Inverted band gap trend through octahedral ordering in Cs_2Au_2X_6 (X=Cl, Br, I)

Double perovskites Cs_2Au_2X_6 (X=Cl, Br, I) are prototypical materials that exhibit charge disproportionation of gold into 1+ and 3+ states. It is known that the disproportionation is resolved under high pressures, and this has stimulated many studies into the pressurization of these materials. At present, the phase changes in these materials are still strongly contested. Here, we use density functional theory to study the pressure-dependent behavior of Cs_2Au_2X_6. We find that a tetragonal–cubic transition occurs directly from the ground state I4/mmm structure. Even so, we also found an intermediate tetragonal P4/mmm structure to be very close in energy, suggesting it to be observable. We also find several other competing metastable phases, which explains some of the controversies in the literature. Focusing on one of the metastable phases, we suggest that Cs_2Au_2X_6 can be prepared in a P4_2/mnm structure, analogous to that of KCuF_3. The band gap in the P4_2/mnm structure widened as atomic number of the halide was increased, which is the inverse trend compared to the ground state structure. We explain this by the different octahedral distortion ordering in the two structural phases. Furthermore, we show that the conduction band in P4_2/mnm is three dimensionally connected, which is favorable for opto-electronic applications. We submit that this work demonstrates that octahedral distortion ordering is a promising avenue for developing new double perovskites and suggests it to be particular effective in tuning the electronic structure properties.