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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^-1K^-1along the in‐planea‐axis, through‐planeb‐axis, and in‐planec‐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 HfTe₅and 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.

Ultrafast time‐domain thermoreflectance (TDTR) is utilized to extract the through‐plane thermal conductivity (Λ_LSCO) of epitaxial La_0.5Sr_0.5CoO_3−δ(LSCO) of varying thickness (<20 nm) on LaAlO₃and SrTiO₃substrates. These LSCO films possess ordered oxygen vacancies as the primary means of lattice mismatch accommodation with the substrate, which induces compressive/tensile strain and thus controls the orientation of the oxygen vacancy ordering (OVO). TDTR results demonstrate that the room‐temperatureΛ_LSCOof LSCO on both substrates (1.7 W m⁻¹K⁻¹) are nearly a factor of four lower than that of bulk single‐crystal LSCO (6.2 W m⁻¹K⁻¹). Remarkably, this approaches the lower limit of amorphous oxides (e.g., 1.3 W m⁻¹K⁻¹for glass), with no dependence on the OVO orientation. Through theoretical simulations, origins of the glass‐like thermal conductivity of LSCO are revealed as a combined effect resulting from oxygen vacancies (the dominant factor), Sr substitution, size effects, and the weak electron/phonon coupling within the LSCO film. The absence of OVO dependence in the measuredΛ_LSCOis rationalized by two main effects: (1) the nearly isotropic phononic thermal conductivity resulting from the imperfect OVO planes when δ is small; (2) the missing electronic contribution toΛ_LSCOalong the through‐plane direction for these ultrathin LSCO films on insulating substrates.