Electric Field Gradient Asymmetry Research Articles

Strength, symmetry and distribution of electric quadrupole interactions at181Ta impurities in hafnium-zirconium alloys

The nuclear quadrupole interaction at substitutional Ta atoms in the alloy system HfxZr1-x has been investigated over the entire range of composition by time differential perturbed angular correlations (TDPAC). Source configurations investigated include oriented and randomised single crystals as well as textured, cold-worked and random polycrystals, all at room temperature. For both pure Zr and pure Hf, a pure electric quadrupole interaction is found of magnitude nu Q=eQVzz/h=303.9(15) MHz and 311.01(11) MHz respectively, which exhibits no electric field gradient (EFG) asymmetry and negligible damping due to finite distribution of the EFG strength. As increasing amounts of impurity are added at either end of the phase diagram, a two-component TDPAC pattern becomes evident: in addition to the undamped pure-metal spin rotation, a second component, exhibiting both damping and asymmetry, is seen at a somewhat higher frequency than that of either pure metal. The fraction of this second component, its frequency, the degree of asymmetry, and the distribution width of the EFG all increase monotonically with increasing impurity concentration.

Niobium-93, lanthanum-139, and tantalum-181 pure NQR in rare earth orthoniobates and lithium tantalate

Abstract 181Ta pure NQR spectra in ferroelectric LiTaO3 were studied from 77 K to room temperature. It was found that the electric field gradient asymmetry parameter has a non-zero value over all temperature ranges. 93Nb nuclear quadrupole interactions in rare earth orthoniobates LaNbO4, YbNbO4, ErNbO4, and in solid solutions K0.5Eu0.5Nb2O6 and K0.5Tm0.5NB2O6 have been investigated from 77 to 450 K. 139La NQR frequencies in LaNbO4 single crystal were also measured at 77 K.

Molecular orbital calculations of electric field gradients at halogen nuclei in solid chlorine and bromine

Using the MO LCAO method in CNDO and INDO approximations, calculations of the electronic structure of polyatomic clusters in solid Cl 2 and Br 2 have been performed. The energies of the molecular orbitals of the clusters are compared with the available experimental data and theoretical results. The effect of d-orbitals on intermolecular bonding is examined in the case of solid chlorine, and the parameters of the electric field gradient tensor at halogen nuclei are evaluated in both cases of solid chlorine and bromine. The calculated quadrupole coupling constants are in satisfactory agreement with the observed values, but the electric field gradient asymmetry parameter is found to be very small in every case, in contradiction of the experimental results. Finally, the possible reasons for such a discrepancy are discussed.

55Mn NQR and infrared intensities of the carbonyl stretching vibrations in group V and olefin derivatives of cyclopentadienylmanganese tricarbonyl

Carbonyl IR intensity and 55Mn NQR measurements are reported for a series of olefin and Group V donor atom derivatives of C 5H 5Mn(CO) 3. The ratio of infrared intensities of the symmetric and antisymmetric carbonyl stretching vibrations for most compounds are rather insensitive to changes in the donor ligand which suggests that the π-bonded ring acts as a buffer of the vibronic contributions that the ligand can make to the CO groups. This observation is consistent with previous results on C 5H 5Fe(CO) 2L and Co(CO) 2(NO)L systems. A simple MO calculation is presented for compounds of the type cis-X 3M(CO) 2L which further supports this explanation. Details of the individual shifts in the intensity data are also presented. 55Mn NQR data for the Group V donor derivatives reveal that the ν(± 5 2 ⇄ ± 3 2 ) transition frequency parallels what is conventionally accepted as the trend in σ-donor ability and π-acceptor ability of the ligands. Within the olefin series the trend in the electric field gradient asymmetry parameter at manganese parallels the ring strain of the olefin.

A study of the distribution of boron sites in glassy B 2O 3 using 11B NMR

A study has been made of the 11B NMR spectrum of glassy B 2O 3 with the aid of computer-simulated NMR spectra. It is shown that the glass contains a distribution of boron sites centered on an average or most probable site characterized by a quadrupole coupling constant of 2.66 MHz and an electric field gradient asymmetry parameter of 0.11. It is concluded that 11B NMR alone cannot determine the exact nature of the distribution. The implications of the NMR data with respect to the suggested existence in the glass of distorted BO 4 “tetrahedra” is discussed.

Experimental study of nuclear electric quadrupole splittings of 27Al in Topaz

The nuclear magnetic resonance spectrum of 27Al in single crystals of natural topaz (Al 2[F,OH] 2SiO 4) has been investigated at room temperature at 10.500 MHz. Observed quadrupolar perturbations have been analyzed by the method of Volkoff, for three orthogonal planes, to yield eight nuclear quadrupole coupling and electric field gradient tensors. Satellite splitting indicated that the crystallographic class of topaz is holohedral. Eigenvalues and electric field gradient asymmetry parameters of the tensors fall into two groups of four, the members of each group having identical parameters. Eigenvectors of all tensors are different. The eight equivalent Al 3+ sites given by the X-ray space group are thus divided into two types of magnetically nonequivalent sites, each type consisting of four magnetically equivalent positions and the term magnetic being used to describe site symmetries as revealed by magnetic resonance. Quadrupole coupling constants and field asymmetry parameters for the two types of sites are: Type α sites Type β sites C z= eQφ zz h 1.5918±0.008MHz 1.6506±0.008MHz ν=(φ χχ−φ yy)φ! zz 0.704±0.007 0.501±0.005

A study of dipole-broadened NMR powder lineshapes in the presence of second-order quadrupole effects with applications to polycrystalline borates

Computer simulations of NMR powder patterns for the m = 1 2 ⇋ m = − 1 2 transition broadened in second order by a quadrupole interaction having a nonzero electric field gradient asymmetry parameter are discussed. The dependence of measurable features of the derivative spectrum on the theoretical parameters is considered. It is found that the frequency dependence of the derivative linewidth is insensitive to relatively large changes in the asymmetry parameter. Thus, methods for determining the quadrupole coupling parameters from the frequency dependence of the NMR linewidth are subject to large uncertainties. In the special case when the dipole broadening is small relative to the quadrupole broadening, it is shown that reference to a library of standard, computer-simulated spectra allows precise determination of the coupling constant and asymmetry parameter without the need for computation of the specific experimental spectrum. This is illustrated with examples of 11B NMR spectra from several polycrystalline borates.

Nuclear Electric Field Gradient and Mean Square Displacement of the Iron Sites in Sodium Nitroprusside

Both the electric-field-gradient (EFG) and mean square displacement (MSD) tensor parameters at the Fe sites in sodium nitroprusside at 23\ifmmode^\circ\else\textdegree\fi{}C have been determined with a high precision by the M\"ossbauer technique. The EFG asymmetry parameter is $\ensuremath{\eta}=0.01\ifmmode\pm\else\textpm\fi{}0.01$. The principal axes of the two tensors do not coincide. Taking proper account of saturation effects was essential, even though the absorbers were only \ensuremath{\sim}0.2 mm thick. For the MSD, proper consideration of polarization effects was also important. A total of 34 runs on an absolutely calibrated mechanical drive gave for the quadrupole splitting $\ensuremath{\Delta}{E}_{Q}=1.7048\ifmmode\pm\else\textpm\fi{}0.0025$ mm/sec.

11B NMR in Polycrystalline Calcium Metaborate

Boron-11 nuclear magnetic resonance has been observed in polycrystalline calcium metaborate doped with paramagnetic Mn2+. The spectrum arises from the m = 12↔m= − 12 transition broadened in second order by a quadrupole interaction having a nonzero asymmetry parameter. The quadrupole coupling constant of 2.58 ± 0.02 MHz is consistent with previously published values for three-coordinated boron. However, the electric field gradient asymmetry parameter of 0.55 ± 0.01 is several times greater than expected, and this large value is attributed to the presence of a nonbridging oxygen in the boron coordination shell. The possibility of using this result for the study of noncrystalline solids is discussed.

Al27 Quadrupole Coupling in Solid AlCl3

The nuclear quadrupole coupling constant of Al27 in solid anhydrous AlCl3 was found to be 472±24 kc/sec with an electric field gradient asymmetry parameter of zero. Assuming the crystal to be completely ionic, the electric field gradient was calculated for the layered AlCl3 structure by summing the contributions of the near ions, and treating the rest of the crystal as being made up of uniformly charged planes. This calculation gives an Al27 quadrupole coupling constant of —2.78 Mc/sec and an electric field gradient asymmetry parameter of zero. The addition of some covalent bonding tends to reduce the magnitude of the coupling constant and bring it into better agreement with the experimental value. It is estimated that the crystal is two-thirds ionic and one-third covalent.

Determination of Nuclear Quadrupole Coupling Constants from Nuclear Magnetic Resonances in Polycrystalline Solids

The line shape of a nuclear magnetic resonance in a polycrystalline solid under the influence of an axially symmetric second-order nuclear quadrupole perturbation has been calculated for 20 values of the quadrupole coupling constant. From these calculations the frequency difference between the peaks in the derivative of the resonance line has been found as a function of the quadrupole coupling constant. This relationship has been shown to be useful in two cases. First, a procedure is described for determining the quadrupole coupling constant by studying the resonance as a function of magnetic field, when it is known that the electric-field-gradient asymmetry parameter is zero. Second, when both first- and second-order quadrupole effects can be observed in a polycrystalline solid, a method is described for determining both the quadrupole coupling constant and the electric field gradient asymmetry parameter for small values of the asymmetry parameter.