Tunable photostriction of halide perovskites through energy dependent photoexcitation*

Photostriction, the name given to volume changes upon illumination, has recently been observed in halide perovskites. However, the microscopic mechanism remains unclear. Using a combination of molecular orbital theory and first principles methods, we propose that the orbital characters of the electronic bands near the Fermi level determine the photostriction behavior. We find that photoexciting electrons from strong antibonding valence states to weaker antibonding conduction states leads to lattice contraction as a result of weakened antibonding interaction. Interestingly, using higher excitation energies promotes electrons from deeper nonbonding valence states to antibonding conduction states, resulting in giant lattice expansion. These results rationalize the experimentally observed tunable photostriction in halide perovskites. Overall, we propose that a detailed knowledge of the electronic structure and the band representations are the key ingredients to quantitatively understand photostriction in general insulators.