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Hafnium pentatelluride (HfTe5) has attracted extensive interest due to its exotic electronic, optical, and thermal properties. As a highly anisotropic crystal (layered structure with in-plane chains), it has highly anisotropic electrical-transport properties, but the anisotropy of its thermal-transport properties has not been established. Here, accurate experimental measurements and theoretical calculations are combined to resolve this issue. Time-domain thermoreflectance measurements find a highly anisotropic thermal conductivity, 28:1:8, with values of 11.3 ± 2.2, 0.41 ± 0.04, and 3.2 ± 2.0 W m-1 K-1 along the in-plane a-axis, through-plane b-axis, and in-plane c-axis, respectively. This anisotropy is even larger than what was recently established for ZrTe5 (12:1:6), but the individual values are somewhat higher, even though Zr has a smaller atomic mass than Hf. Density-functional-theory calculations predict thermal conductivities in good agreement with the experimental data, provide comprehensive insights into the results, and reveal the origin of the apparent anomaly of the relative thermal conductivities of the two pentatellurides. These results establish that HfTe5 and ZrTe5, and by implication their alloys, have highly anisotropic and ultralow through-plane thermal conductivities, which can provide guidance for the design of materials for new directional-heat-management applications and potentially other thermal functionalities.
Bibliographical noteFunding Information:
T.F. and X.W. contributed equally to this work. Theoretical work at Vanderbilt University was supported by the Department of Energy grant DE-FG0209ER46554 and by the McMinn Endowment. Computations were performed at the National Energy Research Scientific Computing Center (NERSC), a Department of Energy, Office of Science, User Facility funded through Contract No. DE-AC02-05CH11231. Computations also used the Extreme Science and Engineering Discovery Environment (XSEDE). TDTR measurements were supported by the National Science Foundation (NSF award No. 1804840) and partially by the Institute on the Environment. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which is partially supported by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013. Sample preparation was conducted in the Minnesota Nano Center, which is supported by the NSF through the National Nano Coordinated Infrastructure Network (NNCI No. ECCS-1542202). Sample synthesis was conducted by P.P.W., L.Y.Z., and X.D. with the supports from Guangdong Innovative and Entrepreneurial Research Team Program (No.2016ZT06D348), NFSC (11874193), and the Shenzhen Fundamental subject research Program (JCYJ20170817110751776) and (JCYJ20170307105434022).
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- thermal conductivities
- time-domain thermoreflectance
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- Period 6