Revisiting the Lower Limit of Crystal Structure Thermal Conductivity*

Phonon picture for thermal transport fails in disordered/amorphous materials as atomic vibrations are thought to carry heat by random walk among uncorrelated Einstein oscillators. Crystalline materials with low thermal conductivity (κ) are often compared with models of this ‘amorphous limit (κ_min)’. Given the periodicity in bulk crystals, κ values are usually much higher than κ_min. Here we identify a moderately simple crystal, Tl₃VSe₄, with calculated κ at 300K lower than κ_min and unusually, lower than measured κ values: 0.16/0.30 W/m-K (calculated/measured). Phonon lifetimes (calculated/measured) are so short that they may be considered “ill-defined” by typical definitions. However, both measured Raman structure and temperature dependent κ show typical phonon-characteristics. We find that measured κ is well-explained when considering two independent conduction channels from (1) typical phonons and (2) uncorrelated Einstein oscillators, suggesting that a measure of κ_min may be more accurate when considering both. This work draws into question the criteria that distinguish crystalline from amorphous vibrational behaviors.