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 BaBiO. The measured κ(T) shows an anomalous glass-like behavior with a ≈ T 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 K 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 BiO 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-BaBiO aside from suggesting it as a candidate building block for functional heterostructures.