Crystals with Ultralow Thermal Conductivity*

In this work, we show the phonon dispersion of (CH₃NH₃)₃Bi₂I₉ single crystals at 300 K measured by inelastic x-ray scattering. The frequencies of acoustic phonons are among the lowest of crystals. Nanoindentation measurements verified that these crystals are very compliant and considerably soft. The frequency overlap between acoustic and optical phonons results in strong acoustic-optical scattering. All these features lead to an ultralow thermal conductivity, as validated by the laser flash method. The fundamental knowledge obtained from this study will accelerate the design of novel hybrid materials for energy applications. This work has been published in Phys. Rev. Lett. 123, 155901 (2019).