Collin Tower, F. S. Razavi, Jeremy Dion, M. Reedyk, Jürgen Nuß, Reinhard K. Kremer
The halide double perovskite Cs 2 AgBiBr 6 has been proposed as a potential replacement for organic halide perovskites for photovoltaic applications. Further investigation of its dielectric response, optical properties, and structural stability is thus warranted. Cs 2 AgBiBr 6 exhibits a well-documented structural phase transition at 120 K but indications for an additional lower-temperature ( ∼ 40 K) phase transition have also been reported. On the basis of measurements of the specific heat capacity, temperature-dependent powder x-ray diffraction, low-frequency capacitance, and infrared reflectivity, we clarify the previously reported splitting of phonon modes in the Raman spectrum at ∼ 40 K as due to a subtle structural phase transition from the tetragonal I 4 / m structure to a monoclinic P 12 1 / n 1 crystal structure. The infrared-active vibrational modes are experimentally investigated in the three structural regimes. In the cubic structure at room temperature, the four infrared-active modes are observed at 135, ∼ 85 , 55, and ∼ 25 cm − 1 . As the symmetry reduces to tetragonal, a minute splitting of these modes is found; however, below 40 K additional mode splitting is observed, indicating a further reduction in symmetry.