Reorientation Of Cations Research Articles

Temperature Dependence of Spontaneous Polarization in Order−Disorder Pyridinium Periodate Extracted from 2H NMR Data

On the basis of occupancy factor data for nitrogen atoms in the pyridinium cation obtained from the structural study of PyHIO4 by neutron diffraction,4 the shape of the potential for cation reorientation about the axis perpendicular to its plane was found. The 2H NMR spectra of d5PyHIO4 taken over a wide range of temperatures have allowed the determination of potential minima populations. These data were used to obtain the temperature dependence of spontaneous polarization. The agreement between the spontaneous polarization calculated in this way and the measured values is very good and indicates that the ferroelectric crystal is of the order−disorder type.

Quasielastic neutron scattering study of pyridinium cation reorientation in thiourea pyridinium nitrate inclusion compound

The dynamics of the pyridinium cation in thiourea pyridinium nitrate inclusion compound has been studied using quasielastic neutron scattering in a wide temperature range (10-350 K). The elastic incoherent structure factor was determined from neutron backscattering and time-of-flight measurements and its analysis allows to describe in detail the geometry of the motions of the pyridinium cation. Our study reveals two types of motion having two different correlation times. The pyridinium cation reorients about the axis perpendicular to its molecular plane over inequivalent threefold potential energy barriers and also executes a faster out-of-plane motion about the axis passing through two opposite atoms of the ring.

Phase Transitions and Molecular Motions in Guanidinium-d6 Trihalogenomercurates(II) [C(ND2)3]HgBr3 and [C(ND2)3]HgI3 as Investigated by Means of 81Br and 127I NQR and 2H NMR

Abstract The temperature dependence of 2H NMR spectra and 81Br and 127I NQR frequencies was investigated for the deuterated compounds of [C(NH2)3]HgBr3 and [C(NH2)3]HgI3 between 77 and ca. 400 K. Due to deuteration, the phase-transition temperatures of the iodine compound were increased by 2 and 6 K for the second-order (251 K) and the first-order (210 K) phase transitions, respectively. Distinctive 127I NQR frequency-shifts of more than 200 kHz were observed in the intermediate phase of the iodine compound on deuteration. The temperature dependences of the 2H NMR spectra for [C(ND2)3]HgBr3 and [C(ND2)3]HgI3 could be explained by invoking C3 reorientation of the guanidinium cation. The activation energies for the reorientation evaluated from 2H NMR were consistent with the former 1H NMR T1 results with the exception of the low-temperature phase of the iodine compound. The quite low activation energy from 2H NMR spectrum for this phase may indicate that the quenching of the orientation disordering of the cations is responsible for the fade-out phenomenon in the 127I NQR resonance lines below ca. 100 K.

Glass-like dielectric relaxation in C 5NH 6ClO 4

Detailed dielectric measurements have been carried out in polycrystalline pyridinium perchlorate C 5NH 6ClO 4 at temperatures of 100 K≤ T≤280 K and frequencies of 10 Hz ≤ ν ≤ 10 5 Hz . Besides the known dielectric anomalies at 248 and 233 K, a glass-like dielectric relaxation is clearly observed at temperatures below 233 K. The frequency dependence of the dielectric constant is interpreted to originate from two different relaxation processes. The relaxation time for the dominant relaxation process is well described by an Arrhenius equation with the activation energy E = 3073 K and τ 0 = 6.2 × 1 0 − 12 s . The other weak relaxation process is also analyzed using the same equation with E = 1160 K and τ 0 = 1.5 × 1 0 − 5 s . The dominant relaxation process is considered to arise from the coupled reorientation of the pyridinium cation and perchlorate anion, while the weak one seems to be attributed to the ferroelectric domain wall motion. This suggests that the phase below 233 K can be described as a mixed phase comprising the dipolar glass and ferroelectric states.

Rotational Displacement of IO4 Tetrahedron in the Phase II of Pyridinium Periodate

In order to investigate the displacement of the IO 4 tetrahedron in the ferroelectric phase II of pyridinium periodate in detail, we performed X-ray structural analysis at 33 temperatures with the use of the estimated site occupancies of carbon and nitrogen atoms. We confirmed the continuous increase of the rotation angle of the IO 4 tetrahedron about the a-axis with decreasing temperature in the phase II. This indicates that the IO 4 tetrahedron acts as the displacive structural unit at the transition from phase I to phase II. It is found that the rotation of the IO 4 tetrahedron is correlated with the reorientation of the pyridinium cation through the N-H─ O type hydrogen bond.

Orientational and translational dynamics in room temperature ionic liquids

The authors investigate the dynamics of a series of room temperature ionic liquids, based on the same 1-butyl-3-methylimidazolium cation with different anions, by means of broadband (10(-6)-10(9) Hz) dielectric spectroscopy and depolarized light scattering in the temperature range from 400 K down to 35 K. Typical ionic conductivity is observed above the glass transition temperature Tg. Below Tg the authors detect relaxation processes that exhibit characteristics of secondary relaxations, as typically observed in molecular glasses. At high temperatures, the characteristic times of cation reorientation, deduced from the light scattering data, are approximately equal to the electric modulus relaxation times related to ionic conductivity. In the supercooled regime and close to Tg, the authors observe decoupling of conductivity from structural relaxation. Overall, room temperature ionic liquids exhibit typical glass transition dynamics, apparently unaltered by Coulomb interactions.

A study of crystal structure and molecular dynamics by XRD and 1H NMR in bis-thiourea N-methylpyridinium iodide

Crystal and molecular structures of two orthorhombic phases of bis-thiourea N-methylpyridinium iodide [(NH2)2CS]2C5H5NI− were determined by a single-crystal X-ray diffraction method. Both phases were discovered earlier by DSC analysis. At 297 K the complex crystallizes in the space group Cmcm with a = 15.742(3) Å, b = 11.349(2) Å, c = 8.360(2) Å, and at 100 K in the space group Pbcn with the cell parameter c tripled. The first-order reversible phase transition occurs at 155 K on cooling and at 160 K on heating. At room temperature all N-methylpyridinium cations are dynamically disordered, while at 100 K the ratio of ordered to statically disordered cations is 2:1. Thiourea-iodide ribbons are well ordered in both crystal phases. 1H NMR analysis performed for the title complex and its deuterated counterpart has revealed a reorientation of methyl group in the low-temperature phase; and in the high temperature phase, a reorientation of N-methylpyridinium cation over inequivalent barriers followed by a probable flip-reorientation of the thiourea molecules. For steric reasons the latter motion is possible only if preceded by a flip-motion of the NH2 group. Activation energy values for the proposed cation and thiourea motions are given.

Structure, phase transitions and dielectric properties of a new inclusion compound of bis-thiourea pyridinium nitrate salt

AbstractA structure of a new inclusion compound of bis-thiourea pyridinium nitrate salt C5H6N+NO3-2[CH4N2S] was determined by single-crystal X-ray diffraction method at 293 K, 250 K and 150 K. In high temperature phase the compound crystallizes in orthorhombic system, the two low-temperature phases are monoclinic. The sequence of phases is:Pbnm→P21/c→P21. In the orthorhombic form (I) theaaxis is doubled in respect to the monoclinic forms and pyridinium and nitrate ions are partially disordered. Both low-temperature monoclinic phases (II) and (III) are ordered but twinned by pseudo-merohedry. In all three phases the host lattice is built of ribbons of hydrogen-bonded thiourea molecules and nitrate anions forming channels in which the guests pyridinium cations are located. Each ribbon consists of two crystallographically independent thiourea molecules and nitrate ions. Four ribbons form a channel parallel to thecaxis with approximately square cross-section. In the channel there is a stack of pyridinium cations, which planes form an inclination angle with the channel axis. The angle varies from about 90° in disordered phase I to 60° in well ordered phase (III).The dielectric spectroscopy measurements were performed on the polycrystalline sample with HP-4291A impedance analyser in the frequency range 1 MHz–1 GHz. The dielectric measurements reveal that two phase transitions occur atT1= 287 K and atT2= 217 K. The phase transition atT2= 287 K is of the first order with the thermal hysteresis of 10 K at cooling and heating. The phase transition at 217 K is of the second order and the dielectric anomaly suggests the ferroelectric-paraelectric phase transition. The dielectric relaxation observed belowT2are caused by the pyridinium cation reorientation. The dielectric relaxation time changes with the temperature according to Arrhenius low and the activation energy of the relaxation process is about 14 kJ/mol.

Structure, phase transitions and molecular dynamics in 4-aminopyridinium hexachloroantimonate(V), [4-NH 2C 5H 4NH][SbCl 6

4-Aminopyridinium hexachloroantimonate(V), [4-NH 2C 5H 4NH][SbCl 6], abbreviated to 4APCA, crystallizes at room temperature in the monoclinic system, space group P2 1/ c (phase III). The crystal undergoes two reversible phase transitions of first order type at 386/346 K (II↔III) and at 467/466 K (I↔II) on heating–cooling runs. Dielectric studies suggest the plastic crystal's behavior above 386 K (phase II). The proton NMR second moment ( M 2) and the 1H NMR spin-lattice relaxation time ( T 1) for 4APCA measured between 77 and 420 K revealed the C 2 reorientation of the 4-aminopyridinium cations. The mechanism of the phase transitions in 4APCA at 386 K is discussed.

Study of phase transitions in Ti and Sn doped Ba W‐type hexaferrites by means of magnetic disaccommodation techniques

The relaxation of the initial permeability has been measured in polycrystalline Ba hexaferrites with the initial composition of W-type (BaMexFe18–xO27, with Me: Ti, Sn, in the substitution range 0 < x < 1). The samples, prepared by means of standard ceramic techniques, correspond to different phases, changing from W phase at lower doping rate to M phase with x = 1. In the temperature range between 80 and 420 K, the magnetic disaccommodation, i. e., the time decay of the initial permeability after sample demagnetization, is represented by means of isochronal curves. They show the presence of up to four relaxation processes whose maxima lie at 375 K, 300 K, 240 K and 180 K (resp. A, B, C and D peaks). With increasing doping content, the C, D and A processes vanish. The results are interpreted in terms of ionic reorientations of ferrous cations and lattice vacancies in the different metallic sites of the hexagonal close packed lattice, and the phase transitions with increasing doping content; in this way magnetic disaccommodation can be a valuable tool in the ellucidation of phase formation in the hexaferrite family. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Single particle dynamics in ionic liquids of 1-alkyl-3-methylimidazolium cations

Ionic dynamics in room temperature molten salts (ionic liquids) containing 1-alkyl-3-methylimidazolium cations is investigated by molecular-dynamics simulations. Calculations were performed with united atom models, which were used in a previous detailed study of the equilibrium structure of ionic liquids [S. M. Urahata and M. C. C. Ribeiro, J. Chem. Phys. 120, 1855 (2004)]. The models were used in a systematic study of the dependency of several single particle time correlation functions on anion size (F-, Cl-, Br-, and PF6-) and alkyl chain length (1-methyl-, 1-ethyl-, 1-butyl-, and 1-octyl-). Despite of large mass and size of imidazolium cations, they exhibit larger mean-square displacement than anions. A further detailed picture of ionic motions is obtained by using appropriate projections of displacements along the plane or perpendicular to the plane of the imidazolium ring. A clear anisotropy in ionic displacement is revealed, the motion on the ring plane and almost perpendicular to the 1-alkyl chain being the less hindered one. Similar projections were performed on velocity correlation functions, whose spectra were used to relate short time ionic rattling with the corresponding long time diffusive regime. Time correlation functions of cation reorientation and dihedral angles of the alkyl chains are discussed, the latter decaying much faster than the former. A comparative physical picture of time scales for distinct dynamical processes in ionic liquids is provided.

Phase transitions and molecular motions in [Cd(H2O)6](BF4)2 studied by DSC, 1H and 19F NMR and FT-MIR

Two solid phase transitions of [Cd(H2O)6](BF4)2 occurring on heating at TC2=183.3K and TC1=325.3K, with 2K and 5K hysteresis, respectively, were detected by differential scanning calorimetry (DSC). High value of entropy changes indicated large orientational disorder of the high temperature and intermediate phase. Nuclear magnetic resonance (1H NMR and 19F NMR) relaxation measurements revealed that the phase transitions at TC1 and TC2 were associated with a drastic and small change, respectively, of the both spin–lattice relaxation times: T1(1H) and T1(19F). These relaxation processes were connected with the “tumbling” motions of the [Cd(H2O)6]2+, reorientational motions of the H2O ligands, and with the iso- and anisotropic reorientation of the BF4− anions. The cross-relaxation effect was observed in phase III. The line width and the second moment of the 1H and 19F NMR line measurements revealed that the H2O reorientate in all three phases of the title compound. On heating the onset of the reorientation of 3 H2O in the [Cd(H2O)6]+2, around the three-fold symmetry axis of these octahedron, causes the isotropic reorientation of the whole cation. The BF4− reorientate isotropically in the phases I and II, but in the phase III they perform slow reorientation only about three- or two-fold axes. A small distortion in the structure of BF4− as well as of [Cd(H2O)6]2+ is postulated. The temperature dependence of the bandwidth of the O–H stretching mode measured by Fourier transform middle infrared spectroscopy (FT-MIR) indicated that the activation energy for the reorientation of the H2O did not change much at the TC2 phase transition.

Barriers for cation reorientation in pyridinium iodide studied by an atom–atom and DFT methods

The heights of barriers for reorientation of the pyridinium cation in a molecular–ionic system of pyridinium iodide have been calculated by the traditional atom–atom approach taking into regard the London dispersion and repulsive interactions and the calculations methods of quantum chemistry DFT (B3LYP/3-21G*) and PM3. The parallel application of the two approaches permitted a very good reproduction of the height of the cation reorientation barriers.

Influence of stoichiometry on the magnetic disaccommodation in M-type Sr hexaferrites

Abstract The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition SrO·nFe2O3 (n=5.7, 6), prepared by means of standard ceramic techniques in air as well as CO2 sintering atmospheres. The isochronal disaccommodation spectra show the presence of different relaxation processes, depending on both the sintering atmosphere and especially the initial composition, and associated to ionic reorientations of ferrous cations and lattice vacancies in the different metallic sites within the spinel (S) and hexagonal (R) blocks of the close packed lattice.

1H NMR Study of Molecular Motions in Thiourea Pyridinium Halide Inclusion Compounds

The proton NMR second moment and spin-lattice relaxation time have been studied for polycrystalline inclusion compounds of thiourea pyridinium chloride, bromide, iodide and their perdeuterated analogues in a wide temperature range. The pyridinium cation reorientation around the pseudohexagonal C6’ symmetry axis over inequivalent barriers and hindered rotation of the thiourea molecule around its C=S bond have been revealed. The activation parameters of the both motions have been found.

Crystal structure, phase transition and ferroelectric properties of the [(CH3)3NH]3[Sb2Cl9(1−x)Br9x](TMACBA) mixed crystals

The paraelectric–ferroelectric phase transition in the[(CH3)3NH]3[Sb2Cl9(1−x)Br9x] (TMACBA) mixed crystals is investigated by the differential scanning calorimetry,dilatometric and dielectric methods. The phase transition is found at363.5, 362.7, 360.5 and 350.9 K (on cooling) for TMACBA crystals withx = 0,0.02, 0.17 and 0.42, respectively. The crystal structures of the pure bromine analogue(x = 1),TMABA, and the mixed crystal TMACBA(x = 0.55)are determined at 297 K. TMABA crystallizes in the trigonalspace group R3̄c: a = 15.098(2) Å, c = 21.906(4) Å,Z = 3,R1 = 0.0393,wR2 = 0.0746.Its structure is built up of discrete Sb2Br93−bioctahedra and disordered trimethylammonium cations. Ferroelectric TMACBA(x = 0.55)crystallizes in the monoclinic space groupPc: a = 10.245(2) Å, b = 9.122(2) Å, c = 15.549(3) Å, β = 90.52(3)°,Z = 2,R1 = 0.0340,wR2 = 0.0712.The crystal is built of two-dimensional layer anions (‘honeycomb-like structure’)and three trimethylammonium cations. Dielectric studies on TMACBA(x = 0.42)in the frequency range 75 kHz–900 MHz indicate relatively fast reorientation of thetrimethylammonium cations over the paraelectric phase. The ferroelectricity ofmixed crystals is preserved for Br concentration smaller than 0.60 ± 0.05.The polar properties are strictly connected with the presence of the polyanioniclayer structure. A possible mechanism for the paraelectric–ferroelectric phasetransition in the TMACBA mixed crystals is discussed on the basis of the resultspresented.

Crystal structure, phase transitions and ferroelastic properties of [(CH 3) 2NH 2] 3[Bi 2Cl 9

A sequence of structural phase transitions in [(CH 3) 2NH 2] 3[Bi 2Cl 9] (DMACB) is established on the basis of differential scanning calorimetry (DSC) and dilatometric studies. Four phase transitions are found: at 367/369, 340/341, 323/325 and 285/292 K (on cooling/heating). The crystal structure of DMACB is determined at 350 K. It crystallizes in monoclinic space group P2 1/ n: a=8.062(2), b=21.810(4), c=14.072(3) Å, β=92.63(3)°, Z=4, R 1=0.0575, w R 2=0.1486. The crystal is built of the double chain anions (“pleated ribbon structure”) and the dimethylammonium cations. Dielectric studies in the frequency range 75 kHz–900 MHz indicate relatively fast reorientation of the dimethylammonium cations over the I, II, III and IV phases. Infrared spectra are recorded in the temperature range 40–300 K and analyzed in region assigned to the symmetric and asymmetric NC 2 stretching vibrations. Optical observations show the existence of the ferroelastic domain structure over all phases below 367 K. The possible mechanisms of phase transitions are discussed on the basis of presented results.

Polarization and energy barriers in ferroelectric pyridinium tetrafluoroborate

Measurements of the 1H and 19F nuclear magnetic resonance (NMR) second moments were performed for a polycrystalline sample of (PyH)BF4, whereas the shape of the 2H NMR line was analysed for a polycrystalline sample of (d5PyH)BF4. Asymmetry parameter δ has been calculated for four models of pyridinium cation reorientation among inequivalent potential energy minima, using the experimental value of the 1H NMR second moment as well as the 2H NMR line width. From knowledge of the potential shape and the population of its minima, the temperature dependence of polarization for all the models has been found. From the comparison of the experimentally determined polarization with the calculated polarization, the most appropriate model of the pyridinium cation reorientation has been chosen.

Analysis of magnetic aftereffects in strontium hexagonal ferrites with W-type stoichiometry

The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition of W phase (SrO⋅9Fe2O3). The samples have been prepared by means of standard ceramic techniques at different temperatures in the 1250 °C&amp;lt;T&amp;lt;1420 °C range in air as well as CO2 sintering atmospheres and then rapidly quenched to provide the presence of crystal vacancies. X-ray diffraction spectra reveal the presence of hexaferrite as the main phase. In the temperature range between 80 and 500 K, the magnetic disaccommodation is represented by means of isochronal curves. The isochronal spectra show the presence of different after-effect processes whose maxima lie at 365 K, 300 K, and 165 K (respectively, A, B, and D peaks). They correspond to thermally activated relaxation processes with activation energies of 1 eV, 0.8 eV, and 0.5 eV, respectively. Processes A and B are connected to ionic reorientations of ferrous cations and lattice vacancies within the octahedral sites in spinel-like S blocks whereas D peak is associated with a similar process in face-sharing octahedral sites located in the hexagonal R blocks of the lattice. Regarding the corresponding isochronal spectra of barium W-type ferrites, we can observe that the C process centered at 240 K does not appear in Sr hexaferrites. This suggests that the relaxation processes which take place in the hexagonal R blocks are not favorable in Sr hexaferrites, probably due to the smaller size of the Sr ion.

Pressure Effect on Molecular and Lattice Dynamics in Pyridinium Nitrate

Measurements of spin-lattice relaxation time T 1 , second moment M 2 and neutron scattering have been performed for a polycrystalline sample of pyridinium nitrate as a function of hydrostatic pressure and temperature. The structure of this compound has also been analysed by the HF/6-31 G method. The results of the measurements and calculations have confirmed that reorientation of the pyridinium cation takes place between the potential minima of different values, and the asymmetry parameter decreases with increasing temperature. The temperature dependence of the asymmetry parameter is modified by the pressure applied. The presence of the hydrogen bond in this compound implies a very small activation volume and is responsible that the crystal does not undergo a phase transition to the disordered phase.