Lone-Pair-Induced Lattice Softness Enables Ultralow Thermal Conductivity in Hybrid Organic-Inorganic Perovskite GuaPbI$_3$

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Original abstract

Thermoelectric cooling efficiency is fundamentally constrained by lattice thermal conductivity, yet conventional inorganic thermoelectrics have approached a performance plateau despite extensive nanostructural engineering. Organic thermoelectrics possess intrinsically low thermal conductivity but often suffer from limited and morphology-sensitive charge transport. Here, we introduce a lone-pair-driven materials design strategy based on chemically induced lattice softness in hybrid organic-inorganic perovskites. A physics-guided symbolic-regression-based machine-learning framework identifies a lone-pair-dominated compositional regime associated with suppressed lattice thermal conductivity and selects GuaPbI3 as a candidate material. Mechanochemical synthesis yields crystalline GuaPbI3 with an ultralow room-temperature thermal conductivity of kappa = 0.088 W m^-1 K^-1. Electrical measurements reveal electronically active, bias-dependent bulk conduction pathways despite strong phonon suppression, while impedance spectroscopy confirms bulk-dominated transport. Density functional theory calculations indicate weakly dispersive valence bands, valence-conduction asymmetry, and localized electrostatic microenvironments from charge redistribution within the lattice. Calculated transport coefficients suggest strong sensitivity of carrier transport to chemical potential, while Lorenz-number analysis indicates deviations from conventional Wiedemann-Franz behavior near the band edges. These results support a picture in which lone-pair-rich hybrid frameworks generate soft and electronically heterogeneous lattice environments that suppress phonon transport while preserving electronically accessible states. This work establishes chemically induced lattice softness as a design principle for ultralow-thermal-conductivity hybrid materials.

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