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

Lead halide perovskites such as methylammonium lead triiodide (CH 3NH 3PbI 3) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution-processable material works so well is currently missing. Previous research has revealed that CH 3NH 3PbI 3 possesses multiple forms of static disorder regardless of preparation method, which is surprising in light of its excellent performance. Using high energy resolution inelastic X-ray (HERIX) scattering, we measure phonon dispersions in CH 3NH 3PbI 3 and find direct evidence for another form of disorder in single crystals: large-amplitude anharmonic zone edge rotational instabilities of the PbI 6 octahedral that persist to room temperature and above, left over from structural phase transitions that take place tens to hundreds of degrees below. Phonon calculations show that the orientations of the methylammonium (CH 3NH 3 +) couple strongly and cooperatively to these modes. The result is a noncentrosymmetric, instantaneous local structure, which we observe in atomic pair distribution function (PDF) measurements. This local symmetry breaking is unobservable by Bragg diffraction but can explain key material properties such as the structural phase sequence, ultralow thermal transport, and large minority charge carrier lifetimes despite moderate carrier mobility. In conclusion,more » from the PDF we estimate the size of the fluctuating symmetry broken domains to be between 1 and 3 nm in diameter.« less
Authors:
 [1] ;  [1] ;  [2] ;  [2] ;  [1] ;  [1] ;  [3] ;  [1] ;  [2] ;  [4]
  1. Columbia Univ., New York, NY (United States)
  2. Univ. of Bath, Bath (United Kingdom)
  3. Argonne National Lab. (ANL), Argonne, IL (United States)
  4. Columbia Univ., New York, NY (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)
Publication Date:
Grant/Contract Number:
AC02-06CH11357
Type:
Accepted Manuscript
Journal Name:
ACS Energy Letters
Additional Journal Information:
Journal Volume: 1; Journal Issue: 4; Journal ID: ISSN 2380-8195
Publisher:
American Chemical Society (ACS)
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
Engineering and Physical Sciences Research Council (EPSRC); National Science Foundation (NSF); USDOE Office of Science (SC)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 36 MATERIALS SCIENCE
OSTI Identifier:
1416017