Nuclear Quadrupole Resonance Study Research Articles

NQR, NMR, and DSC Study of [NH3(CH2)3NH3]CdBr4

Abstract The 79,81Br nuclear quadrupole resonance(NQR) of [NH3(CH2)3NH3]CdBr4 was observed. The results of the temperature dependence of NQR frequencies and differential scanning calorimetry(DSC) showed that phase transitions take place successively at 328, 363, and 495 K. The results of 2D NMR of [ND3(CH2)3ND3]CdBr4 showed that the cations execute the jumping reorientation about their long axes above the transition point of 328 K.

Cu63(2) nuclear quadrupole and nuclear magnetic resonance studies of YBa2Cu4O8in the normal and superconducting states

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NMR and NQR study of Ca-substituted superconducting YBa2Cu4O8.

We report a set of nuclear quadrupole resonance (NQR) measurements including a spin-echo double-resonance (SEDOR) experiment which provides convincing evidence that the additional chain Cu NQR line in Ca-substituted ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ does stem from chain Cu sites disturbed by the ``impurity'' Ca ions. From an analysis of signal intensities we conclude that ``impurity'' Ca ions, responsible for the additional line, substitute ${\mathrm{Y}}^{3+}$. At high Ca doping, the concentration of the substituted Y sites, ${\mathit{x}}^{\ensuremath{'}}$, is appreciably smaller than the nominal Ca concentration, x. Since it is ${\mathit{x}}^{\ensuremath{'}}$ which is responsible for a direct increase of the hole charge-carrier concentration, n, effects connected with the increase of n such as the planar Cu spin-lattice relaxation rate, the NQR frequency, and the magnetic shift are only weakly dependent on x. Thus, the substantial increase of ${\mathit{T}}_{\mathit{c}}$ with x suggests that, besides the increase of n, other effects have to play a role in the ${\mathit{T}}_{\mathit{c}}$ enhancement. One such effect might be the opening of the spin pseudogap. At 150 K, both the main and additional Cu NQR lines in nominal ${\mathrm{YBa}}_{1.9}$${\mathrm{Ca}}_{0.1}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ show distinctly anomalies not seen in pure ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ that point out the Ca doping induced structural phase transition recently observed in specific-heat, elastic-neutron-scattering, and x-ray-diffraction measurements. \textcopyright{} 1996 The American Physical Society.

The combination of X-ray diffraction and nuclear quadrupole resonance studies of crystals

The application of single-crystal Zeeman nuclear quadrupole resonance (NQR) in combination with diffraction studies is discussed. Bond directions and bond angles are found by the Zeeman spectroscopy of NQR, as are symmetry elements of the structures. Centrosymmetry may be differentiated from polar structures. Weak deformations of the valence electron shell by mutual polarization can be recognized and the double-bond character of a bond may be estimated. The method is quite valuable in the study of hydrogen bonds and of dynamics of molecules such as D 2 O and the groups ―CD 3 and ND 3 . Most useful is the method of the combination of X- and N-diffraction because of its sensitivity to the charge distribution around atoms and its location of time scale in dynamical effects.

Nuclear quadrupole resonance and molecular orbital studies of charge distribution in H‐bonded complexes of 2‐chloro‐4‐nitrobenzoic acid

AbstractThe charge distribution in hydrogen bridges of varying strengths in solid 2‐chloro‐4‐nitrobenzoic acid complexes was studied using the 35Cl nuclear quadrupole resonance technique (NQR). The dependence of 35Cl NQR frequencies on the proton affinities was analysed. Molecular orbital calculations using atom superposition and electron delocalization molecular orbital theory showed that the trends of changes in the charge density on the chlorine, nitrogen and oxygen atoms, and on the binding energy and the bond order of NH and OH groups are consistent with the NQR results. Also, the agreement between theoretical and experimental results showed that the 3,5‐dimethylpyridine complex represents the critical point at 50% proton transfer.

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.

Asymmetry parameter studies for systems containing the 35Cl nucleus from Zeeman NQR data

A review of Zeeman nuclear quadrupole resonance (NQR) work on systems containing the 35Cl nucleus is presented. In the case of the 35cl nucleus with spin I = 3 2 , owing to the existence of ± m degeneracy in the absence of a magnetic field (Kramer's degeneracy), there exists only one pure NQR frequency which is related to the two electric field gradient (EFG) parameters, η and e 2 qQ, by the formula, v = ( e 2qQ 2h )(1 + ( η 2 3 )) 1 2 . Hence it is necessary to study the Zeeman effect to obtain both these parameters. Zeeman NQR studies have been carried out by many workers using single crystals and powder specimens, and these are discussed here. The computer simulation method for powder η values is also presented in this paper. The numerical computation program for an IBM 370-158 computer used to simulate the powder Zeeman spectrum is also discussed briefly. The paper also presents two-dimensional Zeeman NQR and Zeeman perturbed spin-echo envelope modulation (ZSEEM) studies on 35Cl systems. Very recently, two-dimensional NQR experiments based on the principle of nutation spectroscopy have been carried out with zero applied magnetic field on 3 2 spin nuclei. This method is also discussed in this review. The relationship between the asymmetry parameter (η) and bond properties is also discussed.

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.

Cu and Ba nuclear quadrupole resonance study of Y 2Ba 4Cu 7O 15 at high pressure

We have measured the pressure dependence of 63Cu and 137Ba NQR frequencies at 100K and 300K. Estimates of the pressure induced charge transfer from CuO-chains to CuO 2-planes are calculated.

Ba nuclear quadrupole resonance study of YBa 2Cu 4O 8

We have measured the temperature dependence of the nuclear quadrupole resonance (NQR) frequency, ν Q. and the spin-lattice relaxation time of 135Ba and 137Ba in the superconductor YBa 2Cu 4O 8 (T c = 81 K). The temperature dependence of ν Q ( T) is given by ν Q ( T) = ν Q (0)(1− AT α ). The relaxation is a quadrupolar origin and can be described perfectly by a two-phonon Raman process with a Debye temperature of 185 K.

14N nuclear quadrupole resonance study of the structural phase transition in phenothiazine

14N nuclear quadrupole resonance frequencies have been measured in both the low-temperature and the high-temperature phase of phenothiazine by a 1H-14N nuclear quadrupole double-resonance technique. The proton spin-lattice relaxation time was measured in both crystallographic phases as well. The NQR data show the presence of large thermal librations of the phenothiazine molecules in both crystallographic phases. The NQR data are consistent with an order-disorder phase transition associated with the reorientations of the phenothiazine molecules around the orthorhombic b axis. The reorientation angle is at Tc equal to 9+or-2 degrees . The long proton spin-lattice relaxation times above and below Tc seem to rule out any large translation or butterfly motions associated with the phase transition.

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.

Structural instability and anion dynamics in the system (Rb1-xCsx)2ZnCl4: a 35Cl nuclear quadrupole resonance study

The dynamics of the ZnCl4 anion in the paraelectric phases of (Rb1-xCsx)2ZnCl4 with x=0.005, 0.01, 0.03 and 0.05 are investigated through 35Cl nuclear quadrupole frequency nu and spin-lattice relaxation time (T1Q) studies. The data, collected as a function of temperature for different lines in these compounds, are analysed to obtain the hindering potential barrier heights and the relative mean square amplitudes for librational motions parallel and perpendicular to the a-axis of the crystal structure. A comparison of these results shows that the presence of impurity affects only perpendicular motions and the corresponding potential becomes anharmonic, probably developing a double-well structure, as the concentration of impurity increases. As the paraelectric-incommensurate transition temperature is suppressed by the presence of impurity, the above results suggest that the perpendicular motions have an important role to play in stabilizing the paraelectric phase in this system.

35Cl nuclear quadrupole resonance study of anion dynamics in the paraelectric phases of Cs2ZnCl4 and Rb2ZnCl4

The dynamics of the ZnCl4 anion in two compounds, one exhibiting a structural transition to an incommensurate phase (Rb2ZnCl4) and another not having any structural transition at all (Cs2ZnCl4), are studied through 35Cl NQR frequency ( nu ) and relaxation time (T1Q) measurements as a function of temperature T in their high-temperature (paraelectric) phases. Estimates of the potential barriers hindering the reorientational motions of ZnCl4 anions along axes parallel and perpendicular to the a axis of the crystal structure are obtained from the T1Q data. These results are analysed in conjunction with a qualitative comparison of relative mean square amplitudes of such motions obtainable from frequency data, in order to investigate the differences in the dynamics and structure of the ZnCl4 anion. There seems to be correlation between the dynamics of the anion in this phase and the possibility that the compound may exhibit a structural instability at low temperatures in this family.

35Cl nuclear quadrupole resonance studies of molecular motions of ClO 3− and water diffusion in Ca(ClO 3) 2·2H 2O

A 35Cl nuclear quadrupole resonance ( 35Cl-NQR) investigation of polycrystalline Ca(ClO 3) 2·2H 2O is described. The 35Cl-NQR frequencies ( ν Q) for two resonance lines ( ν Q1 and ν Q2), the spin lattice relaxation time ( T 1Q) for ν Q2 only and the line width δν Q2 were measured in the temperature range 292–345 K, except for the frequency measured up to 455 K. The observed decrease in the resonance frequencies with increasing temperature permitted the determination of the frequencies of librations of the ClO 3 − ion about two axes perpendicular to the three-fold axis of the ion mainly responsible for this effect. The temperature dependence of the relaxation time T 1Q proved the occurrence of water diffusion and hindered rotation of ClO 3 − ions. The activation energies of these two molecular motions were determined, and their effect on the electric field gradient at the site of a chlorine nucleus was discussed. Temperature measurements of the line width δν Q2 confirmed the conclusions drawn from the analysis of T 1Q( T).

Magnetic phase diagram of lightly doped La2-xSrxCuO4 from 139La nuclear quadrupole resonance.

The magnetic phase diagram of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ in the antiferromagnetic regime (x\ensuremath{\le}0.02) has been derived from $^{139}\mathrm{La}$ nuclear quadrupole resonance studies from 4 to 250 K. The data demonstrate freezing of the doped holes' effective spin degrees of freedom below ${\mathit{T}}_{\mathit{f}}$\ensuremath{\approxeq}(815 K)x into a spin-glass-like state which is superimposed on the antiferromagnetic background. These and previous results allow a detailed magnetic phase diagram to be constructed for x\ensuremath{\le}0.05 and reveal a distinct crossover at x\ensuremath{\approxeq}0.02 in the nature of the spin-glass transition.

A study of activated molecular motion in 2-nitrobenzene sulphonyl chloride by NQR

A comprehensive chlorine nuclear quadrupole resonance study of the temperature and pressure dependences of frequency, line width and spin-lattice relaxation time in 2-nitrobenzene sulphonyl chloride is reported. The results show that each of the two substituted groups on the benzene ring contributes to the spin-lattice relaxation via a different and independent mechanism. In the high-temperature regime, the data are successfully explained assuming reorientation of the SO2Cl group between unequal potential wells and field gradient fluctuations resulting from reorientations of nearby NO2 groups. The activation energy of both motions and the fraction of the electric field gradient influenced by the motion of nitro groups have been estimated, as well as the pressure dependence of the activation enthalpy and the activation volume for these motions.

SQUID-NQR of nitrogen-14 in amino acids and small peptides

We present a technique for the study of nuclear quadrupole resonance of 14N interacting with low electric field gradients as found, for example, in amino acids. A superconducting quantum interference device (SQUID) is used to detect directly small NMR signals via cross relaxation of the 14N polarization to adjacent proton spins, eliminating the need for field cycling. When one nitrogen quadrupolar transition matches the proton Zeeman splitting, the remaining two quadrupolar transitions can be observed by sweeping a saturating rf field through resonance. In addition, signal enhancement by simultaneous excitation of two nitrogen resonances helps to identify connected transitions.

Magnetic and electronic structures of antiferromagnetic La2NiO4+ delta and La2-xSrxNiO4+ delta : 139La nuclear quadrupole resonance study

The authors carried out a nuclear quadrupole resonance study of 139La in La2-xSrxNiO4+ delta ( delta =0.00-0.10; x=0.0-1.3) in the temperature range T=1.4-200 K, paying particular attention to its relationship to the superconducting cuprates. In lanthanum nickel oxide, the occupation of holes on both Ni 3d(x3-y3) and 3d(z2) orbitals with parallel spins (S=1) causes some characteristic properties. (1) The large internal magnetic field (Hint=26-18 kOe) at the La site indicates the existence of a strong transferred exchange interaction between Ni 3d(z2) and La 6s orbitals through the 2pz orbital of apex oxygen. (2) Discontinuities in the magnitude of Hint and electric field gradient observed at delta approximately=0.02 are attributed to the crystal phase transition from a low-T orthorhombic to a high-T tetragonal phase. (3) Up to high Sr doping (close to the phase transition at x approximately=1.0 from antiferromagnetic to paramagnetic), Hint remains almost constant, suggesting that the doped holes first go into the 3d(x2-y2) orbital without producing Ni-Ni spin frustrations. (4) The T dependences of Hint and relaxation rate T1-1 at low T are explained basically by the spin-wave theory for three dimensional antiferromagnets, and the sublattice magnetic moments of Ni2+ are close to the localised spins. (5) The dependence of T1-1 proportional to T2.6 indicates that the 139La relaxation can be attributed to the short-wavelength antiferromagnetic fluctuations notwithstanding the geometrical cancellation at the La site.