Inorganic lead-free double perovskites attract particular interest as a nontoxic and stable material platform for optoelectronic applications. Here, Brillouin light scattering spectroscopy is employed to investigate the elastic properties and structural phase transitions in single crystals of Cs2AgBiBr6 and Cs2AgBiCl6. The complete set of elastic constants is evaluated from the Brillouin scattering measurements performed along three different crystallographic directions. Both materials have similar elastic constants and weak elastic anisotropy in the cubic phase. At low temperatures, the lifting of degeneracy of transverse acoustic phonon modes is attributed to a lowering of crystal symmetry. From the temperature dependence of the acoustic phonon frequencies, we determine the structural phase transition temperature of about 43 K for Cs2AgBiCl6, compared to 122 K in Cs2AgBiBr6.
Nestoklon et al. (Tue,) studied this question.