Glassy Behavior and Anomalous Transport in Chimney Ladder Crystals Induced by Low-Lying Optical Phonons

Nowotny chimney ladder (NCL) crystals present physical properties in between the contrasting paradigms of ideal crystal and amorphous solid, making them promising candidates for thermoelectric applications due to their inherently low thermal conductivity. Here, we report an extensive experimental characterization of the thermodynamic and thermoelectric transport properties of a large class of NCL materials, focusing on the intermetallic compound Ru$_2$Sn$_{3}$. We show that the heat capacity of these NCL compounds exhibits a boson-peak-like glassy anomaly between $8$ K and $14$ K. By combining experimental measurements with density functional theory (DFT), we attribute its microscopic origin to extremely low-energy optical phonons that universally appear as a consequence of the chimney ladder sublattice structure. Additionally, the measured thermal conductivity and the thermoelectric response present distinct anomalous glass-like features that strongly correlate with the dynamics of the low-lying optical phonons revealed by DFT. Our work demonstrates that low-energy optical modes in ordered crystals can induce glassy behavior, outlining a pathway to design metallic materials with low thermal conductivity and unique thermoelectric properties without the need for disorder or strong electronic correlations.