A.H. Vuorimäki

NMR Line shape and cross relaxation of tunneling ammonium protons in (NH4)2ZnCl4

The second moment M2 of the proton NMR spectrum and the time constant TSD of the magnetization transfer between the protons of the A and T symmetry species of NH4+ ions were measured as functions of crystal orientation and temperature down to 4.5 K in (NH4)2ZnCl4. M2 increases in two steps with decreasing temperature and shows, between the steps, a strong dependence on orientation. TSD has a maximum of 3 ms at 12 K. The TSD data and earlier results on T1D, the spin-lattice relaxation time of the dipolar energy, are compared with a model. The results are explained by the presence of two kinds of ammonium sites in this compound.

Spin-lattice relaxation of the nuclear dipolar energy and spin diffusion in NH4ClO4

Abstract The spin-lattice relaxation time of the dipolar energy T 1D and the characteristic time T SD for the spin diffusion between NH 4 + ions of different librational symmetry species were measured as functions of temperature and crystal orientation in NH 4 ClO 4 . T 1D shows shoulders around 44 and 28 K related to the librational tunnelling frequency and the proton resonance frequency, respectively. A broad minimum around 10 K corresponds to the linewidth transition. T SD is less than 0.5 ms above 4 K, showing that the ammonium protons in different symmetry species have, at least locally, a common spin temperature during the magnetization recovery.

Fluorine spin diffusion and relaxation in CF3COOAg

Abstract Besides the Zeeman and dipolar energy, the description of the 19F spin system of rotating CF3 groups involves several quasi-invariant quantities. We measured the lifetimes of two such quasi-invariants in a single crystal of CF3COOAg between 80 and 296 K. The decay of both quantities is much faster than the Zeeman and dipolar-energy relaxation and independent of temperature above 180 K. The time constants, 4 and 0.3 ms, exceed the spin-diffusion times calculated for the system of fast reorienting spin- 1 2 triads. The disagreement is understood by the arrangement of the CF3 groups to proximate pairs in this crystal. The Zeeman relaxation was observed in the temperature range 130–296 K, found non-exponential above 180 K and compared with the Emid—Wind model.

Cross relaxation of the proton magnetization in ammonium compounds

Abstract Expressions are derived for the time constants T1D and TSD of the NH4 protons in tunneling ammonium compounds below the line-width transition temperature. T1D characterizes the speed of the spin-lattice relaxation of the dipolar energy and TSD the speed of the cross relaxation between the A and T symmetry species. The expressions should be valid if all the tunnel splittings between the T species levels are larger than the magnetic dipolar interaction. Predictions are compared with new experimental results on TSD in (NH4)2PbCl6 and with some earlier results on TSD and T1D in (NH4)2 SnBr6 and NH4ClO4. They support the conclusion that for T1D>TSD the T levels are nondegenerate, while the condition T1D

Effect of the grain size on the tunnel distribution in (NH4)2ZnCl4

Abstract The effect of the grain size of a polycrystalline sample on the rotational tunnelling of ammonium ions is studied in (NH4)2ZnCl4 by NMR level crossing. When a single crystal is ground to a powder of 1–2μm grains, the width of the distribution of the tunnel splitting is observed to increase tenfold and the peak position shifts somewhat.

Low-temperature 1H nuclear magnetic resonance study of crystal and electronic structures of the nearly stoichiometric yttrium dihydride

Abstract Proton nuclear magnetic resonance absorption spectra and spin-lattice relaxation rates in yttrium dihydride have been measured in the temperature range from 4.2 to 310K at 36.0MHz. The second moment of the resonance line corresponds to the rigid-lattice regime for YH1.99 and its value agrees with the anticipated CaF2 type of structure. The main contribution to the spin-lattice relaxation rate R 1 arises from conduction electrons and is characterized by R le = 2.77 × 10−3s−1 K−1 × T. Evidence of proton self-diffusion was seen in the linewidth and in R 1 for YH1.99+0.1. The onset temperature, about 250 K, of the self-diffusion, is close to the metal-semiconductor transition reported earlier for that hydride. Below that temperature the linewidth becomes temperature independent and the second moment of the line is explained in terms of different structure models. The fit to the temperature dependence of R 1 in the temperature range 70–310K gives R le = 2.1 × 10−3s−1K−1 × T. The relaxation becomes almost temperature independent below 50 K. Various mechanisms for this behaviour are discussed. In addition, the R 1 data for the sample prepared with yttrium of 99.9% purity are presented. In contrast with the previous case, where pure yttrium from the Ames Laboratory was used, R 1 has a large contribution of spin diffusion to the paramagnetic Gd3+ ions.

Proton double quantum coherence and cross-relaxation in (NH4)2SnBr6

A proton double quantum coherence signal can be observed exclusively from the T species NH4 groups (with the total spin I = 1) in ammonium compounds at low temperatures by the three-pulse sequence 90x degrees - tpr - 90x degrees - tev - 90x degrees - ta, where tpr and ta are of the magnitude of the inverse line width and tev very short. The usefulness of this pulse sequence, preceded by the additional pulse sequence 90x degrees - t1 -90(-x) degrees - t2 for creating an unbalance between the A and T species magnetizations, was demonstrated by applying it to cross-relaxation studies in (NH4)2SnBr6.

Low temperature1H NMR study of electronic structure parameters (T1e T) in zirconium dihydride

Measurements of the proton-spin-lattice relaxation times (T1) are reported for zirconium dihydrides: ZrH1.87, ZrH1.93 and ZrH2.00 at nominal frequency of 41 MHz, with emphasis on the low temperature (4.2–25 K) behaviour of theT1. The dominant contributions are originated from the hyperfine interactions of the protons with the conduction electrons and the paramagnetic impurities. The (T1e 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.