Lattice dynamics in organic ferroelectric PhMDA (2-phenylmalondialdehyde C
9
H
8
O
2
) has been investigated using inelastic neutron scattering (INS) spectroscopy and first principles based ...calculations. Most of the prominent features of the INS spectrum originated from the normal modes of hydrogen bonded malondialdehyde (MDA) units rather than from phenyl rings. It is also found that carbon-hydrogen bonds in the MDA unit are tighter than these in phenyl rings. From the calculated splitting of LO (longitudinal optical) and TO (transverse optical) normal modes, it is predicted that hydrogen bonds in MDA units, which hold the solid state structure, contribute more to the ferroelectric polarisation of PhMDA. Any distortion of H-ions in these hydrogen bonds thus affects ferroelectric properties of this material.
Abstract
Stimuli-responsive behaviors of flexible metal–organic frameworks (MOFs) make these materials promising in a wide variety of applications such as gas separation, drug delivery, and molecular ...sensing. Considerable efforts have been made over the last decade to understand the structural changes of flexible MOFs in response to external stimuli. Uniform pore deformation has been used as the general description. However, recent advances in synthesizing MOFs with non-uniform porous structures, i.e. with multiple types of pores which vary in size, shape, and environment, challenge the adequacy of this description. Here, we demonstrate that the CO
2
-adsorption-stimulated structural change of a flexible MOF, ZIF-7, is induced by CO
2
migration in its non-uniform porous structure rather than by the proactive opening of one type of its guest-hosting pores. Structural dynamics induced by guest migration in non-uniform porous structures is rare among the enormous number of MOFs discovered and detailed characterization is very limited in the literature. The concept presented in this work provides new insights into MOF flexibility.
Inelastic neutron scattering (INS) spectroscopy, contrary to other vibrational spectroscopic techniques such as infrared or Raman spectroscopies, provides much richer microscopic insight into a ...material due to the absence of selection rules induced by the system's symmetry and via its dependence on both energy (E) and momentum (Q) transfer. First-principles density functional theory (DFT) based calculations are now routinely used to interpret infrared and Raman spectra. These calculations can also be used to interpret INS spectra, however, the need to include the neutron scattering cross sections, overtones and combination modes, together with instrument specific E-Q windows make the data analysis challenging. Here we present AbINS: a new generation of software to interpret INS spectra using ab initio phonon data. AbINS is an open-source package implemented as a plugin to the neutron data analysis software, Mantid and offers the facility to plot the full (Q, E) map for powder samples, with the option to extract individual atomic contributions. This option is then applied to analyse the vibrational spectrum of non-hydrogenous K2SiF6 to extract atom-type contributions identifying T1g librational mode of the SiF62− ion together with the T2u F–Si–F bending mode.
We show clear experimental evidence of cooperative terahertz (THz) dynamics observed below 3 THz (∼100 cm^{-1}), for a low-symmetry Zr-based metal-organic framework structure, termed MIL-140A ...ZrO(O_{2}C-C_{6}H_{4}-CO_{2}). Utilizing a combination of high-resolution inelastic neutron scattering and synchrotron radiation far-infrared spectroscopy, we measured low-energy vibrations originating from the hindered rotations of organic linkers, whose energy barriers and detailed dynamics have been elucidated via ab initio density functional theory calculations. The complex pore architecture caused by the THz rotations has been characterized. We discovered an array of soft modes with trampolinelike motions, which could potentially be the source of anomalous mechanical phenomena such as negative thermal expansion. Our results demonstrate coordinated shear dynamics (2.47 THz), a mechanism which we have shown to destabilize the framework structure, in the exact crystallographic direction of the minimum shear modulus (G_{min}).
This paper reports on the development of a collection of dynamical models of one-dimensional peak profile functions used to fit dynamic structure factors
S
(
Q
,
ħω
) of Quasi Elastic Neutron ...Scattering (QENS) data. The objective of this development is to create a maintainable and interoperable Python library with models reusable in other projects related to the analysis of data from Quasi Elastic Neutron Scattering experiments. The ambition is that the library also will serve as a platform where scientists can make their models available for others. We illustrate how the library can be used by newcomers to the field as well as by experts via different examples. These examples, provided as Jupyter notebooks, show how the QENS models can be integrated in the whole QENS data processing pipeline.
Direct phonon excitation in a neutron time‐of‐flight single‐crystal Laue diffraction experiment has been observed in a single crystal of NaCl. At room temperature both phonon emission and excitation ...leave characteristic features in the diffuse scattering and these are well reproduced using abinitio phonons from density functional theory (DFT). A measurement at 20 K illustrates the effect of thermal population of the phonons, leaving the features corresponding to phonon excitation and strongly suppressing the phonon annihilation. A recipe is given to compute these effects combining DFT results with the geometry of the neutron experiment.
Both structural glasses and disordered crystals are known to exhibit anomalous thermal, vibrational, and acoustic properties at low temperatures or low energies, what is still a matter of lively ...debate. To shed light on this issue, we studied the halomethane family CBrn Cl4−n (n = 0, 1, 2) at low temperature where, despite being perfectly translationally ordered stable monoclinic crystals, glassy dynamical features had been reported from experiments and molecular dynamics simulations. For n = 1 , 2 dynamic disorder originates by the random occupancy of the same lattice sites by either Cl or Br atoms, but not for the ideal reference case of CCl4. Measurements of the low-temperature specific heat (Cp) for all these materials are here reported, which provide evidence of the presence of a broad peak in Debye-reduced Cp (T) /T3 and in the reduced density of states (g(ω) /ω2) determined by means of neutron spectroscopy, as well as a linear term in C p usually ascribed in glasses to two-level systems in addition to the cubic term expected for a fully ordered crystal. Being CCl4 a fully ordered crystal, we also performed density functional theory (DFT) calculations, which provide unprecedented detailed information about the microscopic nature of vibrations responsible for that broad peak, much alike the “'boson peak” of glasses, finding it to essentially arise from a piling up (at around 3–4 meV) of low-energy optical modes together with acoustic modes near the Brillouin-zone limits.
In December 1997 the Osiris beamline at the ISIS facility, UK, recorded its first neutron spectrum. The instrument enjoyed a first stint as a cold neutron diffractometer before the spectroscopic ...capabilities were fully commissioned. Osiris soon became a workhorse quasielastic spectrometer as well as a highly successful low-energy spectrometer. The status of the instrument is recognized by the user community with high-impact publications ranging from energy materials over life science to quantum matter. To enhance the existing capabilities a silicon analyzer is under construction. The primary spectrometer will be upgraded with a new supermirror guide providing a factor 10 to 14 increased flux in combination with a new hydrogen moderator. Beyond these developments further improvements of the energy resolution with a combination of a fast pulse shaping chopper and using a direct backscattering geometry are being investigated.
Structure and dynamics of hydrogen bonded organic ferroelectric CBDC (1-cyclobutene-1,2-dicarboxylic acid, C6H8O4) have been investigated using inelastic neutron scattering (INS) spectroscopy and ...first principles lattice dynamics. Vibrational modes of O-H-O bending, related with two different types of hydrogen bonds, inter and intra-molecular, are manifested as two sets of doublets in the range of 900-1400 cm−1 in the INS spectrum. First principles density functional theory (DFT) is used to assign these doublets to out-of-the-plane and in-plane vibrations of these two types of O-H-O bending modes. Correlating structure and dynamics we find that although the local structures of the hydrogen bonded protons are similar, their medium range order may influence their vibrational frequencies. Vibrational modes of non-hydrogen bonded protons also are assigned separately in the spectrum. Comparing the calculated and the experimental INS spectra it is predicted that hydrogen bonded O-H bonds are more anharmonic than non-hydrogen bonded C-H bonds. From the calculated Born effective charge tensor it is predicted that inter-molecular hydrogen bonds contribute more to the ferroelectric polarisation of CBDC than intra-molecular ones. A large LO-TO splitting of the O-H stretching mode at 2471 cm−1 is identified with the mode that has significant contribution to the ferroelectric polarisation. As this mode is sensitive to long range Coulomb interactions and is responsible for ferroelectric properties of the material, vibrational spectroscopy combined with DFT calculations is proposed as a characterisation tool to investigate ferroelectric properties in the CBDC molecular crystal.
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