NQR Signals Research Articles

Cu NQR Study on the “Wipeout” Phenomenon in YBa2Cu3Oy

We have investigated if the wipeout phenomenon observed for the NQR signal of La 2- x Sr x CuO 4 in the region x ∼1/8 at low temperatures, can also be observed in YBa 2 Cu 3 O y system, and found no evidence in the 60 K plateau and higher carrier concentration regions of y . Only in the region of y <6.5, where at least part of hole carriers is localized, the reduction of Cu NQR spectra at low temperatures has been found. The inhomogeneous distribution of these localized carriers seems to cause the wipeout phenomenon. The wipeout phenomenon observed for YBa 2 (Cu 0.98 Zn 0.02 ) 3 O y in the wide region of y can also be understood by the same mechanism.

Circular polarization excitation and detection in (14)N NQR.

Circular polarization excitation and detection of (14)N NQR signal are reported. A theoretical model is presented in terms of fictitious spin-1/2 operators and is compared to experiments performed on a powder crystalline sample of RDX. It is shown that in spin-1 systems with finite asymmetry--unlike previously reported NMR and symmetric spin-3/2 NQR systems (Chen et al., J. Magn. Reson. 54, 324--327, 1983; Weber and Hahn, Phys. Rev. 120, 365--375, 1960)-the circular polarization nature of the signal is due to powder orientation effects in polycrystalline samples. Sensitivity improvements up to a factor of the square root of 2 are reported using the same hardware and switching modes from linear polarization to circular polarization; this also is shown to result from the polycrystalline nature of the samples.

Phase transitions of Cs3Sb2I9, Cs3Bi2I9, and Cs3Bi2Br9 crystals

The phase transitions of Cs3Sb2I9 , Cs3Bi2I9 , and Cs3Bi2Br9crystals were investigated by 133Cs nuclear magnetic resonance. Cs3Bi2Br9was also studied by 87Br nuclear quadrupole resonance. The parameters of the Cs electric-field gradient tensors were determined. The phase transitions were shown to produce significant changes in the nearest neighbor environment of only one of two inequivalent Cs sites. In Cs3Sb2I9 , the tensors remain axisymmetric below the transition at 85 K, in line with the assumption relying on powder x-ray diffraction data that, below the transition temperature, Cs3Sb2I9has a trigonal structure. The observed temperature variation in the shape of the 133Cs NMR and 127I NQR signals suggests that Cs3Bi2I9may undergo a transition to an incommensurate phase.

The valence state of bismuth in BaBiO 3 probed by NQR

We report the NQR on Ba nuclei in four samples of BaBiO 3 prepared in different ways. The spectra of 137Ba consist of broad line centered near 18 MHz at 4.2 K. The broadening of NQR suggests a local distortions of the crystal structure. The area under the line for ceramic sample BaBiO 3 synthesized at 800°C is approximately two times larger comparing to ceramic samples and single crystal prepared at 930–1080°C. The decrease of the NQR signal is attributed to the partial disordering of charge disproportionating Bi ions on the two inequivalent sites.

The direct Cu NQR study of the stripe phase in the lanthanum cuprates

Using Cu NQR in Eu-doped La 2−xSr xCuO 4 we find the evidence of the pinned stripe phase at 1.3K for 0.08≤x≤0.18. The pinned fraction increases by one order of magnitude near hole doping x=1/8. The NQR lineshape reveals three inequivalent Cu positions. A dramatic change of the NQR signal for x>0.18 correlating with the onset of bulk superconductivity corresponds to the depinning of the stripe phase.

35Cl NQR in Glassy Crystal of 2-chlorothiophene

Stable crystalline 2-chlorothiophene has two glass transitions at 164 and 186 K. 35CI NQR measurements were carried out between 77 and 200 K. Two NQR signals with full widths of about 100 kHz at half maximum were observed in this temperature range. The spin-lattice relaxation times T1 were measured at the two peak frequencies. The activation energy obtained from the results of the T1 measurements showed a fairly good agreement with those estimated from calorimetric measurements.

Optimization of NQR Pulse Parameters using Feedback Control

A new method for increasing the probability of detecting nuclear resonance signals is demon-strated experimentally. It is well known that the detection of signals with a low signal to noise ratio (SNR) results in missed detections of false alarms. In situations where the noise is correlated or where limited data is averaging, it may not be possible to achieve a desired SNR through averaging alone. We present an alternative approach in which a feedback algorithm automatically adjusts pulse parameters so that the SNR and probability of correct detection are increased. Experimental results are presented for the detection of 14N NQR signals.

Temperature dependence study of chlorine NQR frequency and spin-lattice relaxation time in 2-amino,-3,5-dichloropyridine

Chlorine-35 NQR frequency and spin–lattice relaxation time measurements as a function of temperature in the range 77–300 K were carried out on 2-amino-3,5-dichloropyridine. Two NQR signals were observed and were assigned to the two chlorines present in the molecule using the additive model for substituent effects. The temperature dependence of the NQR frequency was analysed in terms of the torsional oscillations of the molecule and the torsional frequencies and their temperature dependence were calculated numerically using a two-mode approximation.The temperature dependence of the NQR spin–lattice relaxation time was found to be mainly due to the torsional oscillations of the molecule, with anharmonicity effects showing up at higher temperatures. Copyright © 1999 John Wiley & Sons, Ltd.

Reconstructing Powder NQR Images with Real Gradient Coils

Unlike NMR imaging, where only the components of the magnetic field gradients along the main applied field need be considered, all nonzero components of the gradients must be considered for NQR imaging of powder or polycrystalline materials. Any physically realizable gradient coil will have at least two nonzero components. Since the nonzero components are of comparable magnitude, image reconstruction will not be as straightforward as is found for NMR imaging. Here a practical NQR imaging reconstruction technique based on the use of standard techniques but using a nonrectilinear grid is demonstrated for physically realizable gradients. A two-dimensional image constructed using the35Cl NQR signal from a polycrystalline sample of NaClO3is presented to demonstrate the technique.

Transient NQR signals in the effective field of multipulse trains

This paper is a theoretical analysis of time-dependent nutation and echo signals in the effective field of multipulse trains in NQR. The results of experimental investigations of the same aspects were reported in earlier papers. The various features of dipole-dipole interactions in three-level quadrupole spin systems are discussed. It is shown that, in contrast to NMR, the dipole Hamiltonian in the interaction representation determined by the quadrupole Hamiltonian contains only three diagonal components of the dipole-dipole interaction tensor. On the other hand, the strong inhomogeneous broadening characteristic of NQR hinders exact measurement of these components by ordinary methods. The theoretical analysis suggests that the decay of the echo-signal envelope in the effective field of multipulse trains is determined solely by the dipole relaxation time, which serves as justification of a new experimental method used in measuring the characteristics of the dipole-dipole interaction tensor in spin systems with an inhomogeneously broadened spectrum.

35Cl NQR and structural-dynamic properties of crystalline CCl3C(O)OC6H2Cl3-2,4,6

The temperature dependences of the resonance frequency and quadrupole spin-lattice relaxation time of the chlorine-35 nuclei in crystalline CCl3C(O)OC6H2Cl3-2,4,6 were studied. Owing to the presence of resonant nuclei in various fragments of the molecules, the character of dynamics of these molecular fragments and the entire molecule was established. It is shown that thermal librations of molecules are quasiharmonic over the whole temperature range from 77 K to the melting point of the crystal. The reorientational motion of the CCl3 group bonded to the three-coordinated carbon atom was revealed. This motion causes an exponential increase in the spin-lattice relaxation time of the chlorine nuclei of this group and subsequent damping of NQR signals (chlorine-35 resonance signals of the aryl radical were observed before melting of the sample). The activation energy of the reorientational motion is found; its value, which is 27.3 kJ/mole, is considered in comparison with the activation energies obtained by the Cl NQR method in solids for CCl3 reorientations in similar molecular structures.

Microscopic measurements in 139La-NQR of the ternary carbide superconductor LaNiC 2

139La-NQR measurements have been carried out in the ternary carbide superconductor LaNiC 2. The nuclear quadrupole frequency and the asymmetry parameter of 139La in LaNiC 2 were estimated to be about 1.9 MHz and 0, respectively. In the normal state, the nuclear spin relaxation rate ( 1 T 1 ) in the 139La NQR signal was proportional to temperature ( T) in zero external field above the superconducting transition temperature ( T c) or in an external field larger than the superconducting critical field, which means the system is in the Fermi-liquid state. In the superconducting state, on the other hand, 1 T 1 decreases no more linearly with T, but decreases rapidly exponentially as exp ( −Δ k B T ) at low T with an appreciable enhancement just below T c. The value of the superconducting energy gap, 2Δ, was estimated to be 3.34 k B T c, compared with 3.52 k B T c of the BCS-value. This result strongly suggests that the superconductivity in LaNiC 2 is of a conventional BCS type.

NQR View of Solid Phases of NOCl

Abstract Chlorine NQR studies of solid nitrosyl chloride NOC1 between 110 and 212 K are reported. In the temperature dependence an order-disorder phase transition near 145 K, found by X-ray diffraction, is confirmed. Unusual hysteretic NQR signal behaviour indirectly proves also the low temperature phase transition below 100 K. In the measured temperature region fast relaxing mechanisms are present. The upper limit of the expected 14N NQR frequency is estimated using the Townes-Dailey approach.

Preparation and NMR study of fluorinated Pb-BSCCO-2223

Samples of (BiPb) 2Sr 2Ca 2Cu 3O 10+δ (BSCCO-2223) prepared by the sol-gel route were fluorinated in ClF 3, resulting in approximately e fluorine atoms per formula as measured by fluoride selective-electrode. The samples were characterized by x-rays diffraction (XRD) and magnetization re parameters not T c (114K) changed after the fluorination. Nuclear magnetic resonance (NMR) measurements of 19F in the temperature range of 20–400K show that for T>270K there are 2 types of F nuclei environment. At T<270K only one line is resolved. At T<T c, the fluorine linewidth increases with decreasing temperature. This is attributed to the formation of vortices. At very low temperatures, a new structure appears. This is attributed to an enhanced NQR signal from the Cu nuclei.

X-Ray and 35Cl NQR Studies on the Trichloroacetyl Group in Covalent and Ionic Compounds of L-Valine and DL-Valine

Abstract The crystal structures and 35Cl NQR of trichloroacetyl-DL-valine (1), trichloroacetyl-L-valine (2), as well as the salts of trichloroacetic acid with DL-valine (3) and L-valine (4) have been investigated. Crystal data are for (1): Monoclinic, C2/c, a= 15.835(4) Å, b= 10.481 (3) Å, c = 14.046(4) Å, β = 103.28(1), Z = 8; (2): Orthorhombic, P212121, a = 12.117(3) Å, 6=10.896(3) Å, c= 8.718(2) Å, Z = 4; (3): Triclinic, P1̄, a= 17.269(3) Å, 6 = 8.504(3) Å, c = 10.427(4) A, a = 105.38(2), β = 96.98(2), γ = 96.24(2), Z = 2; (4): Monoclinic, P21, a = 10.378(4) Å, b = 20.349(8) Å, c= 11.890(5) Å, β = 95.28(2), Z = 8. The onset of rotational motion within the trichloroacetyl groups bleaches out 35Cl NQR lines between 115 K and 185 K for (1)-(4). While TCA-L-valine (1), TCA-DL-valine (2), and TCA(-) · DL-valine(+) (3) do not show any phase transition in the temperature range 77 K to 295 K, TCA(-) · L-valine(+) (4) shows more than one phase transition above 77 K before the three NQR signals bleach out at 164 K.

An NQR Study of Inclusion Compounds Formed Between Small Chlorine-containing Molecules and bis(N-alkylimidazolidine-2-thione)Cu(I) Halide Complexes

Abstract Twenty-five inclusion complexes formed by bis(N-alkylimidazolidine-2-thione)cuprous halides with the small, chlorine-containing guest molecules CCl4, CHCl3, CH2Cl2, CH3CCl3 or C2H2Cl2 , have been prepared. Their 63Cu and 35Cl NQR spectra have been measured over the temperature range 77-300 K. For the corresponding bromides the 79Br and/or 81Br spectra have also been investigated. The studies reveal the number and symmetry of different inclusion site. The onset of rapid reorientation of the guest molecules is indicated by the fading out of their NQR signals between 77 and 300 K, whereas the resonances from the host molecules usually persist throughout this temperature range. However, as evidenced by discontinuities in the temperature-dependence of the NQR frequencies of the hosts, sixteen of the complexes show one or more phase-changes in this temperature-range.

NQR study of molecular dynamics in tetramethylammonium hydrogen bis(difluorochloroacetate)

The effect of temperature on the 35Cl NQR spectra and the spin-lattice relaxation time T1Q has been studied in N(CH3)4H(ClF2CCOO)2. The NQR signal disappeared at a temperature of about 110 K. The T1Q of the chlorines is attributed to the sum of two contributions: the reorientation of the CClF2 group and the modulation of the electric field gradient (EFG) produced by the motion of the N(CH3)+4 cation. The activation energies were determined to be 21.2 and 10.7 kJ mol−1 for the CClF2 and N(CH3)+4 motion respectively.

NQR studies in oxygen annealed La 2− xBi xCuO 4+δ

La NMR and NQR studies on polycrystalline samples of La 2CuO 4+δ and La 2− x Bi x CuO 4+δ ( x = 0.05–0.20) Bi doped samples, lower superconducting transition temperature and larger Meissner volume fraction were measured compared with those of La 2CuO 4+δ. No phase separation was observed in X-ray diffraction patterns at room temperature and 7 K. La and Cu NQR signals were observed, showing no evidence of the existence of antiferromagnetic internal field. New resonance lines observed around 57 MHz are probably NQR signals of the Cu nuclei at the vicinity of the excess oxygen atom.

Cation self-diffusion in MAlX 4 (M = Cu, Ag; X = Cl, Br) studied by X-ray diffraction, 63Cu NMR and halogen NQR

Crystal structures of CuAlCl 4 and CuAlBr 4 have been determined by Rietveld analysis of the X-ray powder diffraction data. These crystals belong to a tetragonal system in which both Al and Cu atoms are surrounded by four halogens tetrahedrally. The AlCl(Br) and CuCl(Br) bond lengths are 2.10 Å (2.26 Å) and 2.39 Å (2.48 Å), respectively. Consistent with the structure, 63Cu NMR experiments using a single crystal have suggested that the principal axis of the e 2Qq h tensor is parallel to the c axis. The quadrupole coupling constants were estimated to be 0.89 and 0.35 MHz for chloride and bromide analogs, respectively. Cu + diffusion was confirmed by 63Cu NMR measuring the temperature dependence of linewidth and lineshape. The activation energies for the Cu + diffusion were 48.5 kJ mol −1 and 46.4 kJ mol −1 for CuAlCl 4 and CuAlBr 4, respectively. The spin-lattice relaxation times of the 35Cl and 81Br NQR signals were also affected by Cu + diffusion.

A 81Br NQR study on structural phase transitions in (C nH 2 n+1 NH 3) 2ZnBr 4 crystals ( n = 1−4)

The temperature dependence of the 81Br NQR frequencies in (C n H 2 n+1 NH 3) 2ZnBr 4 ( n = 1−4) was measured above 77K. Four 81Br NQR lines were observed for all the compounds at 273K. It was found that (CH 3NH 3) 2 - . ( n-C 3H 7NH 3) 2 - and ( n-C 4H 9NH 3) 2ZnBr 4 undergo second-order structural phase transitions at 426, 298, and 299K, respectively. Above the transition temperatures three NQR lines were observed for the first two compounds, whereas only one signal could be detected for the last one. However, (C 2H 5NH 3) 2ZnBr 4 showed a first-order phase transition at 299.5K, above which temperature no NQR signal was observable. The nature of these phase transitions is discussed.