Glass-like anomalies and unconventional thermoelectric transport in chimney ladder crystals

Nowotny chimney ladder crystals combine features of ordered crystals and amorphous solids, making them attractive thermoelectric materials because of their intrinsically low thermal conductivity. We investigate the intermetallic compound Ru₂Sn₃ and show that, despite its crystalline order, its heat capacity exhibits a boson-peak-like glassy anomaly at 8-14 K. Combining experiments with first-principles calculations and molecular dynamics simulations, we trace this behavior to low-energy optical phonons emerging from the chimney ladder structure. These modes strongly couple to acoustic phonons, producing hybridization and avoided crossings that reshape the vibrational spectrum and cause the hybridized acoustic branches to contribute directly to the anomaly. Thermoelectric measurements reveal additional glass-like signatures linked to these excitations, while the electrical resistivity displays an extended linear temperature dependence and an anomalously large quadratic contribution at low temperatures. A simple theoretical model based on electron scattering by overdamped phonons qualitatively accounts for these observations.