Symmetry-Breaking Amplifies Lone Pair Expression and Orbital Splitting for Promising Chain-Like Thermoelectrics

The relative impact of electronic instability and quantum confinement on lattice dynamics and transport properties remains elusive. Here, we demonstrate that the influence of lone-pair electrons (LPEs) expression on phonon dissipation surpasses that of dimensional reduction using a quasi-low-dimensional Pb-Sn-S-Se system as a prototype. We experimentally observe that quasi-one-dimensional (1D) PbSnS₃ exhibits higher thermal transport than quasi-two-dimensional (2D) PbSnS₂, attributed to the loss of LPEs. Therefore, we attempt a symmetry-breaking strategy to amplify the expression of LPEs in 1D-Pb₂Sn₂S₅Se. Using first-principles calculations and Boltzmann transport theory, we reveal that amplified LPEs expression in 1D-systems induces more diffuson-like vibrations, as well as enhanced Umklapp and Normal processes. Moreover, phase transition and thermal transport analysis in 2D-PbSnS₂ reveals a stronger correlation between LPEs expression and phonon dissipation compared to the 1D-systems, regardless of variations on structure symmetry and dimension. Apart from the LPEs expression amplification, the symmetry-breaking strategy substantially facilitates orbital splitting in 1D-systems, leading to effective electron-phonon decoupling for promising thermoelectric efficiency. Consequently, our proposed strategy can be extended to optimize other quasi-low-dimensional systems containing LPEs, offering novel physical and chemical insights into the design of advanced thermoelectrics and thermal management materials.