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Recently, superconductivity at high temperatures is observed in bulk La₃Ni₂O_7−δunder high pressure. However, the attainment of high‐purity La₃Ni₂O_7−δsingle crystals remains a formidable challenge. Here, the crystal structure and physical properties of single crystals of Sr‐doped La₃Ni₂O₇synthesized at high pressure (20 GPa) and high temperature (1400 °C) are reported. Through single crystal X‐ray diffraction, it is shown that high‐pressure‐synthesized paramagnetic Sr‐doped La₃Ni₂O₇crystallizes in an orthorhombic structure with Ni─O─Ni bond angles of 173.4(2)° out‐of‐plane and 175.0(2)°and 176.7(2)°in plane. The substitution of Sr alters in band filling and the ratio of Ni²⁺/Ni³⁺in Sr‐doped La₃Ni₂O₇, aligning them with those of “La₃Ni₂O_7.05”, thereby leading to significant modifications in properties under high pressure relative to the unsubstituted parent phase. At ambient pressure, Sr‐doped La₃Ni₂O₇exhibits insulating properties, and the conductivity increases as pressure goes up to 10 GPa. However, upon further increasing pressure beyond 10.7 GPa, Sr‐doped La₃Ni₂O₇transits back from a metal‐like behavior to an insulator. The insulator–metal–insulator trend under high pressure dramatically differs from the behavior of the parent compound La₃Ni₂O_7−δ, despite their similar behavior in the low‐pressure regime. These experimental results underscore the considerable challenge in achieving superconductivity in nickelates.

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.