One-Order Decrease of Thermal Conductivity in Nanostructured ZrTe5 and HfTe5 Crystals

Thermoelectric materials have been hotly explored for potential applications of environmentally friendly electrical generators and freon-free refrigerators. ZrTe5 and HfTe5 crystals have a superior power factor but relative large thermal conductivity. To decrease their thermal conductivity, we here grew ZrTe5 and HfTe5 with nanostructures by optimizing the growth parameters, with a growth mechanism that follows nucleation-then-nuclei-coalescence. Microstructure characterizations verify that nanostructured ZrTe5 and HfTe5 crystals have not only a layered microstructure but also nanostripe morphology. For comparison, we had grown compact ZrTe5 crystals by the flux method that mainly have a layered microstructure. Remarkably, the maximum thermal conductivity of nanostructured ZrTe5 and HfTe5 crystals can be suppressed to as small as 0.45 W m(-1) K-1, one order lower than the theoretical value and three times smaller than that of compact ZrTe5 and HfTe5. Theoretical cumulative thermal conductivities, to simulate the effect of boundary scattering in nanostructured ZrTe5, roughly agree with experimental results. Phenomenological thermal conductivity analysis verifies that the phonon meanfree paths of nanostructured ZrTe5 and HfTe5 reach the thickness of a monolayer (Ioffe-Regel criterion of phonon transport). Raman spectroscopy substantiates that the quasi-particle lifetime of optical phonons in nanostructured ZrTe5 and HfTe5 crystals is nearly four times larger than the phonon transport lifetime, which proves the pronounced effect of the microstructure on thermal conductivity from a spectroscopic viewpoint. Our work may provide an efficient method to modulate thermal conductivity by microstructure engineering through modified crystal growth.