Amorphous-like ultralow thermal conductivity in crystalline Ruddledsen-Popper phases*

Achieving ultralow thermal conductivity (κ) is a pivotal goal in materials science, requiring extensive explorations at the atomic scale. Most ultralow κ have been found in amorphous, van-der-Waals layered, and organic-inorganic composite materials, which sacrifice mechanical strength. Here we report a combined experimental and theoretical investigation of BaZrS₃ and its Ruddlesden-popper (RP) phases, Ba₃Zr₂S₇ and Ba₄Zr₃S₁₀. These phases have superlattice-like structures, featuring half-unit-cell shifts between atomic layers, forming rock-salt-like blocks with interfacial “gaps”. Atomic-resolution scanning-transmission-electron-microscope (STEM) images and density-functional-theory (DFT) results, however, find chemical bonding across the gaps. Time-domain thermoreflectance measurements and DFT-based machine-learning simulations unveil ultralow κ values, comparable to Agne’s diffuson limit. The ratio of Young’s modulus and κ for Ba₃Zr₂S₇ achieves a record-setting high because the rock-salt-like interfaces induce strong anharmonicity and a substantial fraction of localized, low-velocity phonon modes. This work offers profound insights into materials with mechanical strength and ultralow thermal conductivities.