Abstract
Metal-halide perovskites (MHPs) are attracting considerable interest for optoelectronic applications, with Cs2AgBiBr6 one of the main contenders among lead-free systems. Cs2AgBiBr6 crystallizes in a nominally double-perovskite structure, but exhibits a soft lattice with large atomic fluctuations characteristic of MHPs. While crucial to understand electron-phonon and phonon-phonon couplings, the spatiotemporal correlations of these fluctuations remain largely unknown. Here, we reveal these correlations using comprehensive neutron and x-ray scattering measurements on Cs2AgBiBr6 single crystals, complemented with first-principles simulations augmented with machine-learned neural-network potentials. We report the discovery of an unexpected complex modulated ground-state structure containing several hundred atoms, arising from a soft-phonon instability of the low-temperature tetragonal phase. Further, our experiments and simulations both reveal extensive correlated two-dimensional fluctuations of Br octahedra at finite temperature, arising from soft optic phonons that are strongly broadened by anhamonicity, reflecting very shallow potential wells. These results provide new insights into the atomic structure and fluctuations in MHPs, critical to understand and control their thermal and optoelectronic properties.
| Original language | English |
|---|---|
| Article number | 013014 |
| Journal | PRX Energy |
| Volume | 3 |
| Issue number | 1 |
| DOIs | |
| State | Published - Jan 2024 |
Funding
We thank Olle Hellman for providing access to the TDEP software package. X.H., M.K.G., and O.D. were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under Award No. DE-SC0019978. T.L.A. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under Award No. DE-SC0019299. Work at the Materials Science Division at Argonne National Laboratory (characterization, x-ray and neutron scattering measurements and analysis) was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. The synthesis of materials is based upon work supported by the National Institutes of Health (NIH) under award 1R01EB033439. This research used resources at the High Flux Isotope Reactor and Spallation Neutron Source, both DOE Office of Science User Facilities operated by the Oak Ridge National Laboratory. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Theoretical calculations were performed using resources of the National Energy Research Scientific Computing Center, a U.S. DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Olle Hellman for providing access to the TDEP software package. X.H., M.K.G., and O.D. were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under Award No. DE-SC0019978. T.L.A. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under Award No. DESC0019299. Work at the Materials Science Division at Argonne National Laboratory (characterization, x-ray and neutron scattering measurements and analysis) was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. The synthesis of materials is based upon work supported by the National Institutes of Health (NIH) under award 1R01EB033439. This research used resources at the High Flux Isotope Reactor and Spallation Neutron Source, both DOE Office of Science User Facilities operated by the Oak Ridge National Laboratory. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Theoretical calculations were performed using resources of the National Energy Research Scientific Computing Center, a U.S. DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.