Neutron scattering studies of complex lattice dynamics in energy materials
Authors: REN Qingyong , WANG Jianli , LI Bing , MA Jie , TONG Xin ,   Spallation Neutron Source Science Center, Dongguan 523803, China,   Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China,   Guangdong Provincial Key Laboratory of Extreme Conditions, Dongguan 523803, China,   Center for Neutron Scattering and Advanced Light Sources, Dongguan University of Technology, Dongguan 523000, China,   Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China,   School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
Published: November 2024

<sec>Lattice dynamics play a crucial role in understanding the physical mechanisms of cutting-edge energy materials. Many excellent energy materials have complex multiple-sublattice structures, with intricate lattice dynamics, and the underlying mechanisms are difficult to understand. Neutron scattering technologies, which are known for their high energy and momentum resolution, are powerful tools for simultaneously characterizing material structure and complex lattice dynamics. In recent years, neutron scattering techniques have made significant contributions to the study of energy materials, shedding light on their physical mechanisms. Starting from the basic properties of neutrons and double differential scattering cross sections, this review paper provides a detailed introduction to the working principles, spectrometer structures, and functions of several neutron scattering techniques commonly used in energy materials research, including neutron diffraction and neutron total scattering, which characterize material structures, and quasi-elastic neutron scattering and inelastic neutron scattering, which characterize lattice dynamics. Then, this review paper presents significant research progress in the field of energy materials utilizing neutron scattering as a primary characterization method.</sec><sec>1) In the case of Ag<sub>8</sub>SnSe<sub>6</sub> superionic thermoelectric materials, single crystal inelastic neutron scattering experiments have revealed that the “liquid-like phonon model” is not the primary contributor to ultra-low lattice thermal conductivity. Instead, extreme phonon anharmonic scattering is identified as a key factor based on the special temperature dependence of phonon linewidth.</sec><sec>2) Analysis of quasi-elastic and inelastic neutron scattering spectra reveals the changes in the correlation between framework and Ag<sup>+</sup> sublattices during the superionic phase transition of Ag<sub>8</sub>SnSe<sub>6</sub> compounds. Further investigations using neutron diffraction and molecular dynamics simulations reveal a new mechanism of superionic phase transition and ion diffusion, primarily governed by weakly bonded Se atoms.</sec><sec>3) Research on NH<sub>4</sub>I compounds demonstrates a strong coupling between molecular orientation rotation and lattice vibration, and the strengthening of phonon anharmonicity with temperature rising can decouple this interaction and induce plastic phase transition. This phenomenon results in a significant configuration entropy change, showing its potential applications in barocaloric refrigeration.</sec><sec>4) In the CsPbBr<sub>3</sub> perovskite photovoltaic materials, inelastic neutron scattering uncovers low-energy phonon damping of the [PbBr<sub>6</sub>] sublattice, influencing electron-phonon coupling and the band edge electronic state. This special anharmonic vibration of the [PbBr<sub>6</sub>] sublattice prolongs the lifetime of hot carriers, affecting the material's electronic properties.</sec><sec>5) In MnCoGe magnetic refrigeration materials, <i>in-situ</i> neutron diffraction experiments highlight the role of valence electron transfer between sublattices in changing crystal structural stability and magnetic interactions. This process triggers a transformation from a ferromagnetic to an incommensurate spiral antiferromagnetic structure, expanding our understanding of magnetic phase transition regulation.</sec><sec>These examples underscore the interdependence between lattice dynamics and other degrees of freedom in energy conversion and storage materials, such as sublattices, charge, and spin. Through these typical examples, this review paper can provide a reference for further exploring and understanding the energy materials and lattice dynamics.</sec>