Cation Dynamics as Structure Explorer in Hybrid Perovskites-The Case of MAPbI3.

Hybrid organic-inorganic perovskites exhibit remarkable potential as cost-effective and high-efficiency materials for photovoltaic applications. Their exceptional chemical tunability opens further routes for optimizing their optical and electronic properties through structural engineering. Nevertheless, the extraordinary softness of the lattice, stemming from its interconnected organic-inorganic composition, unveils formidable challenges in structural characterization. Here, by focusing on the quintessential methylammonium lead triiodide, MAPbI3, we combine first-principles modeling with high-resolution neutron scattering data to identify the key stationary points on its shallow potential energy landscape. This combined experimental and computational approach enables us to benchmark the performance of a collection of semilocal exchange-correlation functionals and to track the local distortions of the perovskite framework, hallmarked by the inelastic neutron scattering response of the organic cation. By conducting a thorough examination of structural distortions, we introduce the IKUR-PVP-1 structural data set. This data set contains nine mechanically stable structural models, each manifesting a distinct vibrational response. IKUR-PVP-1 constitutes a valuable resource for assessing thermal behavior in the low-temperature perovskite phase. In addition, it paves the way for the development of accurate force fields, enabling a comprehensive understanding of the interplay between the structure and dynamics in MAPbI3 and related hybrid perovskites.

Unraveling the Ordered Phase of the Quintessential Hybrid Perovskite MAPbI3─Thermophysics to the Rescue.

Hybrid perovskites continue to attract an enormous amount of attention, yet a robust microscopic picture of their different phases as well as the extent and nature of the disorder present remains elusive. Using specific-heat data along with high-resolution inelastic neutron scattering and ab initio modeling, we address this ongoing challenge for the case of the ordered phase of the quintessential hybrid-perovskite MAPbI3. At low temperatures, the specific heat of MAPbI3 reveals strong deviations from the Debye limit, a common feature of pure hybrid perovskites and their mixtures. Our thermophysical analysis demonstrates that the (otherwise ordered) structure around the organic moiety is characterized by a substantial lowering of the local symmetry relative to what can be inferred from crystallographic studies. The physical origin of the observed thermophysical anomalies is unequivocally linked to excitations of sub-terahertz optical phonons responsible for translational-librational distortions of the octahedral units.

Spectroscopic Signatures of Hydrogen-Bonding Motifs in Protonic Ionic Liquid Systems: Insights from Diethylammonium Nitrate in the Solid State.

Diethylammonium nitrate, [N0 0 2 2][NO3], and its perdeuterated analogue, [N D D 2 2] [NO3], were structurally characterized and studied by infrared, Raman, and inelastic neutron scattering (INS) spectroscopy. Using these experimental data along with state-of-the-art computational materials modeling, we report unambiguous spectroscopic signatures of hydrogen-bonding interactions between the two counterions. An exhaustive assignment of the spectral features observed with each technique has been provided, and a number of distinct modes related to NH···O dynamics have been identified. We put a particular emphasis on a detailed interpretation of the high-resolution, broadband INS experiments. In particular, the INS data highlight the importance of conformational degrees of freedom within the alkyl chains, a ubiquitous feature of ionic liquid (IL) systems. These findings also enable an in-depth physicochemical understanding of protonic IL systems, a first and necessary step to the tailoring of hydrogen-bonding networks in this important class of materials.

Dynamics & Spectroscopy with Neutrons-Recent Developments & Emerging Opportunities.

This work provides an up-to-date overview of recent developments in neutron spectroscopic techniques and associated computational tools to interrogate the structural properties and dynamical behavior of complex and disordered materials, with a focus on those of a soft and polymeric nature. These have and continue to pave the way for new scientific opportunities simply thought unthinkable not so long ago, and have particularly benefited from advances in high-resolution, broadband techniques spanning energy transfers from the meV to the eV. Topical areas include the identification and robust assignment of low-energy modes underpinning functionality in soft solids and supramolecular frameworks, or the quantification in the laboratory of hitherto unexplored nuclear quantum effects dictating thermodynamic properties. In addition to novel classes of materials, we also discuss recent discoveries around water and its phase diagram, which continue to surprise us. All throughout, emphasis is placed on linking these ongoing and exciting experimental and computational developments to specific scientific questions in the context of the discovery of new materials for sustainable technologies.

Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites.

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.

In silico Raman spectroscopy of YAlO3 single-crystalline film.

The Raman response of the YAlO3 (YAP) perovskite is modeled by means of periodic density functional theory. A number of different approximations to the exchange-correlation functional are benchmarked against the structural and spectroscopic data as imposing all-electron Gaussian-type basis sets. The WC1LYP functional was found to be superior, particularly outperforming other tested approaches in the prediction of the local structure of the AlO subunits, which reflects in the observed lattice-dynamics. The Raman response is further decomposed into the directional spectra, which are due to different components of the polarizability tensor, and confronted with the experimental Raman spectra, recorded in different scattering geometries of the single-crystalline film of YAP. The in silico lattice dynamics provides the unequivocal assignment of the observed bands with an excellent match to the experimental spectra, allowing for a complete analysis of the underlying phonon modes in terms of their energy, symmetry and the directional activity. The presented analysis serves as a high-quality reference, potentially useful in the future studies of other YAP materials, where Raman spectroscopy along with the X-Ray diffraction is the first method of choice.

Nuclear dynamics and phase polymorphism in solid formic acid.

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 ß1 and ß2 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.

Molecular and Vibrational Dynamics in the Cholesterol-Lowering Agent Lovastatin: Solid-State NMR, Inelastic Neutron Scattering, and Periodic DFT Study.

Molecular and vibrational dynamics of a widely used cholesterol-lowering agent, lovastatin, have been studied by combining nuclear magnetic resonance relaxation experiments (1H NMR) with inelastic neutron scattering (INS) and periodic density functional theory modeling (plane-wave DFT). According to a complementary experimental study, lovastatin shows no phase transitions down to cryogenic conditions, while a progressive, stepwise activation of several molecular motions is observed below room temperature. The molecular packing and intermolecular forces were analyzed theoretically, supported by a 13C NMR study and further correlated with observed molecular dynamics. The NMR relaxation experiments combined with theoretical calculations disclose that molecular dynamics in solid lovastatin is related to methyl group motions and conformational disorder in the methylbutanoate fragment. This is precisely assigned and analyzed quantitatively from both experimental and theoretical perspectives. The neutron vibrational spectroscopy further corroborates that the methyl rotors have a classical nature. In addition to the intramolecular reorientations, the vibrational dynamics was analyzed with an emphasis on the low-wavenumber range. For the first time, the terahertz response of lovastatin was studied by confronting neutron and optical techniques and clearly illustrating their complementarity. The consistent picture of the molecular dynamics is provided, which may support further considerations on alternative drug formulations and the amorphization tendency in this important lipid-lowering drug.

Unexpected Cation Dynamics in the Low-Temperature Phase of Methylammonium Lead Iodide: The Need for Improved Models.

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.

In search of the mutual relationship between the structure, solid-state spectroscopy and molecular dynamics in selected calcium channel blockers.

Three isostructural 1,4-dihydropyridines (DHPs), namely, nifedipine, nitrendipine and nimodipine were selected to characterize their structure, intermolecular interactions and molecular dynamics. The studied samples were analyzed using powder X-ray diffraction (XRD), neutron (INS) and infrared spectroscopy (FT-IR) as well as solid-state nuclear magnetic resonance (NMR), where each technique was supported by the state-of-the-art theoretical calculations for solid-state. By combining multiple experimental techniques with advanced theoretical calculations we were able to shed light on the mutual relation between the structure, stabilizing intermolecular interactions and their spectral response. For the first time, unambiguous computationally-supported assignment of the most prominent spectral features in DHPs is presented to give a valuable support for polymorph screening and drug control. Molecular motions were interpreted in details, revealing that a dynamic reservoir of each compound is dominated by intra-molecular reorientations of methyl groups and large-amplitude oscillations in terminal chains. Our study successfully validates the realm of applicability of first-principles solid-state calculations in search of the mutual relation between the structure and spectroscopy in this important class of drugs. Such approach gives a first necessary step to gather combined structure-dynamics data on functionalized DHPs, which are of importance to better understand crystallization and binding tendency. The NMR relaxation experiments reveal that nitro groups significantly hinder the reorientation of methyl rotors and provide the first evidence of low-temperature methyl-group tunneling in DHPs, an intriguing quantum-effect which is to be further explored.

Nuclear dynamics in the metastable phase of the solid acid caesium hydrogen sulfate.

High-resolution spectroscopic measurements using thermal and epithermal neutrons and first-principles calculations within the framework of density-functional theory are used to investigate the nuclear dynamics of light and heavy species in the metastable phase of caesium hydrogen sulfate. Within the generalised-gradient approximation, extensive calculations show that both 'standard' and 'hard' formulations of the Perdew-Burke-Ernzerhof functional supplemented by Tkatchenko-Scheffler dispersion corrections provide an excellent description of the known structure, underlying vibrational density of states, and nuclear momentum distributions measured at 10 and 300 K. Encouraged by the agreement between experiment and computational predictions, we provide a quantitative appraisal of the quantum contributions to nuclear motions in this solid acid. From this analysis, we find that only the heavier caesium atoms reach the classical limit at room temperature. Contrary to naïve expectation, sulfur exhibits a more pronounced quantum character relative to classical predictions than the lighter oxygen atom. We interpret this hitherto unexplored nuclear quantum effect as arising from the tighter binding environment of this species in this technologically relevant material.

Experimental (X-ray, (13)C CP/MAS NMR, IR, RS, INS, THz) and Solid-State DFT Study on (1:1) Co-Crystal of Bromanilic Acid and 2,6-Dimethylpyrazine.

A combined structural, vibrational spectroscopy, and solid-state DFT study of the hydrogen-bonded complex of bromanilic acid with 2,6-dimethylpyrazine is reported. The crystallographic structure was determined by means of low-temperature single-crystal X-ray diffraction, which reveals the molecular units in their native protonation states, forming one-dimensional infinite nets of moderate-strength O···H-N hydrogen bonds. The nature of the crystallographic forces, stabilizing the studied structure, has been drawn by employing the noncovalent interactions analysis. It was found that, in addition to the hydrogen bonding, the intermolecular forces are dominated by stacking interactions and C-H···O contacts. The thermal and calorimetric analysis was employed to probe stability of the crystal phase. The structural analysis was further supported by a computationally assisted (13)C CP/MAS NMR study, providing a complete assignment of the recorded resonances. The vibrational dynamics was explored by combining the optical (IR, Raman, TDs-THz) and inelastic neutron scattering (INS) spectroscopy techniques with the state-of-the-art solid-state density functional theory (DFT) computations. Despite the quasi-harmonic approximation assumed throughout the study, an excellent agreement between the theoretical and experimental data was achieved over the entire spectral range, allowing for a deep and possibly thorough understanding of the vibrational characteristics of the system. Particularly, the significant influence of the long-range dipole coupling on the IR spectrum has been revealed. On the basis of a wealth of information gathered, the recent implementation of a dispersion-corrected linear-response scheme has been extensively examined.

Polymorphism of resorcinol explored by complementary vibrational spectroscopy (FT-RS, THz-TDS, INS) and first-principles solid-state computations (plane-wave DFT).

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.

Experimental and solid-state computational study of structural and dynamic properties in the equilibrium form of temazepam.

Structural properties and rotational dynamics of methyl groups in the most stable form of temazepam were investigated by means of (13)C CP MAS NMR, quasielastic neutron scattering (QENS), and (1)H NMR spin-lattice relaxation methods. The QENS and (1)H NMR studies reveal the inequivalency of methyl groups, delivering their activation parameters. The structural properties of the system were explored in frame of periodic density functional theory (DFT) computations, giving insight into the reorientational barriers and providing understanding of the solid-state NMR results. The theoretical computations are shedding light on the intermolecular interactions along their relation with particular asymmetric structural units.

Solid-state DFT-assisted Raman study of titaniate nanostructures.

The first principle solid-state computations in frame of Density Functional Theory have been employed to analyze the Raman spectra of typical titaniate nanostructures. The Raman scattering studies of the nanotitaniates synthesised hydrothermally at different temperature conditions are reported. Local Density Approximation in combination with linear-response computations have delivered detailed analysis of Raman spectra based on the reference Na2Ti3O7 and NaHTi3O7 structures. The interpretation of the most prominent spectral features commonly reported in the literature have been postulated.

Dynamics and the mesomorphic properties of a novel antiferroelectric liquid crystalline thiobenzoate MHPSBO10: thermal, optical and dielectric spectroscopy study.

The complementary studies of the mesomorphic properties of a novel antiferroelectric liquid crystal (AFLC) (S)-2-octile 4-S-(4'decyloxybiphenyl-4-tiocarboxy)benzoate, known under MHPSBO10 acronym have been undertaken. The polymorphism has been complementary studied in details by Differential Scanning Calorimetry (DSC), Transmitted Light Intensity (TLI) and Polarization Microscopy (POM). The switching characteristics along with multiple macroscopic parameters describing the mesomorphic properties were determined by using electro-optic measurements, both upon cooling and heating. Frequency domain dielectric spectroscopy (DS), covering a wide frequency range, has been applied to characterize the molecular motions. Several collective modes, including the low frequency processes in the condensed hexatic phase were detected, analyzed in details and followed with the temperature. The presented studies deliver a wide report of the phase transitions, molecular dynamics and the macroscopic properties of the novel antiferroelectric thiobenzoate.

Temperature-dependent infrared spectroscopy studies of a novel antiferroelectric liquid-crystalline thiobenzoate.

The temperature-dependent infrared spectroscopy studies of one novel antiferroelectric liquid crystal (AFLC), known under the MHPSBO10 acronym, have been undertaken. The FT-IR measurements have been performed for homeotropic and planar heterogeneous sample geometries. The main order parameters have been determined and followed with temperature. The presented study delivers complex insight into the evolution of the vibrational spectrum upon phase transitions, covering the whole mesophase range. The experimental studies have been supported by theoretical studies of MHPSBO10 in confined geometries.

Complex vibrational analysis of an antiferroelectric liquid crystal based on solid-state oriented quantum chemical calculations and experimental molecular spectroscopy.

The experimental and theoretical vibrational spectroscopic study of one of a novel antiferroelectric liquid crystals (AFLC), known under the MHPSBO10 acronym, have been undertaken. The interpretation of both FT-IR and FT-Raman spectra was focused mainly on the solid-state data. To analyze the experimental results along with the molecular properties, density functional theory (DFT) computations were performed using several modern theoretical approaches. The presented calculations were performed within the isolated molecule model, probing the performance of modern exchange-correlations functionals, as well as going beyond, i.e., within hybrid (ONIOM) and periodic boundary conditions (PBC) methodologies. A detailed band assignment was supported by the normal-mode analysis with SQM ab initio force field scaling. The results are supplemented by the noncovalent interactions analysis (NCI). The relatively noticeable spectral differences observed upon Crystal to AFLC phase transition have also been reported. For the most prominent vibrational modes, the geometries of the transition dipole moments along with the main components of vibrational polarizability were analyzed in terms of the molecular frame. One of the goals of the paper was to optimize the procedure of solid-state calculations to obtain the results comparable with the all electron calculations, performed routinely for isolated molecules, and to test their performance. The presented study delivers a complex insight into the vibrational spectrum with a noticeable improvement of the theoretical results obtained for significantly attracting mesogens using modern molecular modeling approaches. The presented modeling conditions are very promising for further description of similar large molecular crystals.

Experimental (FT-IR and FT-RS) and theoretical (DFT) studies of molecular structure and internal modes of [Mn(NH3)6](NO3)2.

Fourier-transform Raman and infrared spectra of [Mn(NH(3))(6)](NO(3))(2) were recorded and interpreted by comparison with respective theoretical spectra calculated using DFT method. The [Mn(NH(3))(6)](2+) cation equilibrium geometry with C(1) symmetry, harmonic vibrational frequencies, infrared and Raman scattering intensities were determined using B3LYP/LAN2LTZ+/6-311+G(d,p) level of theory. The band assignment was based on potential energy distribution (PED) of normal modes. The computations of NO(3)(-) anion frequencies were performed under assumption of D(3h) symmetry, using 6-311+G(d,p) basis set. In order do draw a comparison, additional calculations were performed separately for the [Cd(NH(3))(6)](NO(3))(2) and [Ni(NH(3))(6)](NO(3))(2). The computations were also carried out using selected modern exchange-correlation functionals. A sufficient general agreement between the theoretical and the experimental spectra has been achieved.

Vibrations and reorientations of H2O molecules and NO3(-) anions in [Ca(H2O)4](NO3)2 studied by incoherent inelastic neutron scattering, Raman light scattering, and infrared absorption spectroscopy.

The vibrational and reorientational motions of H(2)O ligands and NO(3)(-) anions were investigated by Fourier transform middle-infrared Raman scattering (RS) spectroscopy and phonon density of states, calculated from incoherent inelastic neutron scattering, in the high- and low-temperature phases of [Ca(H(2)O)(4)](NO(3))(2). The theoretical IR and RS spectra were also calculated by means of the quantum chemistry method using density functional theory with PBE1PBE functional at 6-311++G(2d,2p) basis set level. The temperature dependences of the full width at half maximum values of nu(s)(H(2)O) bands in both the infrared absorption and the RS spectroscopy suggest that the observed phase transitions (at T(C1) and T(C2)) are not connected with a drastic change in the speed of H(2)O reorientational motions. However, similar Raman nu(4)(NO(3)(-)) band shape measurements as a function of temperature revealed the existence of a fast NO(3)(-) reorientation in phase I, which is abruptly slowed at the phase transition at T(C1). Activation energy values for the reorientational motions of H(2)O ligands and NO(3)(-) anions were calculated.