Evidence of Athermal Metastable Phase in a Halide Perovskite: Optically Tracked Thermal-Breach Memory

Halide perovskite materials have been extensively studied in the last decade because of theirimpressive optoelectronic properties. However, their one characteristic that is uncommon for semi-conductors is that many undergo thermally induced structural phase transitions. The transition ishysteretic, with the hysteresis window marking the boundary of the metastable phase. We have dis-covered that in methylammonium lead iodide, this hysteretic metastable phase is athermal, meaningit shows almost no temporal phase evolution under isothermal conditions. We also show that a largenumber of distinguishable metastable states can be prepared following different thermal pathways.Furthermore, under a reversible thermal perturbation, the states in the metastable phase eithershow return-point memory or undergo a systematic nonrecoverable phase evolution, depending onthe thermal history and the sign of the temperature perturbation. Since the phase fraction canbe probed with extreme sensitivity via luminescence, we have an optically retrievable memory thatreliably records any breach in temperature stability.Such thermal-breach memory in athermalmartensites, of which there are numerous examples, may be useful for tagging packages requiringstrict temperature control during transportation or preservation.