The origin of anharmonic atomic motion in halide perovskite crystals

Halide perovskites attracted much attention in the last years due to their excellent photovoltaic
activity. They are unique in the sense that they exhibit long carrier lifetime despite having many
apparent structural defects. Recent studies in our group concluded that this unique behavior is due
to strong coupling between the electronic band structure and the strongly anharmonic motion of
the atoms within the crystal. Therefore, it is imperative to understand the source of anharmonic
atomic motion in this class of materials. Two kinds of instabilities can give rise to strong
anharmonic behavior in the perovskite crystals. The first is the tilting of the octahedral network,
and second, octahedral distortion due to the B cation lone pair stereoactivity. In order to
understand the anharmonic behavior and its source, we investigated a series of perovskites with
different lone pair stereoactivity and tilting degrees. Using low-frequency Raman spectroscopy, we
quantified the level of anharmonicity and determined the influence of each instability on the
structural dynamics.