Introduction
Our research is driven by outstanding questions in microscopic dynamics at the nano-scale, revealing novel properties of materials for improved energy conversion. We investigate key aspects of transport properties and thermodynamics in important energy materials, including thermoelectrics, solid-state battery materials, photovoltaics, ferroelectrics/multiferroics, and superconductors. To this end, we use state-of-the-art experimental techniques ranging from neutron and x-ray scattering at large-scale facilities in the National Laboratories, as well as optical spectroscopy, and thermodynamic and transport measurements. In parallel, we perform first-principles simulations leveraging large-scale computing to identify the fundamental origins of the effects observed in our measurements. We also develop advanced modeling techniques based on machine learning/artificial intelligence, statistical analysis and supercomputing. This combined experimental and computational approach enables us to gain deeper understanding and ultimately to improve the performance of energy materials.
Phonons and thermal transport in thermoelectrics
Artist rendering of phonon flow in thermoelectric compound Mg3Sb2 (credits: Jill Hemman).
Understanding the microscopic processes involved in the transport and conversion of energy from the atomic scale to the mesoscale is key for the development of next-generation materials for energy sustainability. For example, the interplay between phonons (atomic vibrations) and charge transport is crucial in thermoelectrics. This enables practical devices for “waste-heat harvesting” and heat pumping. At a microscopic level, such couplings result from interactions of phonons with other phonons (phonon-phonon interaction from anharmonicity), or with charge carriers (electron-phonon interaction). In particular, it is key to probe and control phonon scattering mechanisms in thermoelectrics, in order improve the thermoelectric conversion efficiency. The novel approaches we develop provide hitherto unavailable information about mode-resolved phonon scattering rates, leading to a microscopic understanding of thermal transport and guiding rational material design.
We probe phonon dispersions and phonon scattering processes in thermoelectric materials using state-of-the-art inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS) techniques. To rationalize these measurements, we perform first-principles modeling such as density functional theory (DFT) and ab-initio molecular dynamics (AIMD). Our simulations are extended to large-scales (space and time) using machine-learning techniques to develop surrogate interatomic potentials. This combination of experimental and computational approaches provides us unique insights into the microscopic processes of thermal transport.
Atomic structure and dynamics in halide perovskite photovoltaics
Artist rendering of the way PbBr6 octahedra dynamically tilt around hinge-like bromine atoms (red) in CsPbBr3. Image credits: Jill Hemman/ORNL.
Lead halide perovskites (LHPs) are attracting intense interest for their exceptional optoelectronic performance and potential thermoelectric applications. Yet, the origins of their long photocarrier lifetimes, defect tolerance, and ultralow thermal conductivity remain poorly understood and controversial. LHPs are unusually soft and anharmonic materials, and have been characterized as nearly liquid-like or “phonon glasses”, as reflected in their ultralow thermal conductivities, huge thermal expansion coefficients and large atomic displacement parameters. These unique characteristics are thought to impact their electronic and thermal transport properties, but their lattice dynamics have remained puzzling.
The floppy nature of the crystalline lattice has been invoked in both hybrid organic-inorganic perovskites (HOIPs) and all-inorganic variants to account for their long-carrier lifetimes, by enabling large polaron formation, screening defects, and disrupting non-radiative carrier recombination. Yet, the relative importance of the molecular cation dynamics in HOIPs or that of the inorganic lead-halide sublattice remains controversial. It has been pointed out in recent years that the all-inorganic LHPs such as CsPbBr3 exhibit similar behaviors and can achieve excellent optoelectronic properties and enable synthesis routes yielding lower defect concentrations (e.g. for radiation detectors). Recently, theoretical studies pointed out the importance of large-amplitude anharmonic motions on the electron-phonon coupling and optical properties in a wide range of halide perovskites.
Our group uses a state-of-the-art reciprocal-space tomography approach of time-of-flight inelastic neutron scattering (INS) on single-crystals to map out atomic dynamics, including both phonons and quasielastic dynamics. Our results provide full 4D mapping of phonon excitations across comprehensive volumes of (Q,E) reciprocal space. In addition, we map diffuse scattering signals with high Q-resolution using both high-energy x-rays and neutrons to reveal deviations from the average crystalline periodicity (e.g. short range order). The results are rationalized based on our group's atomistic simulations using both first-principles quantum mechanical approaches, and surrogate potentials derived from machine learning.
Atomic dynamics and fast ion diffusion in superionics for rechargeable batteries
An artistic rendition of the intriguing superionic crystalline structure of CuCrSe2, which has copper ions that move like liquid between solid layers of chromium and selenium, giving rise to useful electrical properties. Image credits: Jill Hemman, Oak Ridge National Laboratory
The current push for safer rechargeable batteries with higher power densities is driving a search for all-solid battery designs leveraging solid-state electrolytes. Solid electrolytes with superionic diffusion constants exhibit complex atomic dynamics, featuring simultaneous facile hopping of mobile species and soft lattice dynamics of the host lattice. Importantly, fast diffusion involves large-amplitude motions on strongly anharmonic potential energy surfaces, and such anharmonic effects need to be assessed when investigating phonons and their coupling with the stochastic hopping underlying the diffusion process. Thus, more detailed studies of atomic dynamics connecting mobile ion diffusion and host lattice dynamics are needed.
Our group performs investigations of atomic dynamics in superionic materials for both solid-state electrolytes (rechargeable batteries) and thermoelectric applications, considering on an equal footing the diffusive dynamics of mobile ions and the anharmonic phonons of the overall crystal structure, in order to identify their interactions. We use a combination of inelastic neutron scattering (INS) and quasi-elastic neutron scattering (QENS) experiments to obtain a full picture of the atomic dynamics. To rationalize our measurements and gain atomistic insights, we further perform ab-initio molecular dynamics (AIMD) simulations, and extend these simulations to long time scales using novel machine-learning techniques to access long simulations times (hundreds of picoseconds to nanoseconds). Our results offer detailed microscopic insights into the dynamic mechanism of fast ionic diffusion and provide an avenue to search for further solid electrolytes for next-generation rechargeable batteries, and to understand and control their thermal properties.
Coupling of spin, charge and lattice degrees of freedom in multiferroics and metal-insulator transitions
Understanding the interplay of spin, phonon, and charge degrees-of-freedom is critical to control a wide array of materials and condensed matter systems. These interactions come into sharp focus when considering metal-insulator transitions (MIT) and magnetoelectric materials. Systems with phase transitions intimately involving all these degrees-of-freedom are especially fascinating, and potentially enable greater tunability and external control. Yet, they also open important questions as to which of the spin or lattice channels drives the MIT, and whether concurrency of magnetic and structural transitions is a coincidence or a requirement.
In the candidate magneto-electric material h-FeS ('troilite'), we determined the behaviors and interactions of the lattice, magnetic, and electronic degrees-of-freedom over a wide temperature range across the MIT. We used a combination of state-of-the-art neutron and x-ray scattering measurements, together with first-principles simulations and thermodynamic analysis, to determine the sequence of microscopic couplings triggering the MIT, revealing a new and important connection between magnetic ordering and phonon instabilities. The antiferromagnetic (AFM) ordering enables the emergence of two zone-boundary soft phonons, whose condensation couples to a zone-center mode, with the resulting lattice distortion opening the electronic band gap. Simultaneously, spin-lattice coupling opens a gap in the magnon spectrum that impacts the entropy and thermodynamics of the MIT. The results were published in Nature Physics (2020).
Measurements of phonon dispersions with inelastic x-ray scattering (a-e) in hexagonal iron-sulfide (h-FeS), revealing a zone-boudary instability that couples to the structural distortion and electronic band-gap opening (metal-insulator transition). The anharmonic phonon instability is reproduced in our first-principles simulations of the dynamical structure factor (grey background intensity map in (b-e). (f) photograph of small crystal used in inelastic x-ray scattering experiments. (g) First-principles calculation of the potential energy surface spanned by H1 and K5 phonon modes.
Lattice instabilities and anharmonic phonons in ferroelectrics
The phonon-phonon interaction is also at the heart of soft-mode lattice distortions central to ferroelectrics.