Researchers at Yale University, working with the U.S. Department of Energy's Argonne National Laboratory and Brookhaven National Laboratory, and with contributing researchers from the University of Toledo, the University of Pennsylvania and Cornell University, have identified a previously unknown structural state in the lead-free double perovskite Cs2AgInCl6, a material valued for its ability to emit warm white light across the visible spectrum for LED applications. Triggered by photoexcitation, the new state forms when silver and indium ions swap positions within the crystal lattice, in under a nanosecond, but the material takes several milliseconds to relax back to its ordered ground state.
Cs2AgInCl6's white-light emission relies on self-trapped excitons (STEs): photoexcited charge carriers that distort the crystal lattice around them and release their energy as broadband light rather than a narrow emission line. While STE formation itself has been studied before, the researchers found that earlier measurements, typically limited to nanosecond timescales, had missed a slower process hiding underneath it. Using transient optical spectroscopy extended out to microsecond and millisecond delay times, they detected a broadband photoinduced absorption signal that persisted long after the microsecond-scale STE signal had fully decayed, pointing to a distinct, unidentified structural state activated by light.
To pin down that state, the team turned to time-resolved X-ray diffraction at the Advanced Photon Source, a DOE Office of Science user facility at Argonne, with picosecond time resolution. The measurements revealed a new diffraction peak appearing after photoexcitation, corresponding to a slightly contracted lattice consistent with the formation of indium-rich nanodomains, and this photoinduced phase persisted for milliseconds, far outlasting the microsecond STE states that triggered it. Time-resolved X-ray absorption spectroscopy at the silver K-edge then showed that hole localization within the STEs transiently oxidizes silver ions from Ag+ to Ag2+, narrowing the charge and size mismatch between silver and indium ions enough for them to exchange lattice sites, forming silver-rich and indium-rich domains that sharply reduce the material's electronic bandgap.
The asymmetry in timing traces back to that same redox step: the initial cation swap happens in under a nanosecond, but once the STEs recombine and silver reverts to Ag+, the restored charge and radius mismatch between Ag+ and In3+ creates a high energy barrier to reordering, keeping the disordered phase alive for milliseconds. This behavior has no counterpart in conventional lead halide perovskites, where photoinduced structural changes recover within nanoseconds. The authors note that the mechanism should generalize to other double perovskites combining a Jahn-Teller-active, distortion-prone octahedron with a large ionic-radius mismatch between B-site cations, pointing to compositions such as Cs2AgInCl6 as candidates for similar behavior.
The researchers suggest these long-lived metastable phases could accumulate under continuous illumination, particularly at low temperatures, and that the associated bandgap narrowing, while a drawback for optical gain applications, could be useful for photodetection, including detecting weak infrared signals.
