Ineffectiveness of Formamidine in Suppressing Ultralow Thermal Conductivity in Cubic Hybrid Perovskite FAPbI3

We comprehensively investigate the thermal transport behavior in cubic hybrid perovskite FAPbI3, utilizing a combination of first principles-based anharmonic lattice dynamics and a linearized Wigner transport formula. By employing the Temperature Dependent Effective Potential (TDEP) technique, we stabilize the negative soft modes, primarily dominated by organic cations, at finite temperatures in cubic FAPbI3. We predict an ultra-low thermal conductivity of ~0.63 Wm^(-1) K^(-1) in cubic FAPbI3 at 300 K, characterized by a temperature dependence of T^(-0.740), suggesting a good crystalline nature of phonon transport. Importantly, we find that the ultra-low thermal conductivity in cubic FAPbI3 is primarily attributed to the [PbI3]1- units rather than the FA+ cations. This challenges the prevailing understanding and can be attributed to the presence of Pb(s)-I(p) anti-bonding sates in the [PbI3]1- units. Furthermore, thermal transport in cubic FAPbI3 is predominantly governed by the particle-like phonon propagation channel across the entire temperature range of 300-500 K. This observation can be attributed to the reduced suppression of low-frequency phonons by FA+ cations and the presence of large inter-branch spacings. Finally, our results demonstrate that the anharmonic force constants are highly sensitive to temperature effects, resulting in an underestimation of thermal conductivity when 0-K anharmonic force constants are used. Our study not only elucidates the microscopic mechanisms of thermal transport in cubic FAPbI3 but also establishes a foundational basis for the discovery, design, and understanding of hybrid organic-inorganic compounds with ultra-low thermal conductivity.