Record-Low and Anisotropic Thermal Conductivity of a Quasi-One-Dimensional Bulk ZrTe₅ Single Crystal

Abstract

Zirconium pentatelluride (ZrTe 5 ) has recently attracted renewed interest owing to many of its newly discovered extraordinary physical properties, such as 2D and 3D topological-insulator behavior, pressure-induced superconductivity, Weyl semimetal behavior, Zeeman splitting, and resistivity anomaly. The quasi-one-dimensional structure of single-crystal ZrTe 5 also promises large anisotropy in its thermal properties, which have not yet been studied. In this work, via timedomain thermoreflectance measurements, ZrTe 5 single crystals are discovered to possess a record-low thermal conductivity along the b-axis (through-plane), as small as 0.33 ± 0.03 W m −1 K −1 at room temperature. This ultralow b-axis thermal conductivity is 12 times smaller than its a-axis thermal conductivity (4 ± 1 W m −1 K −1 ) owing to the material's asymmetrical crystalline structure. First-principles calculations are further conducted to reveal the physical origins of the ultralow b-axis thermal conductivity, which can be attributed to: (1) the resonant bonding and strong lattice anharmonicity induced by electron lone pairs, (2) the weak interlayer van der Waals interactions, and (3) the heavy mass of Te atoms, which results in low phonon group velocity. This work sheds light on the design and engineering of high-efficiency thermal insulators for applications such as thermal barrier coatings, thermoelectrics, thermal energy storage, and thermal management.

Thermal Conductivity of HfTe5: A Critical Revisit

“…Figure shows the representative TDTR signals and fitting curves at three modulation frequencies. Surprisingly, we find that the through‐plane (along the b ‐axis) thermal conductivity of HfTe 5 is 0.41 ± 0.04 W m −1 K −1 , which is ≈25% larger than 0.33 ± 0.03 W m −1 K −1 for ZrTe 5 . This result is in sharp contrast with the sole prior data sole theory for the two materials, both of which found much larger average values, with the ZrTe 5 values about 40–50 times larger.…”

“…However, the calculated directionally resolved values revealed only a small degree of anisotropy, 1.65:1.16:1. Both the large values and the virtual absence of anisotropy are surprising in view of the much smaller and highly anisotropic experimental and theoretical values recently reported for ZrTe 5 . We note, however, that the calculations of ref.…”

“…Prior studies of the thermal transport of vdW crystals including multilayer graphene, graphite, and Bi 2 Te 3 have shown the standard DFT could agree well (or even better) with experimental data, when the vdW functional was excluded. Thus, the recently reported excellent agreement between theory and experimental data for the thermal conductivities of ZrTe 5 , for which the DFT calculations eschewed vdW interactions, leaves the question of the thermal conductivities of HfTe 5 wide open.…”

“…Recently, interest in ZrTe 5 and HfTe 5 was boosted by the discovery of extraordinary topological properties and the promise of potential thermoelectric applications . Even more recently, rather exotic thermal‐transport properties were found in single‐crystalline ZrTe 5 . The through‐plane lattice thermal conductivity κ L was found to be 0.33 W m −1 K −1 at room temperature, which is lower than the lowest reported room‐temperature κ L value among bulk crystalline thermoelectric materials (0.47 W m −1 K −1 for SnSe).…”

“…On the theoretical side, we employ DFT calculations without vdW interactions, as in ref. , with solutions of the phonon Boltzmann transport equation (BTE) beyond the relaxation‐time approximation. We also examine the possible effect of four‐phonon scattering, as extensively discussed recently .…”