Distinct critical fluctuations and molecular motions manifest in a model biomembrane

Lattice dynamics in bis-(n-C₁₆H₃₇NH₃)₂SnCl₆, where the hydrocarbon part is analogous to lipid membrane, was investigated by means of 200 MHz ¹H nuclear magnetic resonance. As a result, critical fluctuations and molecular dynamics associated with the phase transitions, an order-disorder and a conformational phase transition, were distinguished in a wide temperature range. The dynamical origin of the critical fluctuations, observed in the long-chain compounds but not in the short-chain compounds, by the laboratory frame spin-lattice relaxation measurements, is revealed and discussed in this work.     

Molecular dynamics of [(CH2OH)3CNH3]2SiF6 by phase transition of 178 K

We have investigated the molecular motions of TRIS⁺ ([(CH₂OH)₃CNH₃]⁺) and ions in the [(CH₂OH)₃CNH₃]₂SiF₆ crystal below room temperature from the measurements of the spin-lattice relaxation time T₁ and the NMR absorption line of ¹H and ¹⁹F nuclei, in order to elucidate the changes of the molecular motions by the phase transition of T_c=178 K. The narrowing of the ¹⁹F-NMR line was observed around T_c=178 K and the reorientation of the anion appears above T_c. Moreover, from the analysis of the temperature dependence of T₁, we have observed that the activation energy of the reorientational motion of ions changes from 0.168 eV (T>T_c) to 0.185 eV (T<T_c). Based on these results, we found that the reorientational motion of ions is closely related to the origin of the phase transition at T_c. In addition, from the measurement of the ¹H-NMR line, we also found that the reorientational motion of H₂ in the -CH₂OH group becomes active accompanied by the phase transition.     

Chain segment ordering in lamellar sublayers of block copolymers: A NMR study

The chain segment dynamics in the bulk lamellar phase of polystyrene-polydimethylsiloxane (PS-PDMS) block copolymers has been probed by NMR. The experiments were performed on a PS-PDMS diblock and on a PS-PDMS-PS triblock with twice the molecular weight. In the diblock, at room temperature, the PDMS block segments undergo uniaxial reorientations around the normal to the lamellae. In the triblock, the reorientational motions exhibit a lower degree of symmetry: deviations from a uniaxial dynamics are observed. Such a behaviour originates in the anchorage of both PDMS chain ends into the PS glassy layers. Received 27 September 2001 and Received in final form 18 January 2002     

H NMR study of the phase transitions of trissarcosine calcium chloride single crystals at low temperature

The ¹H NMR line-width and spin-lattice relaxation time T₁ of TSCC single crystals were studied. Variations in the temperature dependence of the spin-lattice relaxation time were observed near 65 and 130 K, indicating drastic alterations of the spin dynamics at the phase transition temperatures. The changes in the temperature dependence of T₁ near 65 and 130 K correspond to phase transitions of the crystal. The anomalous decrease in T₁ around 130 K is due to the critical slowing down of the soft mode. The abrupt change in relaxation time at 65 K is associated with a structural phase transition. The proton spin-lattice relaxation time of this crystal also has a minimum value in the vicinity of 185 K, which is governed by the reorientation of the CH₃ groups of the sarcosine molecules. From this result, we conclude that the two phase transitions at 65 and 130 K can be discerned from abrupt variations in the ¹H NMR relaxation behavior, and that ¹H nuclei play important roles in the phase transitions of the TSCC single crystal.     

Successive phase transitions of [(PyO)(H/D)][AuCl4] (PyO=C5H5NO)

³⁵Cl NQR as well as heat capacity measurements of [(PyO)H][AuCl₄] and its deuterated analog [(PyO)D][AuCl₄] revealed successive phase transitions at 70.5 and 62.5 K, and at 71 and 63 K, respectively. The NQR frequency varied continuously through the upper transition point while discontinuously through the lower transition point. In the intermediate-temperature phase a remarkable decrease in the signal intensity was observed. These NQR observations as well as the feature of the heat capacity anomaly in which a broad peak is succeeded by a sharp peak with decreasing temperature suggest a possibility of normal-incommensurate-commensurate phase sequence.     

Transferred hyperfine field of Rb2CoCl4 single crystals in the ferroelectric-incommensurate-normal phase by Rb NMR

The molecular susceptibility and paramagnetic shift of Rb₂CoCl₄ single crystals grown using the slow evaporation method were measured, and from these experimental results we obtained the transferred hyperfine interaction due to the transfer of spin density from Co²⁺ ions to Rb⁺ ions. The transferred hyperfine field was obtained for the ferroelectric, incommensurate, and normal phases. In the case of Rb(I), the transferred hyperfine interaction decreases with increasing temperature in the incommensurate phase, and increases with increasing temperature in the normal phase. The value of H_hf in the incommensurate and normal phases increases abruptly with increasing temperature in the case of Rb(II). These results indicate that the effects due to the transfer of spin density from Co²⁺ ions to the Rb(I) and Rb(II) ions are large above T_i. In particular, the effect due to the transfer of spin density to Rb(II) ions in the normal phase is very large; the variations with temperature of the transferred hyperfine interactions of the Rb(I) and Rb(II) nuclei are more or less continuous in T_c1 and T_i, and are not affected by the ferroelectric-incommensurate-normal phase transitions.     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.     

NMR relaxation times and proton motion in HxWO3

The relaxation timesT ₁,T _1q,T _1D, andT ₂ for¹H in the tetragonal-A phase of H_xWO₃ have been measured over the temperature range 190 to 490 K. The¹H relaxation behaviour appears to be governed by diffusion over inequivalent jump distances, approximating to a short range planar diffusion and a long range isotropic diffusion. Parameters for the latter motion are estimated asE _a = 68 kJ/mol and ₀=2.5×10^–13 s. The powder X-ray diffraction pattern for this phase of H_xWO₃ has been studied over the temperature range 300–470 K. The tetragonal distortion diminishes with temperature and H_0.43 WO₃ becomes cubic at about 435 K. Volumetric studies of hydrogen evolution show that decomposition accelerates at approximately this temperature.     

Low temperature1H NMR study of electronic structure parameters (T 1e T) in zirconium dihydride

Measurements of the proton-spin-lattice relaxation times (T ₁) are reported for zirconium dihydrides: ZrH_1.87, ZrH_1.93 and ZrH_2.00 at nominal frequency of 41 MHz, with emphasis on the low temperature (4.2–25 K) behaviour of theT ₁. The dominant contributions are originated from the hyperfine interactions of the protons with the conduction electrons and the paramagnetic impurities. The (T _1e T)^–1/2 values determined from the low temperature range (4.2–25 K) agree with those obtained in the extended range (4.2–300 K) as well as with the values reported by the other researches. The paramagnetic impurity contribution is found to be small and teperature independent.     

A study on new phase transitions in Cs2CaCl4·2H2O single crystals by observation of Cs NMR

The ¹³³Cs 1/2→−1/2 spin-lattice relaxation rate, , and the spin-spin relaxation rate, , for a Cs₂CaCl₄·2H₂O single crystal have been measured in function of temperature. The dominant relaxation mechanism of this crystal over the whole temperature range investigated here proceeds via quadrupole interaction. The changes in the ¹³³Cs spin-lattice relaxation rate near 325 K (=T_c1) and 360 K (=T_c2) correspond to phase transitions in the crystal. The change in the spin-lattice relaxation rate at T_c1 is small because the crystal lattice does not change very much during this phase transition. The change in near T_c2 is due to the critical slowing down of the soft mode that typically occurs in structural phase transitions. The temperature dependence of the spin-lattice relaxation rate for this crystal has maximum values at about 240 K, which is attributable to the effect of molecular motion as described by Bloembergen-Purcell-Pound theory. The phase transition temperatures T_c1 and T_c2 obtained from the temperature dependence of the relaxation rate is also clear from data obtained using differential scanning calorimetry. Therefore, we know that previously unreported phase transitions occur at 325 and 360 K.     

Proton activity and spontaneous strain of Cs3H(SeO4)2 in the phase transition at 369 K

We have investigated the origin of the change in proton activity in the phase transition at T_II-III (=369 K) in Cs₃H(SeO₄)₂ from the viewpoint of its ferroelasticity by using ¹H NMR and X-ray measurements. It is found that the second moment of the ¹H NMR absorption line rapidly decreases at T_II-III with increasing temperature. From this result, we conclude that the hopping motion of a proton, which is the precursor motion in the superprotonic phase, becomes more active above T_II-III. This result is consistent with the fact that the electrical conductivity in phase II is larger than that in phase III. Furthermore, it is also found that the spontaneous strain decreases abruptly at T_II-III. From these results, it is deduced that the decrease in the spontaneous strain at T_II-III causes the increase in the proton activity at T_II-III. In addition, it is deduced that the increase in proton activity and the decrease in the spontaneous strain at T_II-III are closely related with the appearance of the superprotonic phase transition at T_I-II (=456 K).     

Proton magnetic resonance study of the low-temperature phase transition in a LiNH4SO4 single crystal

The proton NMR line width and spin-lattice relaxation times for LiNH₄SO₄ single crystal were studied at low temperature range of 6 and 280 K. The changes in the proton relaxation behavior near the phase transition temperature indicates a change in the state of internal motion at the transition. The molecular motions obtained by the spin-lattice relaxation processes were found to be determined by molecular reorientation of the NH₄ ions in phases III, IV, and V. We also confirmed that the phase transitions occur at 26 and 133 K.     

Nuclear magnetic resonance in [N(CH3)4]2CoCl4 single crystals: transferred hyperfine interaction and spin-lattice relaxation rate

The molecular susceptibility and paramagnetic shift of [N(CH₃)₄]₂CoCl₄ single crystals were measured, and from these experimental results we obtained the transferred hyperfine interaction, H_hf, due to the transfer of spin density from Co²⁺ ions to [N(CH₃)₄]⁺ ions. The transferred hyperfine interaction can be expressed as a linear equation, with H_hf increasing with increasing temperature. The remarkable change in H_hf near T_c5 (=192 K) corresponds to a phase transition. The proton spin-lattice relaxation times of [N(CH₃)₄]₂CoCl₄ single crystals were also investigated, and it was found that the relaxation process can be described by a single exponential function. The variation of the relaxation time with temperature undergoes a remarkable change near T_c5, confirming the presence of a phase transition at that temperature. From the above results, we conclude that the increase in H_hf with increasing temperature is large enough to allow the transfer of spin density between Co²⁺ ions and the nuclear spins of the nonmagnetic [N(CH₃)₄]⁺ ions in the lattice, and thus the increase in the relaxation time with temperature is attributed to an increase in the transferred hyperfine field.     

B nuclear magnetic resonance study of boron nitride nanotubes prepared by mechano-thermal method

We reported ¹¹B nuclear magnetic resonance studies of boron nitride (BN) nanotubes prepared by mechano-thermal route. The NMR lineshape obtained at 192.493 MHz (14.7 T) was fitted with two Gaussian functions, and the ¹¹B nuclear magnetization relaxations were satisfied with the stretched-exponential function, exp[-(t/T₁)^(D+1)/6] (D: space dimension) at all temperatures. In addition, the temperature dependence of spin-lattice relaxation rates was well described by (a: constant, T: temperature) and could be understood in terms of direct phonon process. All the ¹¹BNMR results were explained by considering the inhomogeneous distribution of the paramagnetic metal catalysts, such as α-Fe, Fe-N, and Fe₂ B, that were incorporated during the process of high-energy ball milling of boron powder and be synthesized during subsequent thermal annealing. X-ray powder diffraction as well as electron paramagnetic resonance (EPR) on BN nanotubes were also conducted and the results obtained supported these conclusions.     

Phase composition at surface of Fe-3%Si alloy

The structure, phase and chemical compositions of surface layers in different depths of Fe-3%Si alloy were investigated. According to the X-ray Photoelectron Spectroscopy (XPS) spectrum (penetration depth of up to ∼ 1nm) of the as-prepared sample, a layer of SiO₂ was present on the top. After the subsequent Ar⁺ sputtering (removing the SiO₂ layer), a segregation of Si atoms and two other phases were observed. The phases were described as the cubic c-FeSi and Fe₃Si. The emission⁵⁷Fe M?ssbauer spectra confirmed a presence of these phases. The α-Fe and solid solution of α-Fe + 1wt.%Si were recognized in the Conversion Electron M?ssbauer spectra (penetration depth ∼ 300nm) while the M?ssbauer spectra taken in scattering geometry with detection of 14.4 keV gamma radiation (scanning depth of ∼ 30 μm) indicate Fe-3wt.%Si solid solution as a main phase. Presented at International Colloquium “M?ssbauer Spectroscopy in Materials Science”, Všemina, Czech Republic, June 1–4, 2004. This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (Contract No. IAA1041404).     

Observation of critical behaviors in squaric acid by H nuclear magnetic resonance

¹H nuclear magnetic resonance (NMR) was employed in order to investigate the phase transitions in a two-dimensional antiferroelectric system, squaric acid (H₂C₄O₄). As a result, in addition to the critical behaviors around the long range order phase transition, similar behaviors were observed at higher temperatures where a short range order phase transition presumably takes place.     

Displacive and order-disorder behavior of KDP-type ferroelectrics on the local scale

The modified strong dipole-proton coupling (MSDPC) model, which predicted several static and dynamic dielectric properties of KH₂PO₄ or KDP-type ferroelectrics, was used to investigate the properties of these crystals on the local scale. Results calculated by molecular dynamics (MD) simulation show that both order-disorder and displacive characteristics of one PO₄ dipole are present in KDP and KD₂PO₄ (DKDP). These results correlate with experimental data from NMR and neutron scattering studies of local properties.     

Molecular reorientations of 1-bromo- and 1-iodo-adamantanes 1H N.M.R. relaxation study

Second moments and spin-lattice relaxation times, T ₁ and T _1ρ, have been measured from 100 K to 400 K for the protons in powdered 1-bromo and 1-iodo-adamantanes. Analysis of these data have shown that the reorientations are uniaxial in the low temperature phases. In the high temperature disordered phase of bromo-adamantane, the reorientation is endospherical and a slow molecular translational motion also exists. In the high temperature disordered phase of iodo-adamantane the reorientation is 12-fold uniaxial, in agreement with the Incoherent Quasi-elastic Neutron Scattering (I.Q.N.S.) experiments. All the results correspond to the crystallographic structures deduced from X-ray scattering.     

H and K spin-lattice relaxation, ionic motion and Raman processes in the proton conducting KHSeO4 single crystal

The spin-lattice relaxation rates of ¹H and ³⁹K nuclei in KHSeO₄ crystals were studied in the temperature range 160-400 K. The spin-lattice relaxation recovery of ¹H nucleus in this crystal can be represented with a single exponential function, and the relaxation T₁⁻¹ curve of ¹H can be represented with the Bloembergen-Purcell-Pound (BPP) function. The relaxation process of ³⁹K with dominant quadrupole relaxation can be described by a linear combination of two exponential functions. T₁⁻¹ for the ³⁹K nucleus was found to have a very strong temperature dependence, T₁⁻¹=βT⁷. Rapid variations in relaxation rates are associated with critical fluctuations in the electronic spin system. The T⁷ temperature dependence of the Raman relaxation rate is shown here to be due to phonon-magnon coupling.