Rocky Mountain Solid State Chemistry Workshop

Rocky Mountain Solid State Chemistry Workshop

January 10-11, 2023

University of Colorado Boulder

Gang Cao

“Towards control of quantum states in high-Z materials via electric currents and magneto-synthesis”

High-Z transition metal materials host a unique hierarchy of energy scales defined by comparable spin-orbit and Coulomb interactions (Z = atomic number; spin-orbit interaction ~ Z2). This energy setting generates a delicate interplay between the fundamental interactions and leaves these materials precariously balanced on the border between distinct ground states, and extremely susceptible to small, external stimuli [1,2]. Here I report two new approaches we have developed in recent years to control quantum states/materials via (1) application of small electric currents, as a new external stimulus [1,3,4], and (2) magneto-synthesis or field-editing crystal structures during the formation of single crystals at high temperatures [1,5]. Both are surprisingly effective in controlling the lattice, thus quantum states via the strong spin-orbit interaction and magnetoelastic coupling in these materials [1]. Representative results obtained using the two approaches will be presented and discussed.

References

1. Physics of Spin-Orbit-Coupled Oxides, Gang Cao and Lance E. De Long, Oxford University Press; Oxford, 2021
2. The Challenge of Spin-Orbit-Tuned Ground States in Iridates: A Key Issues Review, Gang Cao and Pedro Schlottmann, Reports on Progress in Physics, 81 042502 (2018)
3. Electrical Control of Structural and Physical Properties via Spin-Orbit Interactions in Sr2IrO4, G. Cao, J. Terzic, H. D. Zhao, H. Zheng, L. E DeLong and Peter Riseborough, Phys. Rev. Lett 120, 017201 (2018); DOI: https://doi.org/10.1103/PhysRevLett.120.017201
4. Control of chiral orbital currents in a colossal magnetoresistance material, Yu Zhang, Yifei Ni, Hengdi Zhao, Sami Hakani, Feng Ye, Lance DeLong, Itamar Kimchi, and Gang Cao, Nature 611, 467–472 (2022), DOI: https://doi.org/10.1038/s41586-022-05262-3
5. Quest for Quantum States via Field-Altering Technology, Gang Cao, Hengdi Zhao, Bing Hu, Nicholas Pellatz, Dmitry Reznik, Pedro Schlottmann and Itamar Kimchi, npj Quantum Materials 5, 83 (2020); https://doi.org/10.1038/s41535-020-00286-2

Ann Greenaway

“Co-design of emerging semiconductor systems for photoelectrochemical applications”

Sustainable fuel generation via a photoelectrochemical (PEC) route has been a major area of research since the first report of solar water-splitting for hydrogen production in 1972. The potential benefits of liquid fuels generated by CO₂ reduction (CO₂R) have recently increased interest in this space. However, delivering on the promise of PEC CO₂R will require substantial advances in the development of photoelectrode materials with high optoelectronic quality that can withstand photoelectrochemical operating conditions, a problem which remains unsolved despite fifty years of research progress. This presentation will highlight recent work on a newly synthesized nitride, ZnTiN₂, selected as a candidate material on the basis of self-passivating surface oxides and potential for integration with other semiconductors. Combinatorial synthesis was used to synthesize and improve the crystal quality of ZnTiN₂, revealing an optical absorption onset below 2 eV, appropriate for PEC applications. Density functional theory computations give insight into the role of cation site disorder in reducing the bandgap from the theoretical value of 3.36 eV. Tracking changes to the ZnTiN₂ surface under electrochemical polarization reveals the development of TiO₂– or ZnO-like character, consistent with the formation of stable solid phases at CO₂R operating conditions. These results highlight the potential of ZnTiN₂ as a photoelectrode material for CO₂R, while demonstrating a new design approach to other candidate materials.

Photoelectrochemical fuel generation is a promising route to sustainable liquid fuels produced from water and captured carbon dioxide with sunlight as the energy input. Development of such technologies requires photoelectrode materials that are both photocatalytically active and operationally stable in harsh oxidative and/or reductive electrochemical environments. Such photocatalysts can be discovered based on co-design principles, wherein design for stability is based on the propensity for the photocatalyst to self-passivate under operating conditions and design for photoactivity is based on the ability to integrate the photocatalyst with established semiconductor substrates. Here we report on synthesis and characterization of zinc titanium nitride (ZnTiN₂) that follows these design rules by having a wurtzite-derived crystal structure and showing self-passivating surface oxides created by electrochemical polarization. The sputtered ZnTiN₂ thin films have optical absorption onsets below 2 eV and n-type electrical conduction of 3 S/cm. The band gap of this material is reduced from the 3.36 eV theoretical value by cation site disorder, and the impact of cation antisites on the band structure of ZnTiN₂ is explored using density functional theory. Under electrochemical polarization, the ZnTiN₂ surfaces have TiO₂– or ZnO-like character, consistent with Materials Project Pourbaix calculations predicting the formation of stable solid phases under near-neutral pH. These results show that ZnTiN₂ is a promising candidate for photoelectrochemical liquid fuel generation and demonstrate a new materials design approach to other photoelectrodes with self-passivating native operational surface chemistry.

“Designing Disorder and Diffusion in Solid-State Electrolytes”

All-solid-state batteries hold the potential to transform electrochemical energy storage technologies. Replacing the flammable liquid electrolyte with a solid-state ion conductor can improve battery safety and may further increase battery energy density when paired with lithium metal anodes. Disorder – both static and dynamic – plays a crucial role in dictating ion diffusion in the solid state. Our work aims to harness disorder across time and length scales as a design principle for the next generation of solid-state electrolytes. The halide argyrodites Li₆PS₅X (X = halide, pseudohalide) present an excellent framework to understand the role of static and dynamic disorder in ion transport. In this work, we have discovered the new solid electrolyte Li₆PS₅CN in which the halide site is occupied by the orientationally-disordered cyanide ion. The new cyanide argyrodite exhibits lower activation barriers for Li-ion conductivity compared to the current champion argyrodite Li₆PS₅Br and comparable room temperature lithium-ion conductivities. Structurally, the similar sizes of cyanide and bromide ions produce nearly identical lithium conduction pathways. Through a suite of computational and experimental studies, we attribute the differences in lithium ion dynamics between Li₆PS₅Br and Li₆PS₅CN to coupled dynamics between cyanide rotations and lithium ion hopping.

Rebecca Smaha

“Probing novel kagome magnets and quantum spin liquid candidates with scattering”

While materials that exhibit a quantum spin liquid (QSL) ground state are highly desirable for potential uses in quantum computing, few viable candidates currently exist because many frustrated materials succumb to a structural distortion that breaks the delicate balance of competing magnetic interactions. The QSL can be described as entangled, frustrated spins that continuously fluctuate around a lattice down to T = 0 K without ever freezing into long range magnetic order. I will discuss the most promising candidates, minerals herbertsmithite and Zn-barlowite, which have a kagome lattice of Cu2+ cations. X-ray and neutron scattering studies—powder and single crystal diffraction as well as anomalous dispersion and inelastic scattering—of materials synthesized hydrothermally reveal that very subtle structural effects play a huge role in the ground state physics observed in this family, which range from ordered, frustrated antiferromagnets to QSL candidates.

“Lithium-ion diffusivity increases with block size in niobium tungsten oxide shear structures”

Nb₁₆W₅O₅₅ emerged as a high-rate anode material for Li-ion batteries in 2018 (Griffith et al., Nature 2018, 559 (7715), 556–563). This exciting discovery ignited research in Wadsley-Roth (W-R) compounds, but systematic experimental studies have not focused on how to tune material chemistry and structure to achieve desirable properties for energy storage applications. In this work, we systematically investigate how structure and composition influences capacity, Li-ion diffusivity, charge-discharge profiles, and capacity loss in a series of niobium tungsten oxide W-R compounds: (3×4)-Nb₁₂WO₃₃, (4×4)-Nb₁₄W₃O₄₄, and (4×5)-Nb₁₆W₅O₅₅. Potentiostatic intermittent titration (PITT) data confirmed that Li-ion diffusivity increases with block size, which can be attributed to an increasing number of tunnels for Li-ion diffusion. The small (3×4)-Nb₁₂WO₃₃ block size compound with preferential W ordering on tetrahedral sites exhibits single electron redox and, therefore, the smallest measured capacity despite having the largest theoretical capacity. This observation signals that introducing cation disorder (W occupancy at the octahedral sites in the block center) is a viable strategy to access multi-electron redox behavior in (3×4) Nb₁₂WO₃₃. The asymmetric block size compounds (i.e., (3×4) and (4×5) blocks) exhibit the greatest capacity loss after the first cycle, possibly due to Li-ion trapping at a unique low energy pocket site along the shear plane. Finally, the slope of the charge-discharge profile increases with increasing block size, likely because the total number of energy-equivalent Li-ion binding sites also increases. This unfavorable characteristic prohibits the large block sizes from delivering constant power at a fixed C-rate more so than the smaller block sizes. Based on these findings, we discuss design principles for Li-ion insertion hosts made from W-R materials.

“Towards the introduction of synthetic feasibility predictions during high throughput computational screening of metal-organic frameworks”

Metal-organic frameworks (MOFs) have garnered substantial attention for use in diverse applications in chemistry, chemical engineering, and materials science. This is due mainly to their highly tunable pore structure and chemistry, which arises from the modular nature of their construction. However, this modularity also results in a combinatorial explosion of possible MOFs compositions, making comprehensive MOF screening challenging. High throughput computational screening has accelerated property prediction of computational MOF prototypes. However, which computational MOF prototypes are synthetically feasible (or likely) is a question that remains unanswered. One question still under debate is to what extent synthetic likelihood is determined by thermodynamic or kinetic factors, respectively. Here I will discuss our efforts developing capabilities to perform high throughput MOF free energy calculations. Leveraging this capability allows I discuss whether thermodynamic factors correlate with the apparent synthesizability of prototypes in a MOF database. Apparent synthesizability was assessed by finding which computational prototypes were also found in experimental MOF repositories, e.g., the Cambridge database. For instance, we find that, in 80% of the cases, the experimentally reported member of an isomorphic series of MOFs has the lowest calculated free energy, revealing that, in the majority of cases, thermodynamics will correctly predict synthetically accessible isomorphs. 

“Magnetism By Design: Validating Computational Predictions of Heusler Skyrmion Hosts”

Gaining control over large magnetic structures, such as skyrmions, cones, and helices, will enable next generation data storage and other technologies. However, we are currently unable to accurately predict the formation of these states in a given material, let alone control their size, mobility, or packing. Heusler intermetallic compounds are known to host a variety of magnetic structures and here we will compare recent computations predicting the formation of skyrmions in a sub-class of Heuslers to experimental results.

“Cracking open the chicken-and-egg correlations in metal halide perovskites: Dynamic local order in CH₃NH₃PbI₃ and CH₃NH₃PbBr₃”

Hybrid lead halide perovskite semiconductors (LHPs) are recently reinvigorated class of materials with impressive performance in optoelectronic devices. Unlike the “classical” semiconductors of Si or GaAs, in LHPs the optoelectronic properties are governed by structural fluctuations within the seemingly well-defined high-temperature cubic phase. Recent work has uncovered potential short-range order arising from these fluctuations. However, the origin and structure of this short-range order are unresolved, impeding our understanding of technologically relevant properties including long carrier lifetimes and facile halide migration. Here, I will present a comprehensive exploration into the nature of short-range order in the prototypical LHPs, CH3NH3PbI3 and CH3NH3PbBr3. The true structure of these LHPs is determined with a combination of diffuse scattering, neutron spectroscopy, and molecular dynamics simulations. We find remarkable collective dynamics consisting of a network of two-dimensional pancake-like regions of dynamically tilting lead halide octahedra (lower symmetry) that induce longer ranger intermolecular correlations on the CH3NH3⁺ sublattice. The impact of this dynamic local structure on charge carrier lifetime and halide migration will be discussed.

“Mechanistically-guided materials chemistry: synthesis of new ternary nitrides, CaZrN₂ and CaHfN₂”

Recent computational studies have predicted many new ternary nitrides, revealing synthetic opportunities in this underexplored phase space. However, synthesizing new ternary nitrides is difficult, in part because intermediate and product phases often have high cohesive energy that inhibits diffusion and/or low thermodynamic stability against decomposition at elevated temperatures. Here, we report the synthesis of two new phases, calcium zirconium nitride (CaZrN₂) and calcium hafnium nitride (CaHfN₂), by solid state metathesis reactions between Ca₃N₂ and MCl₄ (M = Zr, Hf). In situ synchrotron X-ray diffraction studies reveal that stoichiometric reactions produce Zr(III) intermediates early on in the reaction pathway, showing that the excess Ca₃N₂ is necessary to yield a product with Zr(IV). This synthesis pathway contrasts with that of another recently discovered phase, MgZrN₂, in which the reaction proceeds through a ZrNCl intermediate, with a Zr(IV) oxidation state. Applying the hypothesis that the reaction interface locally follows equilibrium, we show that chemical potential diagrams can predict those equilibrium intermediates and rationalize the reaction pathway. Lastly, diffuse reflectance UV-vis measurements shows that CaZrN₂ has a bandgap near 2.0 eV, making it a promising material for photovoltaic applications. In addition to the discovery of a new material, these findings highlight the power of in-situ diffraction studies and chemical potential diagrams, with implications for designing reaction pathways that will enable the discovery of more predicted materials.

“Structure to property relationship explanation of ultra-low thermal conductivity in novel cage-like compounds”

Thermoelectric materials can contribute to enhancing sustainability of global energy systems by converting waste heat to electricity. A good compound for thermoelectric applications is characterized by large Seebeck coefficient accompanied by high electrical and low thermal conductivity. Recent discoveries of materials with extraordinarily suppressed thermal transport open a pathway to significant developments in our field. Here, we conduct a comparative study of three chemically similar antimonides to understand the root causes of their ultra-low thermal conductivity (< 0.6 W/mK). The materials of interest are: unconventional type-XI clathrate K58Zn122Sb207, the tunnel compound K6.9Zn21Sb16, and the type-I clathrate K8Zn15.5Cu2.5Sb28 discovered herein. The smallest value of thermal conductivity is observed for the type-XI clathrate (0.38 W/mK at 525 K); it arises from combination of (i) structural complexity, (ii) soft bonding, and (iii) presence of localized optical phonon modes. The structural complexity results in an abundance of low velocity, strongly overlapping optical modes. In such a scenario, the quasiparticle description no longer applies and a more localized diffuson scenario emerges. The soft bonding yields low acoustic mode group velocities. Finally, the extremely low frequency phonon modes due to rattling of K atoms lead to enhanced scattering throughout the acoustic branches. The type-I clathrate retains the structural complexity and soft bonding. Its rattling modes, however, shift to higher frequencies, which leads to doubling of phonon relaxation time. Thermal conductivity of type-I clathrate is larger with respect to the type-XI counterpart (ca. 0.45 W/mK at 525 K). The absence of any rattling modes in the tunnel compound results in further increase of relaxation time and the highest thermal conductivity among the investigated group (ca. 0.55 W/mK at 525 K). Frequency of rattling is strongly correlated with volume of cavities encapsulating K atoms in the studied structures. Understanding details of thermal transport in complex materials will be crucial for developing the next generation of thermoelectrics.

“Thin films: model systems for stabilizing thermodynamically challenged materials”

In many cases promising functional materials lie outside of thermodynamic equilibrium and the lack of a prescriptive synthesis approach precludes their realization. Unlike molecular synthesis where new materials are routinely achieved through the careful selection of reagents and kinetic control of reaction paths, the synthesis of extended solids usually employs reactions with harsh thermal conditions obliterating any structural or chemical features of reactants in favor of thermodynamically stable phases. However, thin films prepared by condensing ablated or evaporated vapor onto a substrate can help bypass the need for extreme reaction environments and thus offer several advantages when preparing metastable materials. This is because such synthesis affords several difficult-to-achieve features including atomically disperse mixing of reagents, designed diffusion profiles, and altered thermodynamic landscapes during growth. Part results, part review, and part perspective, this talk will use brief vignettes to examine several approaches to metastable thin film materials synthesis. Some reflection on the advantages and disadvantages of thin film systems from a characterization perspective will also be provided.