3D halide perovskites used in light emitters and PV systems exhibit an unusualanharmonicity, which is not accounted for in nowadays descriptions of phonons andelectron-phonon coupling. Therefore, the low frequency lattice vibrations andrelaxations were investigated in single crystals of the four 3D hybrid organoleadperovskites, MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3, at the Brillouin zone centerusing Raman and Brillouin scattering and at the zone boundary using inelasticneutron scattering. The temperature dependence of the PbX6 lattice modes in thefour compounds can be renormalized into universal curves and no soft vibration isobserved excluding a displacive-like transitional dynamics related to structural phasetransitions. The reorientational (pseudospin) motions of the molecular cations exhibita seemingly order-disorder character recalling that of plastic crystals, but attributedto a secondary order-parameter. At ultra-low frequency, a quasi-elastic componentevidenced by Brillouin scattering and associated to the unresolved central peakobserved in neutron scattering, is attributed to center of mass anharmonic motionsand rattling of the molecular cations in the perovskite cavities. 3D halide perovskitesare therefore in a very unusual situation where anharmonicity prevails. It is leading topartial, extensive and ultraslow critical and stochatic dynamics connected or not tostructural instabilities. The extensive disorder of the lattice at normal operationtemperatures for devices is similar to premelting where only long-range acousticnormal modes survive.[1] A.Létoublon, S. Paofai, B. Ruffle, P. Bourges, B. Hehlen, T. Michel, C.Ecolivet, O.Durand, S. Cordier, C. Katan, J. Even. Elastic Constants, Optical Phonons andMolecular Relaxations in the High Temperature Plastic Phase of the CH3NH3PbBr3Hybrid Perovskite. J. Phys. Chem.Lett. 7, 3776 (2016)[2]C. Katan, A. D. Mohite, J. Even, Entropy in halide perovskites, Nature Materials,17, 377-379 (2018)[3]A. Ferreira, A. Létoublon, S. Paofai, S. Raymond, C. Ecolivet, B. Ruffle, S.Cordier, C. Katan, M. I. Saidaminov, A. A. Zhumekenov, O. M. Bakr, J. Even, P.Bourges, Elastic softness of hybrid lead halide perovskites, Phys. Rev. Lett., 121,085502 (2018)[4] A. Ferreira, S. Paofai, A. Létoublon, J. Ollivier, S. Raymond, B. Hehlen, B. Rufflé,S. Cordier, C. Katan, J. Even & P. Bourges, Direct evidence of weakly dispersed andstrongly anharmonic optical phonons in hybrid perovskites, Comm. Phys. 3, 48(2020).[5] B. Hehlen, P. Bourges, B. Rufflé, S. Clément, R. Vialla, A. C. Ferreira, C.Ecolivet, S. Paofai, S. Cordier, C. Katan, A. Létoublon, J. Even, Pseudospin-phononpretransitional dynamics in lead halide hybrid perovskites, Phys. Rev. B, 105,024306 (2022)

3D halide perovskites used in light emitters and PV systems exhibit an unusual anharmonicity, which is not accounted for in nowadays descriptions of phonons and electron-phonon coupling. Therefore, the low frequency lattice vibrations and relaxations were investigated in single crystals of the four 3D hybrid organolead perovskites, MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3, at the Brillouin zone center using Raman and Brillouin scattering and at the zone boundary using inelastic neutron scattering. The temperature dependence of the PbX6 lattice modes in the four compounds can berenormalized into universal curves and no soft vibration is observed excluding a displacive-like transitional dynamics related to structural phase transitions. The reorientational (pseudospin) motions of the molecular cations exhibit a seemingly order-disorder character recalling that of plastic crystals, but attributed to a secondary order-parameter. At ultra-low frequency, a quasi-elastic component evidenced by Brillouin scattering and associated to the unresolved central peak observed in neutron scattering, is attributed to center of mass anharmonic motions and rattling of the molecular cations in the perovskite cavities. 3D halide perovskites are therefore in a very unusualsituation where anharmonicity prevails. It is leading to partial, extensive and ultraslow critical and stochatic dynamics connected to structural instabilities. The extensive disorder of the lattice at normal operation temperatures for devices is similar to premelting where only long-range acoustic normal modes survive.

Lattice dynamics in 3D lead halide hybrid perovskites is analyzed together with the possible consequences for device applications

Poly(N-isopropyl acrylamide) (PNIPAM) is a thermo-responsive polymer which undergoes a reversible coil-to-globule transition in water at temperatures close to the human body temperature [1]. Consequently, PNIPAM-based microgels show a volume phase transition (VPT) under heating. Photo-responsive hybrid microgels have been prepared by incorporating gold nanoparticles in PNIPAM microgels [2]. The outstanding photo-thermal and optical properties of single-walled carbon nanotubes (SWNTs) make them promising candidates to prepare multi-responsive hybrid microgels. However, no thorough studies have been reported so far.
In this work, we report the preparation and properties of aqueous dispersions of PNIPAM/SWNTs hybrid microgels (Figure 1a). We study their structure by transmission electron microscopy and dynamic light scattering. We investigate their optical properties and changes when heated at the VPT by near infrared (NIR) absorption, Raman and photoluminescence (PL) spectroscopies [3]. Furthermore, we use optical microscopy (Figure 1b), light scattering and NIR Raman/PL spectroscopy to show that the VPT transition can be triggered by NIR photo-thermal excitation.

The low frequency lattice vibrations and relaxations are investigated in single crystals of the four 3D hybrid organolead perovskites, MAPbBr3, FAPbBr3, MAPbI3, and α-FAPbI3, at the Brillouin zone center using Raman and Brillouin scattering and at the zone boundary using inelastic neutron scattering. The temperature dependence of the PbX6 lattice modes in the four compounds can be renormalized into universal curves, highlighting a common vibrational dynamics at the cubic to tetragonal transition. In particular, no soft vibration is observed excluding a displacive-like transitional dynamics. The reorientational (pseudospin) motions of the molecular cations exhibit a seemingly order-disorder character recalling that of plastic crystals, but attributed to a secondary order-parameter. At ultra-low frequency, a quasi-elastic component evidenced by Brillouin scattering and associated to the unresolved central peak observed in neutron scattering, is attributed to center of mass anharmonic motions and rattling of the molecular cations in the perovskite cavities. Its partially unexpressed critical behavior at the transition points toward the general importance of defects in HOPs preventing the net divergence of order parameter correlations at the critical temperatures.

The low frequency lattice vibrations and relaxations are investigated in single crystals of the four 3Dhybrid organolead perovskites, MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3, at the Brillouin zone centerusing Raman and Brillouin scattering and at the zone boundary using inelastic neutron scattering. Thetemperature dependence of the PbX6 lattice modes in the four compounds can be renormalized intouniversal curves and no soft vibration is observed excluding a displacive-like transitional dynamics. Thereorientational (pseudospin) motions of the molecular cations exhibit a seemingly order-disordercharacter recalling that of plastic crystals, but attributed to a secondary order-parameter. At ultra-lowfrequency, a quasi-elastic component evidenced by Brillouin scattering and associated to the unresolvedcentral peak observed in neutron scattering, is attributed to center of mass anharmonic motions andrattling of the molecular cations in the perovskite cavities.

Amorphous–amorphous transformations under pressure are generally explained by changes in the local structure from low- to higher-fold coordinated polyhedra1–4. However, as the notion of scale invariance at the critical thresholds has not been addressed, it is still unclear whether these transformations behave similarly to true phase transitions in related crystals and liquids. Here we report ab initio-based calculations of compressed silica (SiO2) glasses, showing that the structural changes from low- to high-density amorphous structures occur through a sequence of percolation transitions. When the pressure is increased to 82 GPa, a series of long-range (‘infinite’) percolating clusters composed of corner- or edge-shared tetrahedra, pentahedra and eventually octahedra emerge at critical pressures and replace the previous ‘phase’ of lower-fold coordinated polyhedra and lower connectivity. This mechanism provides a natural explanation for the well-known mechanical anomaly around 3 GPa, as well as the structural irreversibility beyond 10 GPa, among other features. Some of the amorphous structures that have been discovered mimic those of coesite IV and V crystals reported recently5,6, highlighting the major role of SiO5 pentahedron-based polyamorphs in the densification process of vitreous silica. Our results demonstrate that percolation theory provides a robust framework to understand the nature and pathway of amorphous–amorphous transformations and open a new avenue to predict unravelled amorphous solid states and related liquid phases7,8.