Dielectric constants of MAPbX3 (X = Br, I) in the 1 kHz–1 MHz range show strong temperature dependence near room temperature, in contrast to the nearly temperature-independent dielectric constant of CsPbBr3. This strong temperature dependence for MAPbX3 in the tetragonal phase is attributed to the MA+ dipoles rotating freely within the probing time scale. This interpretation is supported by ab initio molecular dynamics simulations on MAPbI3 that establish these dipoles as randomly oriented with a rotational relaxation time scale of ∼7 ps at 300 K. Further, we probe the intriguing possibility of transient polarization of these dipoles following a photoexcitation process with important consequences on the photovoltaic efficiency, using a photoexcitation pump and second harmonic generation efficiency as a probe with delay times spanning 100 fs–1.8 ns. The absence of a second harmonic signal at any delay time rules out the possibility of any transient ferroelectric state under photoexcitation.

Monte Carlo and molecular dynamics simulations and neutron scattering experiments are used to study the adsorption and diffusion of hydrogen and deuterium in zeolite Rho in the temperature range of 30-150 K. In the molecular simulations, quantum effects are incorporated via the Feynman-Hibbs variational approach. We suggest a new set of potential parameters for hydrogen, which can be used when Feynman-Hibbs variational approach is used for quantum corrections. The dynamic properties obtained from molecular dynamics simulations are in excellent agreement with the experimental results and show significant quantum effects on the transport at very low temperature. The molecular dynamics simulation results show that the quantum effect is very sensitive to pore dimensions and under suitable conditions can lead to a reverse kinetic molecular sieving with deuterium diffusing faster than hydrogen.

The high efficiency of lead organo-metal-halide perovskite solar cells has raised many questions about the role of the methylammonium (MA) molecules in the Pb-I framework. Experiments indicate that the MA molecules are able to 'freely' spin around at room temperature even though they carry an intrinsic dipole moment. We have performed large supercell (2592 atoms) finite temperature ab-initio molecular dynamics calculations to study the correlation between the molecules in the framework. An underlying long range anti-ferroelectric ordering of the molecular dipoles is observed. The dynamical correlation between neighboring molecules shows a maximum around room temperature in the mid-temperature phase. In this phase, the rotations are slow enough to (partially) couple to neighbors via the Pb-I cage. This results in a collective motion of neighboring molecules in which the cage acts as the mediator. At lower and higher temperatures the motions are less correlated.PACS numbers: 61.50. Ah, The spectacular rise of perovskite photovoltaics 1 has sparked much research effort into the physical mechanisms behind these materials' good photovoltaic performance. From a solid state physics perspective, the answer seems simple: the well suited electronic structure of methylammonium lead-iodide (MAPbI 3 ). With a band gap of ∼1.6 eV 2-6 and a high absorption coefficient 2,5,6 , it is expected to be a high efficiency solar cell material for the solar radiation spectrum observed on earth.7 Density Functional Theory (DFT) calculations have shown that an s-p mixture in the valence band maximum (VBM) and p-states in the conduction band minimum (CBM), combined with a direct band gap lead to an absorption coefficient up to an order of magnitude higher than GaAs.8 However, the crystal structure of MAPbI 3 is completely different from GaAs. It even possesses temperature depended dynamical contributions arising from the organic constituent. The perovskite structure of MAPbI 3 is composed of three iodine atoms (monovalent anions) combined with a lead atom (divalent cation) and a CH 3 NH 3 (MA) molecule (monovalent cation). The Pb-I framework forms the perovskite structure out of PbI 6 octahedra and the molecules are trapped in the cavities. Whether the cubic perovskite structure is stable or becomes orthorhombic or tetragonal is determined by the temperature 6 combined with a balance between the size of the molecule and the Pb-I bond length.9 NMR experiments have indicated that at high temperatures (>300 K) the MA molecule exhibits complete orientational disorder.10,11 This means that the MA molecules have enough kinetic energy to overcome the rotational barriers and can rotate in their 'cage'. Around room temperature this process has a typical relaxation time of ∼ 5 ps.12 These rotations apparently do not effect the charge carriers in the system, since very long electronhole diffusion lengths have been reported. 13,14 From an electronic point of view this is not surprising, because the Pb-I framework is electronically decoupled from the m...

We report kinetic molecular sieving of hydrogen and deuterium in zeolite rho at low temperatures, using atomistic molecular dynamics simulations incorporating quantum effects via the Feynman-Hibbs approach. We find that diffusivities of confined molecules decrease when quantum effects are considered, in contrast with bulk fluids which show an increase. Indeed, at low temperatures, a reverse kinetic sieving effect is demonstrated in which the heavier isotope, deuterium, diffuses faster than hydrogen. At 65 K, the flux selectivity is as high as 46, indicating a good potential for isotope separation.