The Nowotny chimney ladders (NCL) binary intermetallic compounds have recently attracted particular interest due to their anomalous glasslike thermal conductivity and the partial breakdown of conventional phonon gas behavior for potential thermoelectric applications. A complete understanding of the underlying unusual heat transport mechanisms of these materials is key to further improving their thermoelectric efficiency. In this work, we present a comprehensive study of thermal conductivity in the typical NCL semiconductor, ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$, based on first-principles molecular dynamics calculations and the Wigner thermal transport model. Our investigation uncovers that anticrossings and twisting modes in ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$ drive the remarkably low and temperature-independent lattice thermal conductivity. The presence of ultra-low-energy optical phonons, anharmonicity, low group velocity, and pronounced anisotropy in thermal conductivity distinguish ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$ from traditional thermoelectric materials. Furthermore, our analysis reveals that the inclusion of lattice defects, such as grain boundaries, enables the predicted thermal transport to align well with experimental observations. Expanding our findings to lower thermal conductivity NCL materials like ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$, we observe similar physical effects and outcomes, such as twisted phonons, anticrossings, and coherent contributions, which contribute to glasslike thermal transport behaviors. This highlights the broad applicability of our findings and the inherent similarities in thermal transport mechanisms across various NCL compounds. Our study challenges traditional understandings of the correlation between structural indicators $\delta$ and thermal conductivity in NCL materials, providing a fresh perspective on this intricate interplay. Our work contributes to a deeper understanding of heat transport in NCL intermetallic compounds, paving the way for advancements in thermoelectric materials with enhanced performance.

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• Anharmonic lattice dynamics
• Heat transfer
• Lattice dynamics
• Lattice thermal conductivity
• 3-dimensional systems
• Thermoelectrics
• Ab initio molecular dynamics
• Boltzmann theory
• Density functional theory
• First-principles calculations