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

D. O. Horiachyi, M. O. Nestoklon, I. A. Akimov, D. R. Yakovlev, V. Vasylkovskyi, O. Trukhina, V. Dyakonov, M. Bayer

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.

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

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.
Paper Structure (9 sections, 4 equations, 5 figures, 2 tables)

This paper contains 9 sections, 4 equations, 5 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Experimantal results
  3. Cubic phase at room temperature
  4. Tetragonal phase at low temperature
  5. Temperature dependence
  6. Discussion
  7. Conclusion
  8. Density
  9. Acoustic phonons velocities

Figures (5)

  • Figure 1: Optical images of (a) Cs$_{2}$AgBiBr$_{6}$ and (b) Cs$_{2}$AgBiCl$_{6}$ single crystals. The as-grown crystals are bounded solely by {111} facets.
  • Figure 2: Powder X-ray diffraction patterns of Cs$_{2}$AgBiBr$_{6}$ and Cs$_{2}$AgBiCl$_{6}$ measured at room temperature $T=293$ K.
  • Figure 3: Brillouin light scattering (BLS) spectra measured at room temperature $T=293$ K. (a) Excitation with 542 nm (green) and 515 nm (brown) of Cs$_{2}$AgBiCl$_{6}$ along $\langle111\rangle$ crystalographic axis. BLS spectra under excitation with 542 nm laser in Cs$_{2}$AgBiCl$_{6}$ (blue) and Cs$_{2}$AgBiBr$_{6}$ (red) measured at room temperature. The data are shown in (b) and (c) for excitation along $\langle111\rangle$ and $\langle110\rangle$ crystallographic directions, respectively.
  • Figure 4: Low temperature BLS spectra at $T\approx 5$ K. BLS spectra under excitation with 542 nm laser for Cs$_{2}$AgBiCl$_{6}$ (blue) and Cs$_{2}$AgBiBr$_{6}$ (red) measured at 5 K for $\mathcal{S}_0$ (a) and $\mathcal{S}_1$ (b) surfaces.
  • Figure 5: Temperature dependence of the phonon frequencies in Cs$_{2}$AgBiCl$_{6}$ along $\langle 111 \rangle$ crystallographic direction under excitation with 542 nm laser. (a) BLS spectra measured at different temperatures. (b) BLS peak positions as a function of temperature. Blue points correspond to LA phonons, red - TA$_1$ phonons, green - TA$_2$ phonons.