High-resolution inelastic neutron scattering and extensive first-principles calculations have been used to explore the low-temperature phase of the hybrid solar-cell material methylammonium lead iodide up to the well-known phase transition to the tetragonal phase at ca. 160 K. Contrary to original expectation, we find that the Pnma structure for this phase can only provide a qualitative description of the geometry and underlying motions of the organic cation. A substantial lowering of the local symmetry inside the perovskite cage leads to an improved atomistic model that can account for all available spectroscopic and thermodynamic data, both at low temperatures and in the vicinity of the aforementioned phase transition. Further and detailed analysis of the first-principles calculations reveals that large-amplitude distortions of the inorganic framework are driven by both zero-point-energy fluctuations and thermally activated cation motions. These effects are significant down to liquid-helium temperatures. For this important class of technological materials, this work brings to the fore the pressing need to bridge the gap between the long-range order seen by crystallographic methods and the local environment around the organic cation probed by neutron spectroscopy.

Synthetic graphene oxide (GO) as well as the product of its reducing performed in the regime of hydrogenolysis (rGO) were studied by both elastic and inelastic neutron scattering at low and room temperature conditions. The neutron diffraction patterns were analyzed to confirm stacking structures of both species consisting of 2−3 and ∼7 layers of microsize lateral dimension and the interlayer distances of 7 and 3.5 Å, respectively. The inelastic incoherent neutron scattering spectra were analyzed in the frame of the computationally supported one-phonon amplitude-weighted density of vibrational states G(ω) approximation. Calculations of G(ω) spectra were performed in the framework of semilocal density functional theory. The computational models were adjusted to the atom mass content of both GO and rGO species. The presented study has revealed the retained water in the freshly made GO, corresponding to the relatively low humidity, which further reacts with the oxygen-containing groups at the GO basal planes. The reaction results in the formation of hydroxyls chemically bound to the GO core in the course of the prolong storage of the product under ambient conditions. The analysis of the rGO G(ω) spectrum has disclosed the chemical composition of its circumference attributing the latter to sets of CH units with a minor presence of atomic oxygen.

The vibrational dynamics of pure and methylammonium-doped formamidinium lead iodide perovskites (FAPbI3) has been investigated by high-resolution neutron spectroscopy. For the first time, we provide an exhaustive and accurate analysis of the cation vibrations and underlying local structure around the organic moiety in these materials using first-principles electronic-structure calculations validated by the neutron data. Inelastic neutron scattering experiments on FAPbI3 provide direct evidence of the formation of a low-temperature orientational glass, unveiling the physicochemical origin of phase metastability in the tetragonal structure. Further analysis of these data provides a suitable starting point to explore and understand the stabilization of the perovskite framework via doping with small amounts of organic cations. In particular, we find that the hydrogen-bonding interactions around the formamidinium cations are strengthened as a result of cage deformation. This synergistic effect across perovskite cages is accompanied by a concomitant weakening of the methylammonium interactions with the surrounding framework.

The polymorphism of resorcinol has been complementary studied by combining Raman, time-domain terahertz, and inelastic neutron scattering spectroscopy with modern solid-state density functional theory (DFT) calculations. The spectral differences, emerging from the temperature-induced structural phase transition, have been successfully interpreted with an emphasis on the low-wavenumber range. The given interpretation is based on the plane-wave DFT computations, providing an excellent overall reproduction of both wavenumbers and intensities and revealing the source of the observed spectral differences. The performance of the generalized gradient approximation (GGA) functionals in prediction of the structural parameters and the vibrational spectra of the normal-pressure polymorphs of resorcinol has been extensively examined. The results show that the standard Perdew, Burke, and Ernzerhof (PBE) approach along with its "hard" revised form tends to be superior if compared to the "soft" GGA approximation.

We apply a unique sequence of structural and dynamical neutron-scattering techniques, augmented with density-functional electronic-structure calculations, to establish the degree of polymorphism in an archetypal hydrogen-bonded system - crystalline formic acid. Using this combination of experimental and theoretical techniques, the hypothesis by Zelsmann on the coexistence of the β and β phases above 220 K is tested. Contrary to the postulated scenario of proton-transfer-driven phase coexistence, the emerging picture is one of a quantitatively different structural change over this temperature range, whereby the loosening of crystal packing promotes temperature-induced shearing of the hydrogen-bonded chains. The presented work, therefore, solves a fifty-year-old puzzle and provides a suitable framework for the use neutron-Compton-scattering techniques in the exploration of phase polymorphism in condensed matter.

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