Perovskite halide materials exhibit fluid-like atomic vibrations
(Nanowerk News) Perovskite Halides are a recently developed class of materials. They have applications in solar energy and radiation detection. They are also potentially useful for heat harvesting—trapping heat from engines and other sources.
Cesium lead bromide (CsPbBr3) is one of the simplest members of the perovskite halide family. Understanding the atomic structure and dynamics of lead-halide perovskites (LHP) is critical for scientists developing these materials for specific applications. LHP exhibits structural instability and large atomic fluctuations that scientists believe affect its optical and thermal properties. However, scientists do not fully understand the motion of the atoms in LHP.
In this study (Natural Ingredients, “A two-dimensional overdamped fluctuation of a soft perovskite lattice in CsPbBr3“), scientists used neutron and X-ray scattering with computational modeling to reveal the unusual behavior of coordinated atomic motion in LHP. In particular, the researchers found that the vibrations of the atoms (phonons) of the bromine octahedron (see figure) have large amplitudes but cannot oscillate for a long time.
On the contrary, the vibrations are very muffled. This is similar to when a guitar player places his palm over the instrument’s strings while strumming them, muffling the sound.
The damped nature of atomic vibrations in LHP can help electrons generated by absorption of photons travel long distances in matter. This will increase the material’s ability to generate electrical power. This discovery provides a new route for designing materials with adjustable optical and thermal behavior.
These materials have many potential applications. For example, they could reshape solar panel technology as well as waste heat harvesting applications. In these applications, halide perovskites can produce devices that are more efficient, cheaper, and easier to manufacture.
Understanding the interaction of phonons and electronic degrees of freedom is essential for controlling a wide range of energized materials. Cesium lead bromide is an interesting material that exhibits extremely complex atomic dynamics in the simplest structure among the large perovskite halide family for photovoltaic, radiation detection, and potential thermoelectric applications.
This research explores the microscopic origin of the highly damped anharmonic phonons in these materials with the combined use of X-ray and neutron scattering experiments at the Spallation Neutron Source and Advanced Photon Source, both user facilities of the Department of Energy’s Office of Science.
The team includes researchers from Duke University, Argonne National Laboratory, Oak Ridge National Laboratory, National Institute of Standards and Technology, University of Maryland, and Northwestern University.
The researchers’ analysis of the experimental results, combined with computer simulations, determined that strong anharmonic lattice vibrations (that is, vibrations with an indeterminate and long-lasting frequency) were associated with unstable cooperative oscillations of bromine octahedra in the crystal structure, as illustrated in the image above. This motion modulates the electronic state near the band gap and is thought to interfere with the recombination of photoexcited charges. Thus, anharmonic phonons and electron-phonon coupling may be key to enabling high performance in photovoltaic and radiation detection applications.
This new understanding of atomic vibrations of the crystal lattice provides new avenues for designing materials with controllable optoelectronic and thermal properties for future energy harvesting and conversion devices.
