Phase transitions are central in much of condensed matter physics and materials science, and the phonon entropy can sometimes play a big role in structural phase transitions. Lattice instabilities impact functional properties in many materials, including ferroelectrics, thermoelectrics, phase-change materials, and superionic conductors. In many materials, the phonon thermodynamics is captured satisfactorily within quasiharmonic approximations. Such approximations, however, can fail spectacularly near soft-mode driven lattice instabilities, owing to strong anharmonic phonon-phonon interactions. Using a combination of experimental scattering techniques (neutrons, x-rays, optical spectroscopy) and first-principles computer simulations, these strong interactions can now be carefully probed and quantitatively rationalized, paving the way for the design of improved materials. I will review some of our recent studies investigating phonon anharmonicity near lattice instabilities in connection with thermal transport in thermoelectrics, and phase stability in ferroelectrics and related oxides.
Funding from US-DOE/BES/MSED, Office of Science Early-Career-Award, and as part of the S3TEC-EFRC.