this issue. CeSbTe, a square-net material, features multiple magnetic-field-controllable topological phases. Here, it is shown that in this material, even higher degrees of tunability can be achieved by changing the electron count at the square-net motif. Increased electron filling results in structural distortion and formation of charge density waves (CDWs). The modulation wave-vector evolves continuously leading to a region of multiple discrete CDWs and a corresponding complex “Devil’s staircase” magnetic ground state. A series of fractionally quantized magnetization plateaus is observed, which implies direct coupling between CDW and a collective spin-excitation. It is further shown that the CDW creates a robust idealized nonsymmorphic Dirac semimetal, thus providing access to topological systems with rich magnetism.

  

• Kinetics and Evolution of Magnetism in Soft-Chemical Synthesis of CrSe₂ from KCrSe₂

  Xiaoyu Song, Sarah N. Schneider, Guangming Cheng, Jason F. Khoury, Milena Jovanovic, Nan Yao, Leslie M. Schoop

  Chemistry of Materials 2021, 33, 20, 8070-8078. DOI: 10.1021/acs.chemmater.1c02620

  Abstract

  Cation deintercalation with soft-chemical methods provides a route to synthesize new layered compounds with emergent physical and chemical properties that are inaccessible by conventional high-temperature solid-state synthesis methods. One example is CrSe₂, a van der Waals (vdW) material that is promising as an air-stable two-dimensional (2D) magnet. Cation deintercalation has rarely been studied mechanistically, and optimized reaction pathways to yield high-quality materials are often poorly understood. In this work, we perform a detailed study of the oxidative deintercalation process of KCrSe₂. We prove for the first time using high-resolution scanning transmission electron microscopy (STEM) that even though CrSe₂ indeed exists in a true vdW-layered structure, K-intercalated crystalline defects exist in the final product, even when an excess of oxidizing agent was used. We then study the kinetics of the oxidative deintercalation process, showing that it is a zeroth-order reaction with an activation energy of 0.27(6) eV, where the solid-state diffusion of K⁺ cations in the potassium deintercalation process is the rate-limiting step. Finally, we study the relationship between Cr–Cr distances and the change in magnetic order by tracking how the properties change as a function of varying potassium content due to deintercalation. These data suggest that it might be possible to switch between magnetic states in CrSe₂ monolayers by varying its lattice parameters with methods, such as applying strain. Our study also provides a deeper understanding of the cation deintercalation process from a mechanistic perspective that will be helpful for the future development of synthetic methodology that can lead to other new layered materials.

  

• A Class of Magnetic Topological Material Candidates with Hypervalent Bi Chains

  Jason F. Khoury, Bingzheng Han, Milena Jovanovic, Ratnadwip Singha, Xiaoyu Song, Raquel Queiroz, Nai-Phuan Ong, Leslie M. Schoop

  Journal of the American Chemical Society 2022, 144, 22, 9785-9796. DOI: 10.1021/jacs.2c02281

  Abstract

  The link between crystal and electronic structure is crucial for understanding structure–property relations in solid-state chemistry. In particular, it has been instrumental in understanding topological materials, where electrons behave differently than they would in conventional solids. Herein, we identify 1D Bi chains as a structural motif of interest for topological materials. We focus on Sm₃ZrBi₅, a new quasi-one-dimensional (1D) compound in the Ln₃MPn₅ (Ln = lanthanide; M = metal; Pn = pnictide) family that crystallizes in the P6₃/mcm space group. Density functional theory calculations indicate a complex, topologically nontrivial electronic structure that changes significantly in the presence of spin–orbit coupling. Magnetic measurements show a quasi-1D antiferromagnetic structure with two magnetic transitions at 11.7 and 10.7 K that are invariant to applied field up to 9 T, indicating magnetically frustrated spins. Heat capacity, electrical, and thermoelectric measurements support this claim and suggest complex scattering behavior in Sm₃ZrBi₅. This work highlights 1D chains as an unexplored structural motif for identifying topological materials, as well as the potential for rich physical phenomena in the Ln₃MPn₅ family.

  

• Ln₃MBi₅ (Ln=Pr, Nd, Sm; M=Zr, Hf): Intermetallics with Hypervalent Bismuth Chains

  Jason F. Khoury, Xiaoyu Song, Leslie M. Schoop

  Zeitschrift für anorganische und allgemeine Chemie 2022, 648, 15, e202200123. DOI: 10.1002/zaac.202200123

  Abstract

  We report five new isostructural compounds in the Ln₃MBi₅ (Ln=Pr, Nd, Sm; M=Zr, Hf) family, and compare them to the recently reported Sm₃ZrBi₅ analogue. Ln₃MBi₅ crystallizes in the P6₃/mcm space group, hosting the anti-Hf₅Sn₃Cu structure type. The one-dimensional structure consists of hypervalent Bi²⁻ chains and face-sharing MBi₆ octahedra that form chains along the c axis. A framework of Ln³⁺ cations charge balances and separates the two motifs. The Bi−Bi and M−Bi bond lengths decrease as the Ln cation becomes smaller across the period, but there is almost no difference in either bond length when the identity of M changes, as expected because Zr and Hf have the exact same radii due to lanthanide contraction. We present a structural stability map showing the limits of cation stability in the structure, with La³⁺ and U³⁺ as the largest cations and Sm³⁺ as the smallest cation. X-ray photoelectron spectra suggest ambiguous valence states in the Ln₃MBi₅ structure depending on the identity of the M atom. This work further expands the Ln₃MBi₅ family and sheds new light on how their bonding behavior may vary based on chemical composition.

  

• Synthesis of an Aqueous, Air-Stable, Superconducting 1T′-WS₂ Monolayer Ink

  Xiaoyu Song, Ratnadwip Singha, Guangming Cheng, Yao-Wen Yeh, Franziska Kamm, Jason F. Khoury, Brianna L. Hoff, Joseph W. Stiles, Florian Pielnhofer, Philip E. Batson, Nan Yao, Leslie M. Schoop

  Science Advances 2023, 9, 12, eadd6167. DOI: 10.1126/sciadv.add6167

  Abstract

  Liquid-phase chemical exfoliation can achieve industry-scale production of two-dimensional (2D) materials for a wide range of applications. However, many 2D materials with potential applications in quantum technologies often fail to leave the laboratory setting because of their air sensitivity and depreciation of physical performance after chemical processing. We report a simple chemical exfoliation method to create a stable, aqueous, surfactant-free, superconducting ink containing phase-pure 1T′-WS₂ monolayers that are isostructural to the air-sensitive topological insulator 1T′-WTe₂. The printed film is metallic at room temperature and superconducting below 7.3 kelvin, shows strong anisotropic unconventional superconducting behavior with an in-plane and out-of-plane upper critical magnetic field of 30.1 and 5.3 tesla, and is stable at ambient conditions for at least 30 days. Our results show that chemical processing can make nontrivial 2D materials that were formerly only studied in laboratories commercially accessible.

• Charge Density Wave-Templated Spin Cycloid in Topological Semimetal NdSb_xTe_2−x−δ

  Tyger H. Salters, Fabio Orlandi, Tanya Berry, Jason F. Khoury, Ethan Whittaker, Pascal Manuel, Leslie M. Schoop

  Physical Review Materials 2023, 7, 4, 044203. DOI: 10.1103/PhysRevMaterials.7.044203

  Abstract

  Magnetic topological semimetals present open questions regarding the interplay of crystal symmetry, magnetism, band topology, and electron correlations. LnSb_xTe_2−x−δ (Ln denotes Lanthanide) is a family of square-net-derived topological semimetals that allows compositional control of band filling, and access to different topological states via an evolving charge density wave (CDW) distortion. Previously studied Gd and Ce members containing a CDW have shown complex magnetic phase diagrams, which implied that spins localized on Ln interact with the CDW, but to this date no magnetic structures have been solved within the CDW regime of this family of compounds. Here, we report on the interplay of the CDW with magnetism in NdSb_xTe_2−x−δ by comparing the undistorted square net member NdSb_0.94Te_0.92 with the CDW-distorted phase NdSb_0.48Te_1.37, via single-crystal x-ray diffraction, magnetometry, heat capacity, and neutron powder diffraction. NdSb_0.94Te_0.92 is a collinear antiferromagnet with T_N∼2.7K, where spins align antiparallel to each other, but parallel to the square net of the nuclear structure. NdSb_0.48Te_1.37 exhibits a nearly fivefold-modulated CDW (q_CDW=0.18), isostructural to other LnSb_xTe_2−x−δ at similar x. NdSb_0.48Te_1.37 displays more complex magnetism with T_N=2.3K, additional metamagnetic transitions, and an elliptical cycloid magnetic structure with q_mag=−0.41b^∗. The magnitudes of q_CDW and q_mag exhibit an integer relationship, 1+2q_mag=q_CDW, implying a coupling between the CDW and magnetic structure. Given that the CDW is localized within the nonmagnetic distorted square net, we propose that conduction electrons “template” the spin modulation via the Ruderman-Kittel-Kasuya-Yosida interaction.

• Acid-Assisted Soft Chemical Route for Preparing High-Quality Superconducting 2M-WS₂

  Xiaoyu Song, Brianna Hoff, Ratnadwip Singha, Joseph W. Stiles, Grigorii Skorupskii, Jason F. Khoury, Guangming Cheng, Franziska Kamm, Ayelet J. Uzan, Stephanie Dulovic, Sanfeng Wu, Florian Pielnhofer, Nan Yao, Leslie M. Schoop

  Chemistry of Materials 2023, 35, 14, 5487-5496. DOI: 10.1021/acs.chemmater.3c00813

  Abstract

  2M-WS₂ is a metastable, superconducting polymorph of the transition metal dichalcogenide (TMD) WS₂, comprised of layers of face-sharing distorted WS₆ octahedra. It is predicted to host non-Abelian quantum states, promising for topological computing. Due to its thermodynamic instability, 2M-WS₂ cannot be synthesized using solid-state synthesis. Rather, it requires a top-down approach in which K⁺ is deintercalated from K_xWS₂; so far, this process has been completed using a strong oxidizer, K₂Cr₂O₇ in dilute H₂SO₄. A disadvantage of such an indirect synthesis is that the harsh reaction condition may cause the crystal quality to suffer. To date, no studies have been performed to optimize the synthesis or understand the chemical nature of this reaction. In this study, we found that the K-deintercalation process from K_xWS₂ is spontaneous, and a non-oxidative acidic reaction environment is sufficient to facilitate the oxidation of K_xWS₂ to 2M-WS₂ while reducing H⁺ to H₂. By analyzing the superconducting transition in the heat capacity, we found that 2M-WS₂ made using less aggressive methods has higher superconducting volume fractions. We describe how to access the thermodynamically unfavorable superconducting 2M phase of WS₂ as high-quality crystals.