Relaxation Mechanisms and Effects of Motion in Albite (NaA1Si308) Liquid and Glass: A High Temperature NMR Study (original) (raw)
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Physics and Chemistry of Minerals, 1987
The nuclear magnetic relaxation of 23Na and z9Si in albite glass and liquid has been studied from 800 K to 1400 K. The dominant spin-lattice relaxation mechanism for 23Na is found to be nuclear quadrupole interaction arising from the Na + diffusion. The activation energy of the Na diffusion is found to be 71 _+ 3 kJ/mol, in close agreement with the results on electrical conductivity and on Na self-diffusion from radio-tracer experiments. The correlation time of the Na motion is estimated to be about 8.5 x 10 11 s near the melting point (~1390 K). Both nuclear dipole-dipole interaction and chemical shift anisotropy interaction are large enough to contribute to the 298i relaxation. However, calculations based on a simplified model which employ single correlation time and exponential correlation function, with interactions typical of crystalline silicates, cannot completely account for the experimental data. N M R relaxation data also reveal that the Si motion is correlated to the Na motion and that the Si is relatively immobile. Several possible motions of SiO 4 tetrahedra that can cause 298i relaxation are suggested. The motion responsible for 29Si relaxation differs from that which is responsible for viscosity: the apparent activation energy for the former is much lower. Measurements of spin-spin relaxation times and linewidths are also presented and the significance of their temperature dependence is discussed.
Molecular motions in thermotropic liquid crystals studied by NMR spin-lattice relaxation
Brazilian Journal of Physics, 1998
Nuclear magnetic resonance relaxation experiments with eld cycling techniques proved to b e a v aluable tool for studying molecular motions in liquid crystals, allowing a very broad Larmor frequency variation, su cient to separate the cooperative motions from the liquidlike molecular di usion. In new experiments combining NMR eld cycling with the Jeener-Broekaert order-transfer pulse sequence, it is possible to measure the dipolar order relaxation time T 1D , in addition to the conventional Zeeman relaxation time T 1Z in a frequency range of several decades. When applying this technique to nematic thermotropic liquid crystals, T 1D showed to depend almost exclusively on the order uctuation of the director mechanism in the whole frequency range. This unique characteristic of T 1D makes dipolar order relaxation experiments specially useful for studying the frequency and temperature dependence of the spectral properties of the collective motions.
Cross-relaxation processes of polarized?-active nuclei in various crystalline solids
Zeitschrift f�r Physik B Condensed Matter and Quanta, 1980
In the present work β-spodumene LiAlSi 2 O 6 and the corresponding glass form a model system highly suitable to study the influence of particle size on Li ion dynamics. The nanostructured samples were prepared by high-energy ball milling of the coarse grained starting material and the corresponding glass, respectively. Diffusion parameters and electrical conductivity were measured by 7 Li spin-lattice relaxation nuclear magnetic resonance (NMR) and impedance spectroscopy. As reported previously, the Li diffusivity in the glassy sample is larger than that in the coarse grained crystalline material of the same chemical composition [Franke et al., Ber. Bunsenges. Phys. Chem. 96, 1674 (1992).] which is quite often observed also for other materials. Decreasing the particle size down to the nm-regime causes an enhancement of the Li conductivity of β-spodumene LiAlSi 2 O 6 reaching an upper limit which is, however, still lower than the conductivity of the corresponding glass. Surprisingly, when the glassy material is mechanically treated under the same conditions, the Li diffusivity is slowed down. The Li conductivity of such a sample resembles that of nanocrystalline LiAlSi 2 O 6. This is astonishing since one might expect that mechanical treatment of a glassy sample does not further influence the transport parameters. A possible structural description trying to explain the observed convergence of the transport parameters of the crystalline and glassy materials as a result of milling is briefly presented.
The Journal of Chemical Physics, 1990
Molecular dynamics calculations have been used to study structural relaxation and dynamical correlations near the glass transition in the system [Ca(N0 3 h]o.4 [KN0 3 ]o.6' As in a typical molten salt, the overall structure is determined by charge ordering. However, the radial distribution function for Ca 2 + ions is unusual in that even at high temperatures it shows a split first peak due to specific spatial correlations of the cations with the nitrate anions. Structural relaxation that accompanies cooling of the system has been characterized with the aid of the van Hove real-space correlation functions G s (r,t) for the constituent atoms (Ca, K, N, 0). The relaxation of the incoherent structure factor Fs (k,t), with a wave vector k near the peak of the static structure factor, has been investigated as a function of temperature. The results clearly reveal both the a and /3 relaxation processes; the former can be well represented by a master curve with a stretched exponential shape. An analysis of the susceptibility, which agrees qualitatively with neutron spin-echo data, suggests that the glass transition for the model occurs around 400 K. The relatively small discrepancy with the experimental transition temperature derived from neutron scattering data (366 K) is likely related to inadequacies in the model employed for the interionic interactions. The functions C1(t) and C 2 (t) , which describe the reorientational relaxation of the threefold symmetry axes of the nitrate ions, are shown to exhibit a scaling behavior analogous to that of the structure factor. In the region of the glass transition, where translational diffusion has essentially stopped, the nitrate ions continue to flip predominantly about their twofold axes.
Journal of Magnetic Resonance, 1999
The temperature dependence of X-band electron spin-lattice relaxation between about 10 and 300 K in magnetically dilute solids and up to the softening temperature in glassy solvents was analyzed for three organic radicals and 14 S ؍ 1 2 transition metal complexes. Contributions from the direct, Raman, local vibrational mode, thermally activated, and Orbach processes were considered. For most samples it was necessary to include more than one process to fit the experimental data. Debye temperatures were between 50 and 135 K. For small molecules the Debye temperature required to fit the relaxation data was higher in 1:1 water:glycerol than in organic solvents. For larger molecules the Debye temperature was less dependent upon solvent and more dependent upon the characteristics of the molecule. The coefficients of the Raman process increased with increasing g anisotropy and decreasing rigidity of the molecule. For the transition metal complexes the data are consistent with major contributions from local modes with energies in the range of 185 to 350 K (130 to 240 cm ؊1 ). The coefficient for this contribution increases in the order 3d < 4d transition metal. For C 60 ؊ anions there is a major contribution from a thermally activated process with an activation energy of about 240 cm ؊1 . For low-spin hemes the dominant contribution at higher temperatures is from a local mode or thermally activated process with a characteristic energy of about 175 cm ؊1 .
Spin-lattice relaxation rates (R 1H and R 1F ) of two nuclear species ( 1 H and 19 F) are measured at different temperatures in the isotropic phase of a liquid crystal (4 -butoxy-3 -fluoro-4isothiocyanatotolane-4OFTOL), over a wide range of Larmor frequency (10 kHz-50 MHz). Their dispersion profiles are found to be qualitatively very different, and the R 1F in particular shows significant dispersion (varying over two orders of magnitude) in the entire isotropic range, unlike R 1H . The proton spin-lattice relaxation, as has been established, is mediated by time modulation of magnetic dipolar interactions with other protons (case of like spins), and the discernable dispersion in the mid-frequency range, observed as the isotropic to nematic transition is approached on cooling, is indicative of the critical slowing of the time fluctuations of the nematic order. Significant dispersion seen in the R 1F extending to very low frequencies suggests a distinctly different relaxation path which is exclusively sensitive to the ultra slow modes apparently present in the system. We find that under the conditions of our experiment at low Zeeman fields, spin-rotation coupling of the fluorine with the molecular angular momentum is the dominant mechanism, and the observed dispersion is thus attributed to the presence of slow torques experienced by the molecules, arising clearly from collective modes. Following the arguments advanced to explain similar slow processes inferred from earlier detailed ESR measurements in liquid crystals, we propose that slowly relaxing local structures representing such dynamic processes could be the likely underlying mechanism providing the necessary slow molecular angular momentum correlations to manifest as the observed low frequency dispersion. We also find that the effects of the onset of cross-relaxation between the two nuclear species when their resonance lines start overlapping at very low Larmor frequencies (below ∼ 400 kHz), provide an additional relaxation contribution.
Journal of Non-Crystalline Solids, 2003
Electrical conductivity relaxation (ECR) experimental data represented by either the complex conductivity or the complex electric modulus are macroscopic in nature. In contrast to ECR, nuclear spin relaxation is a more direct probe of ionic movement and from its result we can infer the microscopic dynamics of the ions. Combined studies of ionic motion using ECR and nuclear spin-lattice relaxation (SLR) in several glassy ionic conductors have shown a large difference between the ECR and SLR times. Any theory that attempts to explain quantitatively the difference faces the dilemma of how to compare the SLR time with the ECR time. In this work we use a recent result [Phys. Rev. B 60 (1999) 9396] to find the ion hopping correlation time from the experimental ECR time. Next, we use the coupling model to calculate the SLR time from the ion hopping correlation time. Good agreements are obtained in three glassy ionic conductors and a crystalline ionic conductor.
Diffusion in solids, which requires the presence of crystal defects or disorder, has both microscopic and macroscopic aspects. Nuclear magnetic resonance techniques provide access to microscopic diffusion parameters like atomic jump rates and activation energies as well as to the tracer diffusion coefficient for macroscopic transport. Microscopic NMR methods include spin-lattice relaxation spectroscopy of stable and beta-radioactive nuclei, spin-spin relaxation or linewidth and spin alignment decay measurements, whereas macroscopic NMR methods are represented by the techniques of static and pulsed field gradient NMR. We recall some basic principles of the mentioned techniques and review case studies for their application to various solids like glassy and crystalline aluminosilicates, nanocrystalline composites, an intercalation compound and a simple bcc metal. Taken together, jump rates in solids are covered over about 10 decades by the microscopic, and diffusion coefficients over 3 decades by the macroscopic NMR methods.
Slow dynamics in a liquid crystal: 1H and 19F NMR relaxometry
The Journal of Chemical Physics, 2011
Spin-lattice relaxation rates (R 1H and R 1F ) of two nuclear species ( 1 H and 19 F) are measured at different temperatures in the isotropic phase of a liquid crystal (4 -butoxy-3 -fluoro-4isothiocyanatotolane-4OFTOL), over a wide range of Larmor frequency (10 kHz-50 MHz). Their dispersion profiles are found to be qualitatively very different, and the R 1F in particular shows significant dispersion (varying over two orders of magnitude) in the entire isotropic range, unlike R 1H . The proton spin-lattice relaxation, as has been established, is mediated by time modulation of magnetic dipolar interactions with other protons (case of like spins), and the discernable dispersion in the mid-frequency range, observed as the isotropic to nematic transition is approached on cooling, is indicative of the critical slowing of the time fluctuations of the nematic order. Significant dispersion seen in the R 1F extending to very low frequencies suggests a distinctly different relaxation path which is exclusively sensitive to the ultra slow modes apparently present in the system. We find that under the conditions of our experiment at low Zeeman fields, spin-rotation coupling of the fluorine with the molecular angular momentum is the dominant mechanism, and the observed dispersion is thus attributed to the presence of slow torques experienced by the molecules, arising clearly from collective modes. Following the arguments advanced to explain similar slow processes inferred from earlier detailed ESR measurements in liquid crystals, we propose that slowly relaxing local structures representing such dynamic processes could be the likely underlying mechanism providing the necessary slow molecular angular momentum correlations to manifest as the observed low frequency dispersion. We also find that the effects of the onset of cross-relaxation between the two nuclear species when their resonance lines start overlapping at very low Larmor frequencies (below ∼ 400 kHz), provide an additional relaxation contribution.