Polar Indirect Valley as a Limiting Factor for Radiative Efficiency in Gold‐Based Mixed‐Valence Double Perovskites

Double perovskites have emerged as promising alternatives to lead halide perovskites, aiming to mitigate challenges related to toxicity and chemical instability. Among them, mixed‐valence gold halides such as Cs 2 Au + Au 3 + Cl 6 , which contain only a single type of metal cation in two oxidation states, stand out due to their unique structural and electronic properties. These materials exhibit strong absorption in the near‐infrared range, making them attractive candidates for optoelectronic applications such as photovoltaics. In this work, temperature‐dependent optical spectroscopy techniques are employed to demonstrate that these compounds exhibit particularly strong polar electron‐phonon coupling, which has a profound impact on their optoelectronic properties. In particular, this coupling induces a temperature‐dependent absorption tail that progressively reshapes the absorption spectrum. It is shown that this tail leads to a forbidden band‐egde recombination, which explains the reported difficulties in detecting a photoluminescence signal from this class of double perovskites.