In the present Letter, we provide direct experimental evidence of the existence of short-range octahedra distortion in MAPbBr3 in the tetragonal and cubic phases. Such distortion is analogous to that found in the ordered orthorhombic phase and increases with temperature and symmetry of the unit cell. Such local distortion is most probably correlated with the presence of hydrogen bonding between MA and the PbBr6 framework, and its evolution with temperature is related to the increase of rotational degrees of freedom of the MA group.

The local structure of MAPbCl3 has been investigated through the combined use of X-ray and neutron pair distribution function (PDF) analysis and computational modeling. The results indicate the presence of a significant level of distortion at the short-range, irrespective of the average structure found by diffraction, and this suggests a possible correlation between the type of distortion and the features of the strongest H–X bond. The distortion of the octahedra decreases by reducing temperature, but the overall tilting of the octahedral system increases, and such tilting depends on the size of the anion. The comparison of the short-range structural data available on the MAPbX3 family strongly indicates a common pattern of the local structural distortion in these systems.

The evolution of the hybrid structure between 9R hexagonal perovskite and palmierite in the entire BaMoW NbO solid solution (where x = 0, 0.25, 0.5, 0.75, and 1) was probed in the 100-900 K range by synchrotron high-resolution powder diffraction. Each sample exhibits a chemical-dependent structural model in the low-temperature regime (from 100 to 500 K) in which 9R and palmierite structures compete each other, the former being progressively favored as tungsten replaces molybdenum. Above 500 K, unit cell parameters and metal site occupancies start to converge toward a similar structural arrangement that is completely reached at 900 K. In fact, at this temperature, the entire solid solution discloses comparable unit cell and an almost enterely occupied M1 site, with a structure that is much closer to palmierite rather than 9R polytype. The present crystallographic results well explain the behavior of the material's bulk ionic conductivity, whose temperature evolution for different compositions depends from the contribution of tetrahedral units proper of the palmierite structure.

Liquid-liquid transitions are interesting to many researchers since they occur in systems 19 as diverse as monoatomic liquids, multicomponent oxides and metallic glass-formers. In 20 some cases, the crossover is accompanied by changes in the dynamical properties, 21 although a direct microscopic evidence of this connection has not been reported so far. 22 By combining state-of-the-art synchrotron techniques, we followed the structure and 23 atomic motion during quasi-static cooling the Au49Cu26.9Si16.3Ag5.5Pd2.3 metallic glass-24 former from the low-temperature supercooled liquid. With this thermal protocol, we were 25 able to lower the glass transition temperature far enough to reveal a liquid-liquid 26 crossover between two amorphous structures corresponding to two ultra-viscous liquids 27 with different kinetic behavior. This transition is in competition with vitrification, which 28 occurs at conventional cooling rates, and is accompanied by structural changes not 29 affecting the average density. Our results provide a direct connection between 30 polyamorphism and dynamical crossover, and an alternative case to add in the highly-31 debated topic on the low-temperature divergence of the dynamics in supercooled liquids. 32 33 2Many systems, ranging from monoatomic liquids [1-3] to amorphous alloys [4][5][6][7], exhibit 34 transitions between liquid phases of different local structure. In glass-formers, liquid-liquid 35 transitions (LLTs) have been reported either in high temperature melts [3,[8][9][10][11][12][13] or in 36 supercooled liquids [4][5][6]14,15], suggesting the existence of intrinsic connections between the 37 kinetic properties of the system and the transition temperature [16]. LLTs are sometime 38 associated to a dynamical crossover with changes in the kinetic fragility [6,10,17]. When 39 observed upon cooling, the liquid evolves from a high temperature fragile phase with a steep 40 temperature dependence of viscosity and structural relaxation time, to a strong phase less 41 affected by temperature changes. For fragile liquids, like molecular and some metallic glass-42 formers, the LLT is expected at such a low temperature that the system may arrest beforehand 43 in the glass [16]. Still, no direct microscopic evidence of this and more in general of a direct 44 connection between LLT and dynamical crossover have been reported so far. This is also due 45 to the extreme difficulty of obtaining experimental measurements of the atomic collective 46 motion in glass-formers. 47 By taking advantage of state-of-the-art synchrotron techniques like x-ray photon correlation 48 spectroscopy (XPCS) and high energy x-ray diffraction (XRD), we present the first 49 experimental measurements of the microscopic dynamics of an ultra-viscous glass former with 50 relaxation times which were not accessible until now, neither experimentally nor with 51 numerical simulations. Our data show the occurrence of a LLT upon quasi-static cooling the 52 Au49Cu26.9Si16.3Ag5.5Pd2.3 metallic glass-former, which can be direct...

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In the present work, the substitution of cesium (Cs+) with rubidium (Rb+) in fully inorganic tin bromide perovskites Cs1–x Rb x SnBr3, has been experimentally demonstrated by synthesizing pure single-phase samples in the CsSnBr3–Cs0.70Rb0.30SnBr3 compositional range. The substitution of Cs with Rb is responsible for structural modification from cubic to orthorhombic symmetry, which has been correlated with optical properties, as the band gap varies from 1.719 to 1.817 eV from CsSnBr3 to Cs0.70Rb0.30SnBr3 sample. Notably, all of the rubidium-embedding alloys present good air stability. All of these results are very straightforward and open the possibility to exploit the electrical and optical capabilities of this very promising family of lead-free materials.

In the last few years the Ba3Mo1-xWxNbO8.5 system is attracting increasing attention because of its potential application as electrolyte in solid state fuel cells. The interest for the system arises...

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