Ineffectiveness of formamidine in suppressing ultralow thermal conductivity in cubic hybrid perovskite FAPbI3

Understanding lattice dynamics and thermal transport mechanisms in cubic hybrid organic-inorganic perovskites remain challenging due to strong anharmonicity and phase transitions. Here, we investigate the thermal transport behavior in benchmark cubic hybrid perovskite FAPbI₃ by coupling first principles-based anharmonic lattice dynamics with a linearized Wigner transport equation. Using the Temperature-Dependent Effective Potential (TDEP) method, we stabilize the negative soft modes, primarily dominated by organic FA⁺ cations. Our calculations predict an ultra-low thermal conductivity of ~ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>0.63</mml:mn> <mml:mspace></mml:mspace> <mml:mi>W</mml:mi> <mml:msup><mml:mrow><mml:mi>m</mml:mi></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> <mml:msup><mml:mrow><mml:mi>K</mml:mi></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> </mml:mrow> </mml:math> at 300 K, following a temperature dependence of <i>T</i> ^-0.740. Contrary to common assumptions, we find that the [PbI₃]^1- units, rather than FA⁺ cations, dominate thermal resistance. Furthermore, we demonstrate that anharmonic force constants are highly temperature-sensitive, relying on 0-K force constants significantly underestimates thermal conductivity. Our study not only elucidates the microscopic mechanisms governing thermal transport in FAPbI₃ but also provides a robust framework for modeling heat conduction in hybrid organic-inorganic compounds.