Room-temperature valence transition in a strain-tuned perovskite oxide

Spin-state crossovers are phenomena where, under changes in temperature or pressure, the spin-state of an ion changes. In some materials, this spin-state crossover occurs simultaneously with a metal-insulator transition, driven by a valence transition. Control over such valence, spin-state, and metal-insulator transitions has much technological appeal, but, thus far, materials displaying this have been limited to cryogenic temperatures. Here, the authors show that in strained films of (Pr1-yYy)1- xCaxCoO3-δ, these transitions can be promoted to room temperature.