The discovery of hybrid halide perovskites materials heralded a new era in optoelectronic technologies, with an unprecedented rise to above 20% in the efficiencies of photovoltaic devices in just a few years. However, for the highest thermal stability, an all inorganic perovskite would be preferable. To date it has been proven very difficult to stabilize the room-temperature polymorph of the inorganic lead halide CsPbI3. Moreover, the unexpected functionality of the lead halide perovskites has driven the research community towards the attempt to discover new metal-halide based compounds with improved functionality.

Recently, a new class of candidate photovoltaic materials – the double halide perovskites with chemical formula A₂BB'X₆ (A, B=monovalent cations, B’=trivalent cation, X=halogen) – has been identified as a possible alternative to the better-known APbX₃ perovskites. In these materials, the B/B’ cations (average valence 2+) substitute for lead and are fully ordered in a double perovskite superlattice. Of particular interest is Cs₂AgBiBr₆, which does not contain toxic elements, and is highly stable both structurally and chemically. Several studies exploring its implementation in devices have started to appear in the literature, yet very little is known about the relationship between the crystal structure and the opto-electronic properties of this double perovskite compound. Hence, there is an urgent need for a detailed understanding of these materials at a more fundamental level.

We have investigated the temperature-dependent structural behaviour of Cs₂AgBiBr₆ using heat capacity measurements, X-ray powder and single-crystal diffraction and neutron powder diffraction and discovered that this compound undergoes a low-temperature structural phase transition (T_S~122 K) from cubic to tetragonal. The crystal structures of both high- and low-temperature phases were refined based on our diffraction data. The temperature dependence of the exciton energy in proximity to the direct band gap was determined using reflectivity measurements. We found a direct, linear relationship between the tetragonal strain and the exciton energy, demonstrating that the latter is controlled by the BBr₆ octahedral rotation. Meanwhile time-resolved photoluminescence measurements indicated a qualitative change in the charge-carrier recombination mechanisms at a temperature that correlates well with the phase transition. Further absorption and photoluminescence spectral measurements probing the temperature dependence of the indirect bandgap transition have also been reported.