Small-angle scattering is a commonly used tool to analyze the dispersion of nanoparticles in all kinds of matrices. Besides some obvious cases, the associated structure factor is often complex and cannot be reduced to a simple interparticle interaction, like excluded volume only. In recent experiments, we have encountered a surprising absence of structure factors (S(q) = 1) in scattering from rather concentrated polymer nanocomposites [A.-C. Genix et al, ACS Appl. Mater. Interfaces 11 (2019) 17863]. In this case, quite pure form factor scattering is observed. This somewhat “ideal” structure is further investigated here making use of reverse Monte Carlo simulations in order to shed light on the corresponding nanoparticle structure in space. By fixing the target “experimental” apparent structure factor to one over a given q-range in these simulations, we show that it is possible to find dispersions with this property. The influence of nanoparticle volume fraction and polydispersity has been investigated, and it was found that for high concentrations only a high polydispersity allows reaching a state of S = 1. The underlying structure in real space is discussed in terms of the pair-correlation function, which evidences the importance of attractive interactions between polydisperse nanoparticles. The calculation of partial structure factors shows that there is no specific ordering of large or small particles, but that the presence of attractive interactions together with polydispersity allows reaching an almost “structureless” state.

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)