Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH₃NH₃⁺ ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH₃NH₃PbI₃ crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1–200-ps time window. Monte Carlo simulations of interacting CH₃NH₃⁺ dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH₃NH₃⁺ in either antiferroelectric or ferroelectric domains. Collective realignment of CH₃NH₃⁺ to screen a device’s built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1–1 ms, faster than most observed hysteresis.

Understanding the molecular mechanism of proton conduction is crucial for the design of new materials with improved conductivity. Quasi-elastic neutron scattering (QENS) has been used to probe the mechanism of proton diffusion within a new phosphonate-based metal–organic framework (MOF) material, MFM-500(Ni). QENS suggests that the proton conductivity (4.5 × 10^–4 S/cm at 98% relative humidity and 25 °C) of MFM-500(Ni) is mediated by intrinsic “free diffusion inside a sphere”, representing the first example of such a mechanism observed in MOFs.

Little is presently known about the unique nanostructure of deep eutectic solvents (DES). The order of the liquid–solid phase transition is contended and whether DES-water mixtures are merely aqueous solutions, or have properties dominated by the eutectic pair, is unclear. Here, we unambiguously show the structure of choline chloride-malic acid (malicine) as a liquid, and also in solid and hydrated forms, using neutron total scattering on D/H isotope-substituted samples, and quasi-elastic neutron scattering (QENS). Data were refined using empirical potential structure refinement. We show evidence for a stoichiometric complex ion cluster in the disordered liquid, with strong choline–chloride bonding and a hydrogen bond donor (HBD) contribution. The 1:1 eutectic stoichiometry makes these ionic domains more well-defined, with less HBD clustering than seen previously for reline. There is minimal structural difference for the solidified material, demonstrating that this DES solidification is a glass transition rather than a first order phase change. QENS data support this by showing a gradual change in solvent dynamics rather than a step change. The DES structure is mostly retained upon hydration, with water acting both as a secondary smaller HBD at closer range to choline than malic acid, and forming transient wormlike aggregates. This new understanding of DES structure will aid understanding of the properties of these novel green solvents on the molecular length scale in chemical processes, as well as giving an insight into the apparent role of natural DESs in plant physiology.

Despite extensive efforts in experimental and computational studies, the microscopic understanding of dynamics of biological macromolecules remains a great challenge. It is known that hydrated proteins, DNA and RNA, exhibit a so-called “dynamic transition.” It appears as a sharp rise of their mean-squared atomic displacements $⟨r^2⟩$ at temperatures above $200–230\mathrm{K}$ . Even after a long history of studies, this sudden activation of biomolecular dynamics remains a puzzle and many contradicting models have been proposed. By combining neutron and dielectric spectroscopy data, we were able to follow protein dynamics over an extremely broad frequency range. Our results show that there is no sudden change in the dynamics of the protein at temperatures around $\sim 200–230\mathrm{K}$ . The protein’s relaxation time exhibits a smooth temperature variation over the temperature range of $180–300\mathrm{K}$ . Thus the experimentally observed sharp rise in $⟨r^2⟩$ is just a result of the protein’s structural relaxation reaching the limit of the experimental frequency window. The microscopic mechanism of the protein’s structural relaxation remains unclear.