Anomalous lattice thermal conductivity of quasi-one-dimensional palladium thiophosphate A2PdPS4I (A = K, Rb, Cs)

The quest for intrinsically low lattice thermal conductivity (KL) has gained widespread attention in thermoelectrics and refractories. The reduction of KL in low-dimensional systems typically arises from anharmonicity and anisotropic chemical bonding. Due to the structural instability and the technical complexity, the thermal functional application of practical low-dimensional materials confronts great challenges. In this context, the investigations of quasi-low-dimensional bulk materials, rather than the real low-dimensional materials, hold paramount importance from both fundamental and practical perspectives. Here, we employ the abinitio self-consistent phonon (SCPH) theory in combination with the compressive sensing lattice dynamics method to explore the lattice dynamics and microscopic mechanisms of thermal transport in emerging pentanary thiophosphates A2PdPS4I (A = K, Rb, Cs). These compounds feature covalent one-dimensional (1D) chains, providing bonding heterogeneity and resulting in flat phonon bands and low phonon group velocities (Vph). The strengthened anharmonicity stems from the cooperative rattlinglike vibrations of the interchain alkali metal atoms and I atoms. In K- and Rb-containing systems, the cooperative vibrations of the rattling unit (K/Rb-I) result in strong anharmonicity, while in Cs-containing system, the abnormally enhanced particlelike KL (Kp) is due to the antibonding interaction breaking the cooperative vibration of the rattling unit (Cs-I), thereby weakening the anharmonicity. Upon heating, the Kp show decreasing behavior. However, the wavelike KL (Kc) exhibits a positive temperature dependence, highlighting the nonnegligible coherent phonon contributions to the KL and triggering anomalous temperature-dependent behavior of KL. Our study uncovers the multifaceted microscopic mechanisms behind the low KL in three quasi-one-dimensional (Q-1D) systems of A2PdPS4I and provides fresh insights for the discovery and design of Q-1D low KL materials.