Optical Properties of Cs₂AgBiBr₆

Cs₂AgBiBr₆ is a halide double perovskite and has been proposed as a more stable and less toxic alternative to the high photovoltaic efficiency lead based perovskites.[1] We have synthesized 4mm single crystals of Cs₂AgBiBr₆ by a controlled cooling method.[2] The crystals were characterized with X-ray Diffraction and the phase transition from the room temperature cubic structure (Fm-3m) to the low temperature tetragonal structure (I4/m) was detected at near 120K through differential scanning calorimetry. Investigated optical properties near the band gap of Cs₂AgBiBr₆ via transmission measurements, temperature dependent resonant Raman scattering, and photoluminescence will be presented. Of note, the Stokes Raman spectrum exhibited resonant enhancement with 532.8 nm laser excitation. The A_1g/A_g phonon at 5.36 THz exhibits anomalies in the temperature dependence of its Lorentzian fit parameters at the structural phase transition and a characteristic temperature of 40 K whose origin remains to be investigated further.

[1] A. H. Slavney et.al. Journal of the American Chemical Society 2016. 138 (7), 2138-2141

[2] Y. Lixiao et. al. Advanced Optical Materials 2019. 7 (19), 1900491.