De-mixing is an important phenomenon in material science referring to the phase separation occurring in mixtures, including metallic alloys and ionic solids. Under dark at equilibrium, thermodynamic variables, such as temperature and composition, play an important role on determining the phase stability of a mixture. Interestingly, in 2015 Hoke et al. reported that reversible phase separation in a mixed-conducting mixture of metal halide perovskites, (e.g. MAPb(Br_xI_1-x)₃) can be induced with illumination[1], opening important questions on the thermodynamics of energetically pumped system. As the light induced evolution of different phases involves significant ion transport, improved understanding of the underlying defect chemical mechanisms involved in this photo-induced phase separation (photo de-mixing) and in the re-mixing occurring under dark(dark re-mixing)  is necessary for both understanding and controlling this effect. However, to date, there are few reports devoted to investigating thermodynamic parameters in these energetically pumped systems, including the evaluation of the composition for the photo de-mixed phases. This is further complicated by the light induced degradation affecting halide perovskites.  The improved stability of 2D perovskites[2] makes them suitable model systems for investigating photo de-mixing. Furthermore, the suppressed ion transport in 2D halide perovskites compared with classic 3D hybrid perovskites observed experimentally raises questions on the influence of dimensionality on the defect chemistry and the phase properties of these mixtures.

Here, we consider Dion-Jacobson mixtures based on (PDMA)Pb(Br_0.5I_0.5)₄ (PDMA: 1,4-phenylenedimethanammonium spacer) as a model system for our study.  We demonstrate that 2D mixed halide perovskites undergo photo de-mixing with direct transformation from the pristine phase to the de-mixed phases. Almost complete re-mixing of these phases occurs when the sample is left in the dark, showing that the process is reversible in terms of optical and structural properties. We have further conducted temperature-dependent absorption measurements under light to extract the photo de-mixed compositions and to map the photo-miscibility-gap with three different methods.[3] Lastly, we propose a model that describes photo de-mixing in mixed lead halide perovskites where we emphasize that, in addition electronic effects, the energetics related with potential ionic defect formation should be considered for the overall energetic balance that drive photo de-mixing.[4, 5]