Structural phase transitions in double perovskite crystals studied by Brillouin light scattering

TL;DR

This study uses Brillouin light scattering to analyze elastic properties and phase transitions in lead-free double perovskite crystals, revealing their structural changes at low temperatures.

Contribution

It provides detailed elastic constants and phase transition temperatures for Cs2AgBiBr6 and Cs2AgBiCl6 using Brillouin scattering, a novel application for these materials.

Findings

Both materials have similar elastic constants in the cubic phase.

Structural phase transition occurs at 43 K for Cs2AgBiCl6.

Transition temperature for Cs2AgBiBr6 is about 122 K.

Abstract

Inorganic lead-free double perovskites represent particular interest as non-toxic and stable material platform for optoelectronic applications. Here, we employ Brillouin light scattering spectroscopy to investigate the elastic properties and structural phase transitions in single crystals of Cs2AgBiBr6 and Cs2AgBiCl6. A complete set of elastic constants is determined from the Brillouin scattering measurements performed along three different crystallographic directions. Both materials exhibit 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 for the cubic-to-tetragonal phase transition in Cs2AgBiBr6.