Much recent attention has been devoted towards unraveling the microscopic optoelectronic properties of hybrid organic-inorganic perovskites. Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: MAPbBr_{3}, FAPbBr_{3}, MAPbI_{3}, and α-FAPbI_{3} (MA: methylammonium, FA: formamidinium). We report a complete set of elastic constants characterized by a very soft shear modulus C_{44}. Further, a tendency towards an incipient ferroelastic transition is observed in FAPbBr_{3}. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft α-FAPbI_{3}.

The paper will discuss the possibility for light spectroscopies, e.g. Brillouin Light Scat- tering or Raman Scattering, to locally monitor the densification of amorphous silica. It is indeed well-known that silica significantly densifies under high stresses, either elastically as for example during a high-pressure experiment in a diamond-anvil cell below about 10 GPa or plastically above that limit. Further, this elastic limit strongly depends on the temperature. Irradiation-induced density changes are also observed using ultra-short laser pulses, neutrons or other particles. All these different routes lead to substantial structural modifications which in turn alter the response of silica to light spectroscopy in a complex manner.

High-pressure Brillouin [1] and polarized Raman spectra [2] of vitreous silica are measured up to 8 GPa in diamond-anvil cell at room temperature. On one hand, Brillouin frequencies show that the well-known anomalous maximum in the pressure dependence of the compressibility is suppressed by He incorporation into the silica network. This shows that the elastic anomaly relates to the collapse of the largest interstitial voids in the structure. On the other hand, the combined use of either a nonpenetrating pressurizing medium-argon-or a penetrating one-helium and Neon, allows one to separate density from stress effects on the Raman frequencies.[1]C. Weigel, A. Polian, M. Kint, B. Ruffl ́e, M. Foret, and R. Vacher, Phys. Rev. Lett. 109, 245504 , 2012[2]C. Weigel, M. Foret, B. Hehlen, M. Kint, S. Cl ́ement, A. Polian, R. Vacher, and B. Ruffl ́e, Phys. Rev. B 93, 224303 , 2016

Contact-resonance atomic force microscopy (CR-AFM) is of great interest and very valuable for a deeper understanding of the mechanics of biological materials with moduli of at least a few GPa. However, sample surfaces can present a high topography range with significant slopes, where the local angle can be as large as ± 50°. The non-trivial correlation between surface slope and CR-frequency hinders a straightforward interpretation of CR-AFM indentation modulus measurements on such samples. We aim to demonstrate the significant influence of the surface slope on the CR-frequency that is caused by the local angle between sample surface and the AFM cantilever and present a practical method to correct the measurements. Based on existing analytical models of the effect of the AFM set-up's intrinsic cantilever tilt on CR-frequencies, we compute the non-linear variation of the first two (eigen)modes CR-frequency for a large range of surface angles. The computations are confirmed by CR-AFM experiments performed on a curved surface. Finally, the model is applied to directly correct contact modulus measurements on a durum wheat starch granule as an exemplary sample.

Much recent attention has been devoted towards unraveling the microscopic optoelectronic properties of hybrid organic-inorganic perovskites. Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: MAPbBr3, FAPbBr3, MAPbI3, and α−FAPbI3 (MA: methylammonium, FA: formamidinium). We report a complete set of elastic constants characterized by a very soft shear modulus C44. Further, a tendency towards an incipient ferroelastic transition is observed in FAPbBr3. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft α−FAPbI3.

Hybrid organic-inorganic perovskites have been intensively studied as promising materials for photovoltaic applications, taking advantage of their high efficiency and low cost processing. Although much recent attention has also been devoted towards unraveling their microscopic optoelectronic properties, the origin of their softness, by comparison to classic semiconductors, is currently still lacking a comprehensive understanding and systematic experimental studies. Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: MAPbBr3, FAPbBr3, MAPbI3 and α-FAPbI3. We report a very small shear C44 elastic constant for all the compounds and a considerable elastic anisotropy. The extremely low bulk modulus and negative C12 in α-FAPbI3 substantiates its very unstable nature and in FAPbBr3, a tendency towards an incipient ferroelastic transition, is interpreted as further evidence of the influence of plasticity in hybrid perovskites. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft α-FAPbI3.