Nuclear Quadrupole Resonance Spectra Research Articles

14N nuclear quadrupole resonance studies of 1,8-bis(dimethylamino)naphthalene

The presence of four resonance peaks from two nitrogen atoms in the 14N nuclear quadrupole resonance (NQR) spectrum of 1,8-bis(dimethylamino)naphthalene (DMAN) confirms the asymmetry of this molecule in the crystal. Spectroscopic parameters such as transition frequencies as well as the asymmetry parameters of the electric field gradient (EFG) tensors and differences in sigma bond occupation numbers have been assigned to particular nitrogen atoms in the molecule.

Effect of Preparative Temperatures on Nuclear Quadrupole Resonance Spectra in Tantalum Pentaiodides*

Effect of Preparative Temperatures on Nuclear Quadrupole Resonance Spectra in Tantalum Pentaiodides^*

Influence of hydrogen on 123 and 124 HTc superconductors studied by Cu NMR and NQR

The antiferromagnetic compounds produced by the reaction of the 123 and 124 HTc superconductors with hydrogen are studied by nuclear resonancee of copper. For the 123 compounds two reactions are possible, the reduction by hydrogen which leads to the normal antiferromagnetic YBa2Cu3O6, and the solution of hydrogen in YBa2Cu3O7 which forms a different antiferromagnetic structure with hydrogen concentration of more than 2 H mol−1. Both structures can be distinguished by the nuclear quadrupole resonance spectra of the Cu(1) sites and the nuclear magnetic resonance spectra in the hyperfine field of the antiferromagnetic state. By using samples containing Gd at the place of Y we can prove that only the Cu(2) sites carry permanent magnetic moments. In the hydrogen loaded YBa2Cu4O8 the situation is similar with the antiferromagnetic order limited to the Cu(2) sites.

Low-temperature single-crystal X-Ray diffraction,1H and13C solid-state NMR and14N NQR studies of 1,8-bis(dimethyl-amino)naphthalene

Low-temperature 13C and 1H MAS NMR and 14N NQR spectra of solid 1,8-bis(dimethylamino)naphthalene (DMAN) have been measured. X-Ray diffraction shows that the molecule is more symmetric than at room temperature. The low-temperature geometry of DMAN is very similar to the ab initio optimized geometry. Solid-state NMR and nuclear quadrupole resonance (NQR) confirm the symmetry of the molecule at low temperature despite the asymmetry of intermolecular interactions in the crystal. Experimental values of the principal components of electric field gradients at 14N, which characterize the properties of the lone pair electrons of the nitrogen atoms, are in good agreement with the ab initio calculated values.

MOLECULAR STRUCTURES OF PCl4F, PCl3F2, AND PCl2F3: PURE CHLORINE NUCLEAR QUADRUPOLE RESONANCE AND LOW TEMPERATURE F19 NUCLEAR MAGNETIC RESONANCE SPECTRA

Abstract Chlorine nuclear quadrupole resonance spectra determined at 77°K. and F19 n.m.r. data obtained as a function of temperature for the molecular forms of PCl4,F, PCl3F2, and PCl2F3 were correlated with previous infrared and Raman spectra. The data support the trigonal bipyramid as the structural model for the halides with fluorine atoms showing a preference for axial positions. The symmetry of PCl4F is C3v and that of PCl3F2 is D3h. For PCl2F3 the data are best interpreted in terms of the C2v point group. Further, the data give no evidence for any significant variation in molecular structure between the gas, liquid, and solid states. Symmetry considerations and the F19 n.m.r. data support the presence of axial P‒F π-bonding. The chlorine quadrupole data indicate a lesser importance of π-bonding in the P‒Cl bonds.

The incommensurate phase of 4,4'-dichlorobiphenyl sulphone reinvestigated by 35Cl NQR

In order to acquire new information about the normal-incommensurate (IC) phase transition and to check the existence of a lock-in transition in 4,4'-dichlorobiphenyl sulphone (4,4'-DCBS), a comprehensive 35Cl nuclear quadrupole resonance (NQR) study is reported. Detailed NQR measurements of frequency, line width and spin relaxation time (Tl) were obtained in the temperature range 80-270 K. The results show that the IC phase is present from 150 K down to 80 K. The behaviour of the NQR spectrum in the transition region suggests that the IC modulation is dominated by the first-order term in the order parameter. The tl values at the peaks of the IC spectrum are quite different, suggesting strong anharmonic effects. No lock-in transition was detected at 115 K. Small changes in the temperature behaviour of the frequency and the tl of the low-frequency IC peak were observed from temperatures lower than 110 K. Neither impurities nor thermal hysteresis effects were detected.

NMR and NQR study in HgBa 2CuO 4+δ

The temperature dependences of the Cu nuclear spin-lattice relaxation rate (1/ T 1) and the Knight shift at the planar Cu site in the as-sintered HgBa 2CuO 4+δ with T c ≈ 84K have been measured by the nuclear quadrupole resonance(NQR) and the nuclear magnetic resonance(NMR) techniques. The as-sintered sample seems to be a lightly doped material because of the increase of T c by annealing in the oxygen atmosphere. From the broadness of the NQR spectrum and the distribution of the (1/ T 1), we conclude that the as-sintered sample has an inhomogeneity of the atomic location, especially oxygen. The temperature dependence of (1/ T 1) behaves as T n ( n ≈ 3−4) and there is no Hebel-Slichter peak below T c , which is re-confirmed the common feature in high- T c cuprate superconductor.

Observation of Magnetic Ordering in La2-xSrxCuO4aroundx=0.115–La-NQR Study–

The nuclear quadrupole resonance (NQR) study of 139 La in La 2- x Sr x CuO 4 was systematically carried out in the Sr-content range from 0.085 to 0.15 at 1.4 and 4.2 K in zero field. The linewidth of the 139 La NQR spectrum for the (±1/2→±3/2) transition was found to increase especially at x =0.115, associated with the occurrence of magnetic ordering. This is hence considered to be responsible for the decrease of the superconducting transition temperature around x =0.115. Since the spectrum does not broaden with for the (±5/2→±7/2) transition down to 1.4 K, the ordered Cu-moments are considered to exist in the Cu-O 2 sheets.

Low Frequency Nuclear Quadrupole Resonance with SQUID Amplifiers

Abstract The dc SQUID (Superconducting QUantum Interference Device) can be configured as an ampli­fier of spin-echos with a noise temperature of approximately 10 mK (f/1 M Hz) at an operating temperature of 1.5 K. A Fourier transform spectrometer based on a SQUID with a superconducting input circuit and operated in a flux-locked loop is used to obtain nuclear quadrupole resonance (NQR) spectra in a broadband m ode over the bandwith 0 -1 M Hz. Spin-echo spectra of 14N in NH4ClO4 reveal sharp NQR resonances, obtained simultaneously, at 17.4, 38.8 and 56.2 kHz. At 1.5 K, the measured longitudinal and transverse relaxation times T1 and T2 for the 38.8 kHz transition are 63 ± 3 ms and 22±2 ms, respectively.

35Cl NQR Study of Thiourea-CCl4 and Thiourea-CCl3Br Inclusion Compounds

The 35 Cl nuclear quadrupole resonance (NQR) spectrum of the (thiourea) 3 -CCl 4 inclusion compound has been determined at several temperatures in the range 5-20 K. The ratio of intensities of the two resonances was found to be 1:1, indicating that the tetrahedral CCl - molecule sites in the trigonal thiourea channel with the molecular and site 2-fold axes coinciding. NQR frequency-swept spin-echo measurements of the (thiourea) 3 -3CCl 3 Br inclusion compound reveal broad resonances at 40.995 and 40.690 MHz at 9.9 K, with a 1:2 ratio of intensities. The ∼300-kHz separation and the temperature dependence of the resonance frequencies are the same as that observed in the thiourea-CCl - compound, indicating that the two compounds are isomorphic at low temperatures

AN EFFECTIVE METHOD FOR SIGNAL ENHA-NCEMENT IN SOLID-STATE MAGNETIC RESONANCE SPECTROSCOPY

That the multiple single pulse exci tation method proposed in the present articlecan largely increase the signal-to- noise ratio of the solid-state magnetic resonancespectroscopy is analyzed and confirmed experimentally.In addition,the applicationof this technique to the measurements of the central lines of half- integer quadrupo- le nuclei and ￣14N remote nuclear quadrupole resonance spectra is also discussed indetail.

5254950 DC superconducting quantum interference device usable in nuclear quadrupole resonance and zero field nuclear magnetic spectrometers

A spectrometer for measuring the nuclear quadrupole resonance spectra or the zero-field nuclear magnetic resonance spectra generated by a sample is disclosed. The spectrometer uses an amplifier having a dc SQUID operating in a flux-locked loop for generating an amplified output as a function of the intensity of the signal generated by the sample. The flux-locked loop circuit includes an integrator. The amplifier also includes means for preventing the integrator from being driven into saturation. As a result, the time for the flux-locked loop to recover from the excitation pulses generated by the spectrometer is reduced.

Spin dynamics in CuO and Cu1-xLixO from 63Cu and 7Li nuclear relaxation.

$^{63}\mathrm{Cu}$ nuclear quadrupole resonance (NQR), nuclear antiferromagnetic resonance (AFNMR), and spin-lattice relaxation, as well as $^{7}\mathrm{Li}$ NMR and relaxation measurements in CuO and in ${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$O, are used to study the spin dynamics in the paramagnetic and in the antiferromagnetic (AF) phase, and in particular the effects induced by lithium doping. It is argued that Li enters in CuO lattice up to a doping x around 4%; from a comparison of the electric-field gradient (EFG) and of the dipolar field at the nucleus with theoretical estimates, it appears that ${\mathrm{Li}}^{+}$ is slightly displaced from the Cu lattice site. From NQR, NMR, and AFNMR spectra, information on the Cu hyperfine interaction and the EFG are derived. The N\'eel temperature ${\mathit{T}}_{\mathit{N}}$ is affected by Li doping to a moderate extent, decreasing from 226 K in CuO to ${\mathit{T}}_{\mathit{N}}$=183 K for x=3.7%. The relaxation rates, driven by the time-dependent part of the magnetic electron-nucleus Hamiltonians, indicate that the ${\mathrm{Cu}}^{2+}$ spin dynamics is almost unaffected by Li doping for T\ensuremath{\gg}${\mathit{T}}_{\mathit{N}}$ and possibly controlled by valence fluctuations. The Li doping modifies in a dramatic way the spin fluctuations in the AF phase.The relaxation rates in ${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$O, in fact, increase by orders of magnitude (similarly to the case of copper-oxide precursors of high-${\mathit{T}}_{\mathit{c}}$ superconductors) and display temperature behaviors quite different from those in CuO. These effects are associated with an effective low-frequency spectral density of the spin fluctuations negligible in CuO, which strongly increases upon Li doping. This increase is related to the fluctuations induced in the local ${\mathrm{Cu}}^{2+}$ spin configuration as a consequence of the diffusionlike motion of the holes in the AF matrix, implying the insurgence of low-frequency spin excitations. The orders of magnitude of the relaxation rates, their dependences on x and on T, as well as on the measuring frequency, are satisfactorily explained by a picture in which the correlation time ${\mathrm{\ensuremath{\tau}}}_{\mathit{h}}$ for the hole-hopping motion is controlled by an energy gap E(x) between localized and itinerant states. Quantitative information on ${\mathrm{\ensuremath{\tau}}}_{\mathit{h}}$ and E(x) is thus derived.

Nuclear-quadrupole double-resonance study of RbH2PO4 in the supercooled high-temperature monoclinic phase.

Rubidium nuclear-quadrupole-resonance (NQR) spectra have been measured in the high-temperature monoclinic phase of ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ above ${\mathit{T}}_{\mathit{p}}$=86 \ifmmode^\circ\else\textdegree\fi{}C and also in the supercooled high-temperature phase below ${\mathit{T}}_{\mathit{p}}$. At the transition into the high-temperature phase, the rubidium quadrupole-coupling constants drop by a factor of 5. The high-temperature phase is metastable below ${\mathit{T}}_{\mathit{p}}$. The kinetics of the transformation of the metastable phase into the stable tetragonal phase in a polycrystalline sample is determined at room temperature from the time dependence of the rubidium NQR spectra. A comparison of the present rubidium NQR data in the high-temperature monoclinic ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ with the rubidium NQR data in monoclinic ${\mathrm{RbD}}_{2}$${\mathrm{PO}}_{4}$ suggests that at low temperatures, supercooled monoclinic ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ may be isostructural with the phase III of monoclinic ${\mathrm{RbD}}_{2}$${\mathrm{PO}}_{4}$ whereas above ${\mathit{T}}_{\mathit{p}}$ ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ may be isostructural with phase II of ${\mathrm{RbD}}_{2}$${\mathrm{PO}}_{4}$.

Nuclear quadrupole double resonance in AHSO4 (A identical to K, Rb, Cs, Tl, NH4)

Nuclear quadrupole resonance (NQR) spectra of 17O, 39K, 85Rb, 87Rb and 133Cs have been measured in the low-temperature phases of KHSO4, RbHSO4, NH4HSO4 and TlHSO4 with the nuclear quadrupole double resonance technique based on magnetic field cycling. The NQR results are related to the crystal structures of the low-temperature phases of these compounds.

NQR study of local structure and cooling rate-dependent superconductivity in La2CuO4+δ

Preliminary results of structural studies in oxygen-annealed La2CuO4+δ(δ∼0.03) using 139La nuclear quadrupole resonance (NQR) spectroscopy are reported. Superconducting critical temperatures were found to depend on the rate of cooling through a narrow temperature range near 195 K. Analysis of the 139La NQR spectra reveal that lanthanum atoms in the oxygen-rich metallic region occupy two sites having different local environments. One site has a structural configuration closely related to the stoichiometric oxygen-poor compound. The second appears only below 200 K and exhibits a large shift and broad distribution of NQR frequency νQ. Shifts in νQ in the vicinity of the superconducting transition were also observed for both metallic sites. Possible origins of these anomalies and their connection to superconductivity are discussed.

Chlorine-nuclear-quadrupole-resonance study of the neutral-to-ionic transition in tetrathiafulvalene-chloranil.

The temperature dependence of the chlorine-35 nuclear-quadrupole-resonance (NQR) parameters has been studied in the mixed-stack charge-transfer complex tetrathiafulvalene-chloranil and discussed with reference to spectroscopic and neutron-diffraction results. The NQR spectrum changes conspicuously at the neutral-to-ionic transition temperature, ${\mathit{T}}_{\mathit{N}\mathrm{\ensuremath{-}}\mathit{I}}$=79.2 K. The changes have been analyzed in terms of the symmetry breaking occurring at the transition. Below, coexistence of the neutral and ionic phases has been observed down to the limit of metastability of the neutral phase (${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}69 K). Spin-lattice relaxation rates reveal the existence of highly anisotropic critical fluctuations with a predominantly one-dimensional character.

35Cl nuclear quadrupole resonance and crystal structures of small guest molecules in tris(2,4-pentanedionate)M(III) inclusion compounds

Tris(acetylacetonate)M(III) complexes, where M = AI, V, Cr, Mn, Fe, Co and Ga, form inclusion compounds with different solvent molecules. The compounds have a host/guest ratio of 1:2 for the small guest molecules CHCl 3 and CH 2Cl 2 and of 1:1 for the slightly bigger guest molecules trans-CHClCHCl, CH 2ClCH 2Cl and CCl 4. The 35Cl nuclear quadrupole resonance spectra of these clathrates have been measured in the temperature range 4–200 K. The site symmetry, the existence of different crystal phases, the molecular disorder and molecular motions of the guest molecules in the host cavities, and the host-guest interactions, were evaluated from the NQR frequency measurements. The crystal structures of the M(AcAc) 3/ trans-CHClCHCl compounds were measured at room temperature and confirm the NQR results.

35Cl nuclear quadrupole resonance in 2,4‐ and 2,6‐dichlorobenzyl alcohols

Abstract35Cl nuclear quadrupole resonance (NQR) was studied in 2,4‐dichlorobenzyl alcohol (2,4‐DCBA) and 2,6‐dichlorobenzyl alcohol (2,6‐DCBA). The NQR spectrum of 2,4‐DCBA show four lines and that of 2,6‐DCBA two lines. The temperature dependence of the NQR frequencies from 77 K up to the respective melting points was studied. An order–disorder transition was observed in 2,6‐DCBA around 215 K. Torsional frequencies and their temperature coefficients were calculated using the temperature dependence of the NQR frequencies, and were compared with the laser Raman spectra obtained at room temperature.

Variation of the Pr^3+ nuclear quadrupole resonance spectrum across the inhomogeneous optical line in Pr^3+:LaF_3

We report nuclear quadrupole resonance (NQR) frequency and linewidth variations of the Pr3+ ground-state hyperfine levels for various positions within the 3H4(Γ1)–1D2(Γ1) inhomogeneous optical line and satellite transitions of Pr3+:LaF3. A linear increase in the broadening of the Pr3+ hyperfine transitions, without a shift of central frequency, is measured by the Stark modulated optical pumping hole-burning technique as the laser is tuned toward the wings of the inhomogeneous optical transition. The linear variation is attributed to electric quadrupole broadening caused by changes in the local density of point defects across the Stark-broadened optical transition. The optically detected NQR spectra of the satellite transitions associated with Pr ion pairs are shifted in frequency and for most satellites are narrower than the NQR spectra found in the wings of the main inhomogeneous optical line.