  Mixing iodide and bromide in halide perovskite semiconductors is an effective
strategy to tune their bandgap, therefore mixed-halide perovskites hold great
promise for color-tunable LEDs and tandem solar cells. However, the bandgap of
mixed-halide perovskites is unstable under (sun-)light, since the halides
segregate into domains of different bandgaps. Using pressure-dependent
ultrafast transient absorption spectroscopy, we show that high external
pressure increases the range of thermodynamically stable halide mixing ratios.
Chemical pressure, by inserting a smaller cation, has the same effect, which
means that any iodide-to-bromide ratio can be thermodynamically stabilized by
tuning the crystal volume and compressibility. We interpret this stabilization
by an alteration of the Helmholtz free energy via the largely overlooked
PdeltaV term.
