Non-monotonic Thermal Conductivity of FA x MA1-x PbI3 Achieving Ultralow Values: The Role of Anharmonic Low Energy Rotation of Organic Moieties

Stability issues of MAPbI(3) and FAPbI(3) have led to the exploration of more stable FA(x)MA(1-x)PbI(3) solid solutions for many energy applications. Thermal properties are important in view of the inevitable heat generation in energy conversion processes. MAPbI(3) is reported to have an ultralow thermal conductivity, kappa (similar to 0.34 W<middle dot>m(-1)<middle dot>K-1). Surprisingly, there is no report of kappa for FA/MA mixed compositions, despite their obvious relevance. Here, we report an intriguing observation of a non-monotonic reduction of kappa with FA substitution, while most other physical properties, such as the lattice parameters and bandgap, vary monotonically. This leads the asymmetric composition FA(0.3)MA(0.7)PbI(3) to become the lowest kappa (similar to 0.2 W<middle dot>m(-1)<middle dot>K-1) material among all three-dimensional metal halide perovskites. We show this non-monotonic kappa is a consequence of two monotonic and opposing trends with increasing FA compositions, namely, an increasingly facile rotation of a decreasing number of MA ions. Raman studies, together with ab initio theoretical results, show that the heterogeneous bonding environment in FA(x)MA(1-x)PbI(3) results in numerous low-energy, highly anharmonic, and localized optical phonon modes strongly coupled to acoustic phonons related exclusively to MA, but not FA, rotations, leading to very short phonon lifetimes (0.3-2 ps) and, therefore, to the ultralow kappa for these compounds.