Unprecedented mechanical response of the lattice thermal conductivity of auxetic carbon crystals

Lattice thermal conductivity (κ) of bulk materials usually increases under compression and decreases under tension, while there are still some unusual systems, exhibiting reduced κ when compressed. However, to date it has never been reported for a bulk material, whose κ is substantially enhanced under tensile strain. In this paper, we have studied thermal transport of three auxetic carbon crystals: cis-C, trans-C and hin-C for short, and their strain responses by performing first-principles calculations. It is intriguing to find that their κ are much lower than those of their allotropes, and further decrease abnormally under compression. More strikingly, κ of trans-C (cis-C) anomalously increases with tensile strain up to 7% (6%) with maximum κ of almost 7 (5) times larger than the unstrained value. The abnormal strain dependent κ are attributed to the dominant role of the enhancement of phonon lifetime under stretching, which can be further explained from the unique atomic structure of the main chain of polydiacetylene in trans-C and cis-C. The weakening of phonon anharmonicity is reflected by the enhancement of root mean-square displacement values. The reported giant augmentation of κ may inspire intensive research on auxetic carbon crystals as potential materials for emerging nanoelectronic devices.

Despite mechanical strain has been widely used to engineer the lattice thermal conductivity (κ) of materials, only three (out of four) possibilities of κ vs. strain have been found. Herewith, we identified the fourth (and the last) possibility in auxetic carbon crystals, namely cis-, trans-hinged polydiacetylene, and hinged polyacetylene (cis-C, trans-C and hin-C). Unexpectedly, κ of trans-C (cis-C) increases with tensile strain up to 7% (6%) with maximum κ of 7 (5) times larger than the unstrained value. The unusual strain dependent κ is attributed to the unique atomic structure of the main chain of polydiacetylene in cis-C and trans-C.216