Incoherent Inelastic Neutron Research Articles

Water dynamics in calcium silicate hydrates probed by inelastic neutron scattering and molecular dynamics simulations

Calcium-silicate-hydrate (C-S-H) is a disordered, nanocrystalline material, acting as a primary binding phase in Portland cement. C-S-H and C-A-S-H (an Al-bearing substitute present in low-CO2 cement) contain thin films of water on solid surfaces and inside nanopores. Water controls multiple chemical and mechanical properties of C-S-H, including drying shrinkage, ion transport, creep, and thermal behavior. Therefore, obtaining a fundamental understanding of its properties is essential. We applied a combination of inelastic incoherent neutron scattering and molecular dynamics simulations to unravel water dynamics in synthetic C-(A)-S-H conditioned at five hydration states (from drier to more hydrated) and with three Ca/Si ratios (0.9, 1, and 1.3). Our results converge towards a picture where the evolution from thin layers of interfacial water to bulk-like capillary water is dampened by the structure of C-(A)-S-H. In particular, the hydrophilic Ca2+ sites organize the distribution of interfacial C-(A)-S-H water.

Helical Organic and Inorganic Polymers.

Despite being a staple of synthetic plastics and biomolecules, helical polymers are scarcely studied with Gaussian-basis-set ab initio electron-correlated methods on an equal footing with molecules. This article introduces an ab initio second-order many-body Green's function [MBGF(2)] method with nondiagonal, frequency-dependent Dyson self-energy for infinite helical polymers using screw-axis-symmetry-adapted Gaussian-spherical-harmonics basis functions. Together with the Gaussian-basis-set density-functional theory for energies, analytical atomic forces, translational-period force, and helical-angle force, it can compute correlated energy, quasiparticle energy bands, structures, and vibrational frequencies of an infinite helical polymer, which smoothly converge at the corresponding oligomer results. These methods can handle incommensurable structures, which have an infinite translational period and are hard to characterize by any other method, just as efficiently as commensurable structures. We apply them to polyethylene (2/1 helix), polyacetylene (Peierls' system) and polytetrafluoroethylene (13/6 helix) to establish the quantitative accuracy of MBGF(2)/cc-pVDZ in simulating their (angle-resolved) ultraviolet photoelectron spectra and of B3LYP/cc-pVDZ or 6-31G** in reproducing their structures, infrared and Raman band positions, phonon dispersions, and (coherent and incoherent) inelastic neutron scattering spectra. We then predict the same properties for infinitely catenated chains of nitrogen or oxygen and discuss their possible metastable existence under ambient conditions. They include planar zigzag polyazene (N2)x (Peierls' system), 11/3-helical isotactic polyazane (NH)x, 9/4-helical isotactic polyfluoroazane (NF)x, and 7/2-helical polyoxane (O)x as potential high-energy-density materials.

Vibrational characterization of the various forms of (solvated or unsolvated) mobile proton in the solid state. Advantages, limitations and open questions

A didactic review of (Raman, infrared and neutron) vibrational spectroscopy procedures to study mobile protonic species (H2O, H3O+, H5O2+, “H+”, NH4+, etc.) in solid hydrates, crystals and ceramics is proposed on the basis of paper published since 1970s. Three materials are taken as representative examples: hydrated uranyl phosphate (HUP), oxonium, hydroxonium and ammonium beta- and beta“-aluminas and nominally anhydrous strontium/barium zirconate perovskite. Particular attention is given to the advantages of isotopic substitution and dilution measurements as a function of temperature and partial ion exchange. The vibrational signatures that we consider as being able to serve as references for the different kinds of proton species observed in the proton conductors are presented. We also discuss the signature of protonic species giving no vibrational signature - or whose Raman and infrared signatures are too weak to be clearly detected – that need to be better characterized and understood. The presence of a strong incoherent inelastic neutron scattering background appears characteristic of (mobile) proton conductors.

Incoherent Neutron Scattering and Terahertz Time-Domain Spectroscopy on Protein and Hydration Water

Incoherent inelastic and quasi-elastic neutron scattering (INS) and terahertz time-domain spectroscopy (THz-TDS) are spectroscopy methods that directly detect molecular dynamics, with an overlap in the measured energy regions of each method. Due to the different characteristics of their probes (i.e., neutron and light), the information obtained and the sample conditions suitable for each method differ. In this review, we introduce the differences in the quantum beam properties of the two methods and their associated advantages and disadvantages in molecular spectroscopy. Neutrons are scattered via interaction with nuclei; one characteristic of neutron scattering is a large incoherent scattering cross-section of a hydrogen atom. INS records the auto-correlation functions of atomic positions. By using the difference in neutron scattering cross-sections of isotopes in multi-component systems, some molecules can be selectively observed. In contrast, THz-TDS observes the cross-correlation function of dipole moments. In water-containing biomolecular samples, the absorption of water molecules is particularly large. While INS requires large-scale experimental facilities, such as accelerators and nuclear reactors, THz-TDS can be performed at the laboratory level. In the analysis of water molecule dynamics, INS is primarily sensitive to translational diffusion motion, while THz-TDS observes rotational motion in the spectrum. The two techniques are complementary in many respects, and a combination of the two is very useful in analyzing the dynamics of biomolecules and hydration water.

Hydrogen vibration excitations of ZrH1.8 and TiH1.84 up to 21 GPa by incoherent inelastic neutron scattering

Hydrogen vibration excitations of fluorite-type $\mathrm{Zr}{\mathrm{H}}_{1.8}$ and $\mathrm{Ti}{\mathrm{H}}_{1.84}$ were investigated at pressures up to 21 and 4 GPa, respectively, by incoherent inelastic neutron scattering experiments. The excitations of both the samples were well described by quantum harmonic oscillator (QHO) over the entire pressure region of this study. The first excitation energies increased with increasing pressure, as described by the equations ${E}_{1}(\mathrm{meV})=141.4(2)+1.02(2)P$ (GPa) and ${E}_{1}\phantom{\rule{0.16em}{0ex}}(\mathrm{meV})=149.4(1)+1.21(8)P$ (GPa) for $\mathrm{Zr}{\mathrm{H}}_{1.8}$ and $\mathrm{Ti}{\mathrm{H}}_{1.84}$, respectively. Coupling with pressure dependence of lattice parameters determined by diffraction experiments, the relations between metal-hydrogen distance $({d}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{H}})$ and ${E}_{1}$ of $\mathrm{Zr}{\mathrm{H}}_{1.8}$ and $\mathrm{Ti}{\mathrm{H}}_{1.84}$ at high pressures are found to be well described by the equations ${E}_{1}(\mathrm{meV})=1.62(9)\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}{d}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{H}}^{\ensuremath{-}3.32\phantom{\rule{0.16em}{0ex}}(7)}(\AA{})$ and ${E}_{1} (\mathrm{meV})=1.47(21)\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}{d}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{H}}^{\ensuremath{-}3.5(2)}(\AA{})$, respectively. The slopes of these curves are very steep compared to the previously reported trend in various fluorite-type metal hydrides at ambient pressure, suggesting that pressure and chemical substitution affect the hydrogen vibration excitations differently. The hydrogen wave function spreading estimated from the ${E}_{1}$ value assuming the QHO model showed the preferential shrinkage of the wave function to the tetrahedral sites, suggesting that the local potential field for a hydrogen atom shrinks more intensively than the tetrahedral site. The preferential shrinkage of the hydrogen wave function and the steep rise in the ${E}_{1}$ at a small ${d}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{H}}$ under pressure are likely caused by the rigid metal ion core compared to hydrogen atoms and the resulting confinement of the hydrogen atom in the narrower potential field at high pressures.

Quantum thermodynamics of hydrogen in nano-structured materials—H2 in carbon nanotubes

Abstract Modern quantum theory of correlations predicts, and reveals, new counter-intuitive dynamical effects of hydrogen in nano-structured materials, being of considerable importance for basic research as well as for technological applications. To support this claim, here the focus is on H2 molecules in carbon nanotubes and other nanocavities, as experimentally investigated with the well established technique of incoherent inelastic neutron scattering (INS). In particular, the experimentally determined momentum and energy transfers from an H2 molecule in a carbon (C-) nanotube resulting in roto-translational motion along the nanotube axis, appear to (1) either violate the standard conservations laws, or (2) attribute the translating H2 molecule a highly reduced inertia, as quantified by the effective mass M eff ≈ 0.64 a.m.u. (atomic mass units), instead of 2 a.m.u. for an isolated molecule. This counter-intuitive INS-observation has no conventional interpretation, but it can be qualitatively interpreted “from first principles” in the frame of modern theories of quantumness of correlations and Quantum Thermodynamics (QTD). This analysis also provides a surprising new physical insight: The nano-structured cavities do represent quantum systems which contribute to the quantum dynamics of the hydrogen translational dynamics, and thus to the hydrogen transport in the studied materials. This insight may also have far-reaching consequences for technological applications and material sciences (e.g. fuel cells, H storage materials, etc.), since it concerns the choice and design of “optimized” materials adsorbing or carrying hydrogen.

Spectroscopic Identification of Hydrogen Bond Vibrations and Quasi-Isostructural Polymorphism in N-Salicylideneaniline.

The ortho-hydroxy aryl Schiff base 2-[(E)-(phenylimino)methyl]phenol and its deutero-derivative have been studied by the inelastic incoherent neutron scattering (IINS), infrared (IR) and Raman experimental methods, as well as by Density Functional Theory (DFT) and Density-Functional Perturbation Theory (DFPT) simulations. The assignments of vibrational modes within the 3500–50 cm−1 spectral region made it possible to state that the strong hydrogen bond in the studied compound can be classified as the so-called quasi-aromatic bond. The isotopic substitution supplemented by the results of DFT calculations allowed us to identify vibrational bands associated with all five major hydrogen bond vibrations. Quasi-isostructural polymorphism of 2-[(E)-(phenylimino)methyl]phenol (SA) and 2-[(E)-(phenyl-D5-imino)methyl]phenol (SA-C6D5) has been studied by powder X-ray diffraction in the 20–320 K temperature range.

Polymorphism and Conformational Equilibrium of Nitro-Acetophenone in Solid State and under Matrix Conditions.

Conformational and polymorphic states in the nitro-derivative of o-hydroxy acetophenone have been studied by experimental and theoretical methods. The potential energy curves for the rotation of the nitro group and isomerization of the hydroxyl group have been calculated by density functional theory (DFT) to estimate the barriers of the conformational changes. Two polymorphic forms of the studied compound were obtained by the slow and fast evaporation of polar and non-polar solutions, respectively. Both of the polymorphs were investigated by Infrared-Red (IR) and Raman spectroscopy, Incoherent Inelastic Neutron Scattering (IINS), X-ray diffraction, nuclear quadrupole resonance spectroscopy (NQR), differential scanning calorimetry (DSC) and density functional theory (DFT) methods. In one of the polymorphs, the existence of a phase transition was shown. The position of the nitro group and its impact on the crystal cell of the studied compound were analyzed. The conformational equilibrium determined by the reorientation of the hydroxyl group was observed under argon matrix isolation. An analysis of vibrational spectra was achieved for the interpretation of conformational equilibrium. The infrared spectra were measured in a wide temperature range to reveal the spectral bands that were the most sensitive to the phase transition and conformational equilibrium. The results showed the interrelations between intramolecular processes and macroscopic phenomena in the studied compound.

Proton Dynamics in Palladium-Silver: An Inelastic Neutron Scattering Investigation.

Proton dynamics in Pd77Ag23 membranes is investigated by means of various neutron spectroscopic techniques, namely Quasi Elastic Neutron Scattering, Incoherent Inelastic Neutron Scattering, Neutron Transmission, and Deep Inelastic Neutron Scattering. Measurements carried out at the ISIS spallation neutron source using OSIRIS, MARI and VESUVIO spectrometers were performed at pressures of 1, 2, and 4 bar, and temperatures in the 330–673 K range. The energy interval spanned by the different instruments provides information on the proton dynamics in a time scale ranging from about 102 to 10−4 ps. The main finding is that the macroscopic diffusion process is determined by microscopic jump diffusion. In addition, the vibrational density of states of the H atoms in the metal lattice has been determined for a number of H concentrations and temperatures. These measurements follow a series of neutron diffraction experiments performed on the same sample and thus provide a complementary information for a thorough description of structural and dynamical properties of H-loaded Pd-Ag membranes.

Symmetry/Asymmetry of the NHN Hydrogen Bond in Protonated 1,8-Bis(dimethylamino)naphthalene

Experimental and theoretical results are presented based on vibrational spectra and motional dynamics of 1,8-bis(dimethylamino)naphthalene (DMAN) and its protonated forms (DMANH+ and the DMANH+ HSO4− complex). The studies of these compounds have been performed in the gas phase and solid-state. Spectroscopic investigations were carried out by infrared spectroscopy (IR), Raman, and incoherent inelastic neutron scattering (IINS) experimental methods. Density functional theory (DFT) and Car–Parrinello molecular dynamics (CPMD) methods were applied to support our experimental findings. The fundamental investigations of hydrogen bridge vibrations were accomplished on the basis of isotopic substitutions (NH → ND). Special attention was paid to the bridged proton dynamics in the DMANH+ complex, which was found to be affected by interactions with the HSO4− anion.

Inter- vs. Intramolecular Hydrogen Bond Patterns and Proton Dynamics in Nitrophthalic Acid Associates.

Noncovalent interactions are among the main tools of molecular engineering. Rational molecular design requires knowledge about a result of interplay between given structural moieties within a given phase state. We herein report a study of intra- and intermolecular interactions of 3-nitrophthalic and 4-nitrophthalic acids in the gas, liquid, and solid phases. A combination of the Infrared, Raman, Nuclear Magnetic Resonance, and Incoherent Inelastic Neutron Scattering spectroscopies and the Car–Parrinello Molecular Dynamics and Density Functional Theory calculations was used. This integrated approach made it possible to assess the balance of repulsive and attractive intramolecular interactions between adjacent carboxyl groups as well as to study the dependence of this balance on steric confinement and the effect of this balance on intermolecular interactions of the carboxyl groups.

Terahertz collective dynamics of DNA as affected by hydration and counterions

Local and propagating vibrations occurring in DNA in the terahertz domain are essential for processes at the basis of cellular metabolism. Here we report the results of a study on the B-DNA terahertz dynamics where we combined high-resolution inelastic x-ray experiments and incoherent inelastic neutron scattering. By using two different high-hydration conditions we could study the effect of packing interactions between double helices, while by selecting Na and Cs counterions we could inspect how the mass loading affects the DNA low-frequency modes. The pattern of coherent excitation energies is well represented in terms of two branches, an acoustic-like one, with an associated propagation velocity of 3000 ± 100 m/s, and the other almost dispersionless at ~2 meV. This picture is also supported by the vibrational density of states projected on the hydrogen atoms. The acoustic-like mode is assigned to DNA excitations, despite its intriguing similarity with the analogous acoustic mode of bulk water at low wave-vector transfers. We also provide evidence for the intrahelical optic-like character of the low-energy mode, that we ascribe to large scale relative motions of DNA sub-domains.

Weak Values and Quantum Information in Scattering Physics — New Theoretical and Experimental Effects

Weak Values (WV) and Two-State-Vector Formalism (TSVF) provide novel insights in various quantum physical and technological fields. In the first part of the paper we consider a new quantum effect of scattering accompanying an elementary collision of two quantum systems A and B, in which the latter interacts with a quantum environment. In clear contrast to a classical environment, the quantum case can exhibit counter-intuitive effects of momentum- and energy-transfer which contradict conventional expectations. Experimental evidence of a new effect—deficit of momentum transfer (equivalently: reduced effective mass) in a neutron-atom collision—is presented and theoretically interpreted. Here, non-relativistic incoherent inelastic neutron scattering (INS) is applied. INS on single H2 molecules confined in multi-walled carbon nanotube channels has been experimentally investigated. Interpreted within conventional theory, the results reveal a counter-intuitive reduced effective mass of the recoiling H2 molecule, i.e. M = 0.64 a.m.u. (atomic mass units). In contrast, this finding has a simple qualitative interpretation within WV and TSVF theory. In the second part of the paper we report on current experimental and theoretical investigations in the field of X-ray diffraction (XRD), which belongs to coherent scattering. Preliminary XRD results from cubic crystalline materials show a surprising variation of the measured lattice parameter (usually called “lattice constant”) with momentum transfer. A first theoretical model of the effect in the light of the new theory is presented. These findings give further evidence for the broad character and significance of the novel WV and TSVF theory.

Effects of Quantum Confinement of Hydrogen in Nanocavities – Experimental INS Results and New Insights

Current developments of non-relativistic quantum mechanics appear to predict, and reveal, counter-intuitive dynamical effects of hydrogen in nanostructured materials that are of considerable importance for basic research as well as for technological applications. In this review, the experimental focus is on HO and H molecules in carbon nanotubes and other nanocavities that have been experimentally investigated using the well-established technique of incoherent inelastic neutron scattering (INS). For instance, the momentum and energy transfers, as obtained from the commonly used standard data analysis techniques, from a (I) H2 molecule in a C-nanotube resulting in a roto-translational motion along the nanotube axis seems to (1) either violate the standard conservation laws or (2) to attribute to the H molecule undergoing translation the effective mass a.m.u. (atomic mass units) instead of the expected 2 a.m.u. A similar striking anomalous effect has been found in the neutron-H scattering from the (II) H2O molecules in nano-channels of some solid materials, in which O-H stretching vibrations along the channel axis are created. The results of this scattering process seem to once again either violate the standard conservation laws or to attribute to the effective mass of the struck H2 molecule as a.m.u. instead of the expected value of 1 a.m.u. We show that these counterintuitive observations from the INS studies have no conventional interpretation within the standard non-relativistic scattering theory. However, they can be qualitatively interpreted “from first principles” within the framework of modern theories of (III) time-symmetric quantum dynamics, as provided by the weak values (WV) and two-state- vector formalism (TSVF), and/or (IV) quantum correlations, especially quantum discord (QD) and quantum thermodynamics (QTD). The theoretical analysis provides an intuitive understanding of the experimental results, gives strong evidence that the nano-structured cavities do represent quantum systems which participate significantly in the dynamics of the neutron-H scattering, and, surprisingly, shows that new physical information can be derived from the experimental data. This latter point may also have far-reaching consequences for technology and material sciences (e.g., fuel cells, H storage materials, etc.). Moreover, novel insights into the short-lived quantum dynamics and/or quantum information theory can be gained.

Low frequency vibrational density of state of highly permeable super glassy polynorbornenes - the Boson peak.

Inelastic incoherent neutron time-of-flight scattering was employed to measure the low frequency density of states for a series of addition polynorbornenes with bulky side groups. The rigid main chain in combination with the bulky side groups give rise to a microporosity of these polymers in the solid state. The microporosity characterized by the BET surfaces area varies systematically in the considered series. Such materials have some possible application as active separation layer in gas separation membranes. All investigated materials show excess contributions to the Debye type density of states characteristic for glasses known as Boson peak. The maximum position of the Boson peak shifts to lower frequency values with increasing microporosity. Data for PIM-1 and Matrimid included for comparison are in good agreement to this dependency. This result supports the sound wave interpretation of the Boson peak.

Weak Values and Two-State Vector Formalism in Elementary Scattering and Reflectivity—A New Effect

The notions of Weak Value (WV) and Two-State Vector Formalism (TSVF), firstly introduced by Aharonov and collaborators, provide a quantum-theoretical formalism of extracting new information from a system in the limit of small disturbances to its state. Here, we explore two applications to the case of non-relativistic two-body scattering with one body weakly interacting with its environment. We present a physically compelling analysis of a new quantum effect: momentum transfer deficit and an accompanying enhanced energy transfer; or, equivalently, an apparent mass-deficit of the struck body. First, incoherent inelastic neutron scattering (INS) from protons of H 2 molecules in C-nanotubes is investigated. The data of the H 2 translational motion along the nanotube shows that the neutron apparently exchanges energy and momentum with a fictitious particle with mass of 0.64 atomic mass units (a.m.u.), which is in blatant contradiction with the expected value of 2 a.m.u. Second, the same theory is applied to neutron reflectivity—which is elastic and coherent—from the interface of (single crystal) Si with H 2 O-D 2 O liquid mixtures. The data shows a striking enhanced reflectivity in a wide range of momentum transfers, which is tantamount to a momentum-transfer deficit with respect to conventional expectations. However, these effects find a natural interpretation within the WV-TSVF theoretical analysis under consideration. In summary, both scattering effects contradict conventional theoretical expectations, thus also supporting the novelty of the theoretical framework of WV and TVSF. Additionally, it should be pointed out that the two dynamical variables in the interaction Hamiltonian of the theoretical model belong to two different physical bodies.

Weak measurement and weak values — New insights and effects in reflectivity and scattering processes

Recently, the notions of Weak Measurement (WM), Weak Value (WV) and Two-State-Vector Formalism (TSVF), firstly introduced by Aharonov and collaborators, have extended the theoretical frame of standard quantum mechanics, thus providing a quantum-theoretical formalism for extracting new information from a system in the limit of small disturbance to its state. Here we provide an application to the case of two-body scattering with one body weakly interacting with its environment — e.g. a neutron being scattered from a H2 molecule physisorbed in a carbon nanotube. In particular, we make contact with the field of incoherent inelastic neutron scattering from condensed systems. We provide a physically compelling prediction of a new quantum effect — a momentum transfer deficit; or equivalently, an enhanced energy transfer; or an apparent reduction of the mass of the struck particle. E.g., when a neutron collides with a H2 molecule in a C-nanotube and excites its translational motion along the nanotube, it apparently exchanges energy and momentum with a fictitious particle with mass of 0.64 atomic mass units. Experimental results are shown and discussed in the new theoretical frame. The effect under consideration has no conventional interpretation, thus also supporting the novelty of the quantum theoretical framework of WV and TVSF. Some speculative remarks about possible applications being of technological interest (fuel cells and hydrogen storage; Li+ batteries; information and communication technology) are shortly mentioned.

Shear viscosities of liquid sodium and potassium using Green-Kubo and “first principles” pseudopotential formalisms

This article presents an original work on velocity and stress autocorrelation functions, memory functions, spectral densities and atomic transport properties of two liquid alkali metals (sodium and potassium). For this, we carried out molecular dynamics (MD) simulations using Green-Kubo relation under the condition of very long-time duration to obtain reliable and accurate results.An originality of the present work is that the interatomic forces are described by pair potentials built within the “first principles” pseudopotential formalism using the non-local and energy dependent model potential of Shaw. It is completely free of adjustable parameters since the energy dependent parameters are determined self-consistently at the Fermi energy on an absolute energy scale. The Green-Kubo relationship is the main theoretical tool that allows us determining the atomic transport coefficients from appropriate time-autocorrelation functions. An important new result concerns the memory function. We demonstrate, for the first time to our knowledge, that it can be depicted as a sum of wavelets. The wavelets with their amazing features emphasize the nature of dynamic processes at the microscopic level. We also compared, for the first time to our knowledge, the spectral density associated to the velocity autocorrelation function (VACF), to experimental values obtained by incoherent inelastic neutron scattering. Finally, the temperature dependence of the self-diffusion coefficient (our previous study) and that of shear viscosity (present work) are both in excellent agreement with experiment.

Low-temperature phase transition in [Cd(OS(CH3)2)6](ClO4)2 studied by inelastic and elastic neutron scattering, Raman light scattering, and infrared absorption spectroscopy

The vibrational and reorientational dynamics of CH3 groups from (CH3)2SO ligands in the high- and low-temperature phases of [Cd(OS(CH3)2)6](ClO4)2 were investigated by Fourier transform infrared (FT-IR) and Raman (FT-Raman) spectroscopy, and quasielastic and inelastic incoherent neutron scattering (QENS and IINS) methods. The results show that in the high temperature phases the CH3 groups and ClO4− anions perform fast (τR ≈ 10−12s) reorientational motions. These molecular motions do not change significantly at low-temperature phase transition (detected earlier by differential scanning calorimetry (DSC) at TC4c=235 K on cooling and at TC4c=242 K on heating). Neutron powder diffraction (NPD) measurements, performed simultaneously with QENS and IINS, indicated that this phase transition is associated with a change of the crystal structure. The comparison of FT-IR, FT-Raman and the proton-weighted phonon density of states function G(ν) calculated from IINS experimental spectra at low temperature phase of [Cd(OS(CH3)2)6](ClO4)2 was made.

Phase transition and NH3 dynamics in [Ni(NH3)4](ReO4)2 studied by infrared absorption, X-ray powder diffraction and neutron scattering methods

The phase transition in [Ni(NH3)4](ReO4)2 detected previously by differential scanning calorimetry (DSC) at Tch=188K was now investigated by infrared absorption (FT-IR), incoherent inelastic and elastic neutron scattering (IINS, ND), and X-ray powder diffraction (XRPD) methods. The reorientational dynamics of NH3 groups was investigated by infrared band shape analysis (IRBS) and quasielastic neutron scattering (QENS) methods. The infrared data show that some of the bands split in the vicinity of the phase transition temperature, which suggests a change in the crystal structure. The systematic narrowing of particular bands at cooling is also observed, but reorientational dynamics of NH3 is not stopped at the phase transition temperature, which is fully confirmed by the QENS analysis. The broadening of the quasielastic neutron scattering peak is clearly visible below the phase transition temperature. Both NPD and XRPD measurements indicate that a small change of crystal structure is associated with the phase transition.