We provide a semi-empirical model based on in situ degradation measurements to predict the durability of hybrid perovskite materials under simulated thermal operation conditions. In the model, the degradation path of MAPbI3 layers is proved to follow an Arrhenius-type law. The predictive role is played by the activation energy combined with its pre-exponential factor. Our comparative study under moisture conditions with respect to vacuum and nitrogen treatments has assessed the occurrence of an intrinsic dynamic exchange of protons between the organic cations and the inorganic cage with a direct impact on the lattice stability, for which the presence of water molecules is not mandatory. This mutual interaction produces defects inside the material and volatile species, such as HI, CH3NH2 or MAI, with an associated experimental activation energy of 1.54 eV measured under vacuum conditions in dark. This value is comparable to that calculated by the density functional theory for defect generation in MAPbI3. In air, the action of water molecules reduces the activation energy for proton exchanges in dark to 0.96 eV. As an alternative solution to increase the material stability, we demonstrate that the substitution of methylammonium (MA(+)) with the formamidinium (FA(+)) cations inside the inorganic cage gives greater robustness to the overall lattice and extends the material durability due to a different interaction between the organic molecules and the inorganic cage. This definitely supports the use of FAPbI3 in applications, provided its structure can be stabilized in the dark phase at room temperature.

The role of chloride in the MAPbI3-xClx perovskite is still limitedly understood, albeit subjected of much debate. Here, we present a combined angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and first-principles DFT modeling to investigate the MAPbI3-xClx/TiO2 interface. AR-XPS analyses carried out on ad hoc designed bilayers of MAPbI3-xClx perovskite deposited onto a flat TiO2 substrate reveal that the chloride is preferentially located in close proximity to the perovskite/TiO2 interface. DFT calculations indicate the preferential location of chloride at the TiO2 interface compared to the bulk perovskite due to an increased chloride-TiO2 surface affinity. Furthermore, our calculations clearly demonstrate an interfacial chloride-induced band bending, creating a directional "electron funnel" that may improve the charge collection efficiency of the device and possibly affecting also recombination pathways. Our findings represent a step forward to the rationalization of the peculiar properties of mixed halide perovskite, allowing one to further address material and device design issues.

We study the mechanisms of CH3NH3PbI3 degradation and its transformation to PbI2 by means of X-ray diffraction and the density functional theory. The experimental analysis shows that the material can degrade in both air and vacuum conditions, with humidity and temperature-annealing strongly accelerating such process. Based on ab initio calculations, we argue that even in the absence of humidity, a decomposition of the perovskite structure can take place through the statistical formation of molecular defects with a non-ionic character, whose volatility at surfaces should break the thermodynamic defect equilibria. We finally discuss the strategies that can limit such phenomenon and subsequently prolong the lifetime of the material.

Tunable single-molecule magnets: The spin-level landscape in a series of Fe(III) (4) single-molecule magnets with propeller-like structure was analyzed by means of high-frequency EPR spectroscopy. The zero-field splitting parameter D of the ground S=5 spin state correlates strongly with the pitch of the propeller gamma (see picture), and thus provides a simple link between molecular structure and magnetic behavior.We report three novel tetrairon(III) single-molecule magnets with formula [Fe(4)(L)(2)(dpm)(6)] (Hdpm=2,2,6,6-tetramethylheptane-3,5-dione), prepared by using pentaerythritol monoether ligands H(3)L=R'OCH(2)C(CH(2)OH)(3) with R'=allyl (1), (R,S)-2-methyl-1-butyl (2), and (S)-2-methyl-1-butyl (3), along with a new crystal phase of the complex containing H(3)L=11-(acetylthio)-2,2-bis(hydroxymethyl)- undecan-1-ol (4). High-frequency EPR (HF-EPR) spectra at low temperature were collected on powder samples in order to determine the zero-field splitting (zfs) parameters in the ground S=5 spin state. In 1-4 and in other eight isostructural compounds previously reported, a remarkable correlation is found between the axial zfs parameter D and the pitch gamma of the propeller-like structure. The relationship is directly demonstrated by 1, which features both structurally and magnetically inequivalent molecules in the crystal. The dynamics of magnetization has been investigated by ac susceptometry, and the results analyzed by master-matrix calculations. The large rhombicities of 2 and 3 were found to be responsible for the fast magnetic relaxation observed in the two compounds. However, complex 3 shows an additional faster relaxation mechanism which is unaccounted for by the set of spin Hamiltonian parameters determined by HF-EPR.

We investigate the degradation path of MAPbI3 (MA=methylammonium) films over flat TiO2 substrates at room temperature by means of X-ray diffraction, spectroscopic ellipsometry, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The degradation dynamics is found to be similar in air and under vacuum conditions, which leads to the conclusion that the occurrence of intrinsic thermodynamic mechanisms is not necessarily linked to humidity. The process has an early stage, which drives the starting tetragonal lattice in the direction of a cubic atomic arrangement. This early stage is followed by a phase change towards PbI2 . We describe how this degradation product is structurally coupled with the original MAPbI3 lattice through the orientation of its constituent PbI6 octahedra. Our results suggest a slight octahedral rearrangement after volatilization of HI+CH3 NH2 or MAI, with a relatively low energy cost. Our experiments also clarify why reducing the interfaces and internal defects in the perovskite lattice enhances the stability of the material.