A thermodynamic unification of jamming

Fragile materials ranging from sand to fire-retardant to toothpaste are able to exhibit both solid and fluid-like properties across the jamming transition. Unlike ordinary fusion, systems of grains, foams and colloids jam and cease to flow under conditions that still remain unknown. Here we quantify jamming via a thermodynamic approach by accounting for the structural ageing and the shear-induced compressibility of dry sand. Specifically, the jamming threshold is defined using a non-thermal temperature that measures the 'fluffiness' of a granular mixture. The thermodynamic model, casted in terms of pressure, temperature and free-volume, also successfully predicts the entropic data of five molecular glasses. Notably, the predicted configurational entropy avoids the Kauzmann paradox entirely. Without any free parameters, the proposed equation-of-state also governs the mechanism of shear-banding and the associated features of shear-softening and thickness-invariance.Comment: 16 pgs double spaced. 4 figure

A new potential for methylammonium lead iodide.

We present a new set of interatomic potentials for modelling methylammonium lead iodide. The potential model uses existing potentials for lead iodide and methylammonium, and new functions are fitted to enable these pre-existing potentials to be used together, while still being capable of modelling lead iodide and methylammonium iodide as separate materials. Fitting was performed using a combination of ab initio and experimental reference data. Our simulations are in agreement with experiment and reveal the short and long range ordering of the molecular cations and lead iodide octahedra

Principles of Chemical Bonding and Band Gap Engineering in Hybrid Organic–Inorganic Halide Perovskites

The performance of solar cells based on hybrid halide perovskites has seen an unparalleled rate of progress, while our understanding of the underlying physical chemistry of these materials trails behind. Superficially, CH3NH3PbI3 is similar to other thin-film photovoltaic materials: a semiconductor with an optical band gap in the optimal region of the electromagnetic spectrum. Microscopically, the material is more unconventional. Progress in our understanding of the local and long-range chemical bonding of hybrid perovskites is discussed here, drawing from a series of computational studies involving electronic structure, molecular dynamics, and Monte Carlo simulation techniques. The orientational freedom of the dipolar methylammonium ion gives rise to temperature-dependent dielectric screening and the possibility for the formation of polar (ferroelectric) domains. The ability to independently substitute on the A, B, and X lattice sites provides the means to tune the optoelectronic properties. Finally, ten critical challenges and opportunities for physical chemists are highlighted

Inversion symmetry and bulk Rashba effect in methylammonium lead iodide perovskite single crystals

Methylammonium lead iodide perovskite (MAPbI_3) exhibits long charge carrier lifetimes that are linked to its high efficiency in solar cells. Yet, the mechanisms governing these unusual carrier dynamics are not completely understood. A leading hypothesis—disproved in this work—is that a large, static bulk Rashba effect slows down carrier recombination. Here, using second harmonic generation rotational anisotropy measurements on MAPbI_3 crystals, we demonstrate that the bulk structure of tetragonal MAPbI_3 is centrosymmetric with I4/mcmspace group. Our calculations show that a significant Rashba splitting in the bandstructure requires a non-centrosymmetric lead iodide framework, and that incorrect structural relaxations are responsible for the previously predicted large Rashba effect. The small Rashba splitting allows us to compute effective masses in excellent agreement with experiment. Our findings rule out the presence of a large static Rashba effect in bulk MAPbI_3, and our measurements find no evidence of dynamic Rashba effects

Thermal Behavior of Water in Sephadex^® G25 Gels at Low Temperatures Studied by Adiabatic Calorimetry

Water in a crosslinked dextran gel, Sephadex® G25, is known to remain partially unfrozen during cooling and undergoes ice crystallization during rewarming. However, the mechanism of ice crystallization during rewarming is still unclear. To elucidate the frozen state of water in the gel, thermal behavior at low temperatures was investigated by using adiabatic calorimetry. Heat capacities and enthalpy-relaxation rates of the gel-containing water of mass ratio h (=g H2O/g dry G25) = 1.00 were measured between 80 and 300 K during rewarming, where the gel was intermittently heated at the rate of 50–100 mK min−1. Although an exotherm indicating ice crystallization during rewarming was confirmed with the gel precooled rapidly, at 5 K min−1, it disappeared when precooled slowly, at 20 mK min−1. During rewarming after the rapid cooling, two glass transitions were observed at ca. 175 K and 240–242 K. A higher-temperature glass transition due to the water trapped by the polymer network was not so clear, as it was overlapped with an endotherm due to the melting of small ice crystals, which indicates that the ice crystals formed have a broad size-distribution and that water in the gel is vitrified when ice crystals of even the smallest size cannot be formed