Multi-physics investigations on the thermal effects in giant photostriction in MAPbBr₃ crystals

Photo-induced lattice expansion in metal-halide perovskites is an important phenomenon affecting the physical properties. However, there is extensive controversy over whether it is a direct non-thermal photostrictive effect or a photothermal effect. In this work, we present a strategy combining a high-resolution digital holographic interferometry (DHI) system with a simulation-guided thermal calibration strategy, the convection heating method, which leads us to successfully and unambiguously decouple the photothermal and photostrictive contributions to the lattice expansion. Under 532 nm laser illumination, a giant and localized relative deformation of 4.7 × 10^-4 is directly observed using the three-dimensional DHI method. And crucially, in situ thermal imaging confirms that this large deformation occurs with a negligible accompanying temperature rise. A novel natural convection heating method is used to obtain a linear expansion coefficient of 3.3 × 10^-5 K^-1 for MAPbBr₃. Therefore, to produce equivalent lattice expansion as the photo-induced lattice expansion, the temperature needs to be increased by more than 14 °C, which is in contrast to our direct experimental observations. Our results definitively demonstrate that the giant lattice expansion is a dominant, non-thermal photostrictive effect, which can be identified as an intrinsic electronic-strain response caused by the direct interaction of photo-generated carriers with the MAPbBr₃ crystal lattice. This work provides a clear resolution to the ongoing debate and establishes a reliable methodology for investigating photo-induced dynamics in advanced materials.