Phase transitions, structural fluctuations and temperature dependent thermal conductivities of semiconductors from phonon anharmonicity

Abstract

Anharmonicity of many material behaviors including thermal transport properties, ferroelectricity, multiferroicity, superconductivity or soft-mode driven structural phase transitions arise from the couplings between microscopic degrees-of-freedom, such as phonons,spins and electrons. Understanding these microscopic processes in the transport and conversion of energy from atomic scale is of vital importance for development and improvement of next-generation materials for energy sustainability. While harmonic phonon picture provides insight into the basic understanding and modeling of atomic vibration, the anharmonicity is indeed essential to explain material behaviors including thermal expansion and structural phase transitions that cannot be thoroughly described by harmonic phonon theory. Therefore, it is important to better understand the anharmonicity of phonons and their coupling to other degrees-of-freedom in order to realize the controllable design and property improvement of energy materials characterized as halide perovskites. Recent progress has been made both experimentally and theoretically to probe and rationalize the microscopic processes that determine the phonon scattering rates and phonon lifetime of different solid materials. In particular, inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS) provide direct measurements of the phonon dispersions, scattering rate and dynamical structure factor S(Q,E). Developments in first-principles simulations have also enabled the computation of phonon scattering processes and provide detailed accounts of anharmonic scattering channels that is crucial in understanding material properties. Combining these experimental and theoretical methods, the goal of this research is to systematically explore the influence of the anharmonic phonons to solid state material properties related to energy transport. As a prototypical system with strong temperature-dependent anharmonicity, elemental bismuth has long served as a platform for exploring phonon-phonon and electron-phonon interactions. Prior studies have demonstrated strong softening of interatomic potentials in photoexcited Bi, raising interest in how intrinsic anharmonic phonon scattering evolves with temperature. In this work, we use INS and first-principles simulations to probe thetemperature dependence of acoustic phonon energies and lifetimes in Bi, shedding light on the microscopic mechanisms governing its lattice thermal conductivity. In parallel, we explore the anharmonic lattice dynamics in inorganic halide perovskites, including CsMX3 (M = Ge, Sn, Pb) and the double perovskite Cs2AgBiBr6, which have shown excellent performance in photovoltaic and radiation detector applications. These materials are characterized by soft lattices, low thermal conductivities, and large thermal expansion—features that arise from strong phonon anharmonicity. Phase transitions in these systems are typically associated with octahedral tilts and rotations driven by zoneboundary soft modes. Recent studies suggest that such structural fluctuations can couple to the electronic structure, facilitating exciton dissociation and enhancing carrier lifetimes. Using INS, X-ray diffuse scattering, and molecular dynamics simulations, we identify 2D overdamped diffuse rods in the high-temperature phases of CsSnBr 3 and CsPbBr3, highlighting the universal nature of these dynamic instabilities across halide perovskites. Extending this investigation to hybrid and double perovskite systems, we focus on methylammonium lead iodide (MAPbI3, or MAPI) and the inorganic double perovskite Cs2AgBiBr6, both of which exhibit soft lattice dynamics and intriguing structural phase behaviors. In Cs2AgBiBr6, we identify an incommensurately modulated ground state that emerges upon cooling from the tetragonal phase, as revealed by single-crystal neutron diffraction. At higher temperatures, quasi-elastic diffuse rods associated with anharmonic octahedral rotations are observed in both the cubic and tetragonal phases, similar to those found in other halide perovskites. In MAPI, dynamic structural fluctuations are also evident in the form of diffuse rods in the cubic phase, which vanish abruptly below the cubic-totetragonal transition. These measurements were made possible by combining inelastic neutron scattering, diffuse X-ray scattering, polarized neutron techniques, and first-principles simulations. Together, our results reveal distinct temperature-dependent lattice dynamics in both systems and offer new insight into the role of phonon anharmonicity and structural fluctuations in shaping the electronic and thermal behavior of halide perovskites.