Anomalous Glassy Thermal Conductivity in a Perovskite Bismuthate Induced by Structural Dynamic Instability

Unraveling the underlying mechanisms at the origin of ultra-low thermal conductivity in pristine crystals is of central relevance to designing new functional materials. Here, a study of thermal conductivity κ(T) in an extended temperature range (1.5-400 K) is reported in the single-crystalline bismuthate perovskite BaBiO₃. The measured κ(T) shows an anomalous glass-like behavior with a ≈ T² dependence at low temperatures and a plateau in a wide temperature range from about 20 to 260 K, surprisingly recovering the expected downturn for a crystal only for T > 300 K. The measured room temperature κ ≈ 1.6 W m^-1 K^-1 agrees with the calculated value, including three-phonon and isotope disorder scattering. However, the departure of the experimental from the calculated κ(T) for T < 300 K, at the onset of the plateau, indicates that an additional scattering mechanism comes into play at lower temperatures. It is proposed that a tunneling two-level system associated with the BiO₆ octahedra rotation offers an additional phonon scattering mechanism and may explain the observed κ(T) suppression. The findings may have significant implications for the pairing mechanism and unconventional superconductivity of doped-BaBiO₃ aside from suggesting it as a candidate building block for functional heterostructures.