Elastic Constants, Optical Phonons, and Molecular Relaxations in the High Temperature Plastic Phase of the CH3NH3PbBr3Hybrid Perovskite.
Low frequency dynamics has been studied in a CH3NH3PbBr3hybrid perovskite single crystal by using four different spectroscopy techniques: coherent inelastic neutron, Raman and Brillouin scatterings, and ultrasound measurements. Sound velocities were measured over five decades in energy to yield the complete set of elastic constants in a hybrid halide perovskite crystal in the pseudocubic plastic phase. The C44shear elastic constant is very small, leading to a particularly low resistance to shear stress. Brillouin scattering has been used to study the relaxation dynamics of methylammonium cations and to evidence translation-rotation coupling associated with the cubic to tetragonal phase transition at Tc≈ 230 K. Low frequency and highly damped optical phonons observed using both Raman and inelastic neutron below 18 meV, do not present softening close to Tc. The critical dynamics at Tc≈ 230 K is compatible with an order-disorder character, dominated by relaxational motions of the molecules.
UMR FOTON, CNRS, INSA-Rennes, F- 35708 Rennes, France.
Antoine
A
Létoublon
Institut des Sciences Chimiques de Rennes, UMR 6226, CNRS, Université de Rennes 1 , F-35042 Rennes, France.
Serge
S
Paofai
Laboratoire Charles Coulomb (L2C), UMR 5221 CNRS- Université de Montpellier , F-34095 Montpellier, France.
Benoît
B
Rufflé
LLB, CEA, CNRS, Université Paris-Saclay , CEA Saclay, F-91191 Gif-sur-Yvette, France.
Philippe
P
Bourges
Laboratoire Charles Coulomb (L2C), UMR 5221 CNRS- Université de Montpellier , F-34095 Montpellier, France.
Bernard
B
Hehlen
Laboratoire Charles Coulomb (L2C), UMR 5221 CNRS- Université de Montpellier , F-34095 Montpellier, France.
Thierry
T
Michel
Institut de Physique de Rennes, UMR 6251, CNRS, Université de Rennes 1 , F-35042 Rennes, France.
Claude
C
Ecolivet
UMR FOTON, CNRS, INSA-Rennes, F- 35708 Rennes, France.
Olivier
O
Durand
Institut des Sciences Chimiques de Rennes, UMR 6226, CNRS, Université de Rennes 1 , F-35042 Rennes, France.
Stéphane
S
Cordier
Institut des Sciences Chimiques de Rennes, UMR 6226, CNRS, Université de Rennes 1 , F-35042 Rennes, France.
Claudine
C
Katan
UMR FOTON, CNRS, INSA-Rennes, F- 35708 Rennes, France.
Jacky
J
Even