Direct Observation of Dynamic Symmetry Breaking above Room Temperature in Methylammonium Lead Iodide Perovskite

Abstract

Lead halide perovskites such as methylammonium lead triiodide (MAPI) have\noutstanding optical and electronic properties for photovoltaic applications,\nyet a full understanding of how this solution processable material works so\nwell is currently missing. Previous research has revealed that MAPI possesses\nmultiple forms of static disorder regardless of preparation method, which is\nsurprising in light of its excellent performance. Using high energy resolution\ninelastic X-ray (HERIX) scattering, we measure phonon dispersions in MAPI and\nfind direct evidence for another form of disorder in single crystals: large\namplitude anharmonic zone-edge rotational instabilities of the PbI_6 octahedra\nthat persist to room temperature and above, left over from structural phase\ntransitions that take place tens to hundreds of degrees below. Phonon\ncalculations show that the orientations of the methylammonium couple strongly\nand cooperatively to these modes. The result is a non-centrosymmetric,\ninstantaneous local structure, which we observe in atomic pair distribution\nfunction (PDF) measurements. This local symmetry breaking is unobservable by\nBragg diffraction, but can explain key material properties such as the\nstructural phase sequence, ultra low thermal transport, and large minority\ncharge carrier lifetimes despite moderate carrier mobility.\n