Value Of Electric Field Gradient Research Articles

14N and 2H NMR Study of the Mesophases of Cetyltrimethylammonium Bromide in Formamide

Two mixtures of cetyltrimethylammonium bromide (CTAB) and formamide (FA), (60/40 wt % (CTAB60) and 78/22 wt % (CTAB78)), which form lyotropic mesophases, were studied by using 2H and 14N NMR spectroscopy at varying temperatures. Both CTAB and FA give a 14N resonance, whereas the 2H resonance arises from a small amount of FA-d3 in the samples. From low to high temperature, the following phases were observed: liquid + crystal → hexagonal → isotropic and liquid + crystal → hexagonal → cubic → cubic + lamellar → lamellar for CTAB60 and CTAB78, respectively. The line shapes of the 2H NMR patterns of deuterated formamide are explained with a model in which formamide is moving fast compared to its nuclear quadrupolar coupling constants in uniaxial domains oriented in different directions. For the calculation of the orientation parameters of formamide, nuclear quadrupolar coupling constants corresponding to both gas and liquid phases were utilized. The gas phase values of electric field gradient (EFG) tensors were obtained by first principles calculations applying both coupled cluster doubles and density-functional theories. The 14N EFG tensor was calculated for a model of CTAB to compare the orientation of formamide and CTAB. The average location and orientation of formamide among the micelles is discussed on the basis of the best suited nuclear quadrupole coupling constants and orientation parameters.

Study of the EFG Trends in Zr2T (T = Fe, Co, Ni) Intermetallic Compounds

Abstract We use the linear muffin-tin orbital formalism, in the atomic sphere approximation, to investigate the trends of the electric field gradient (EFG) at the nucleus for the non-equivalent sites in Zr2T (T = Fe, Co and Ni) intermetallic compounds. As all those compounds crystallize in the same C16 crystallographic structure, they offer a rare opportunity to investigate electronic structure effects coming from transition metals on the EFG at Zr site. Those results are compared with EFG values obtained from quadrupole coupling constant measurements performed with the time differential perturbed angular correlation (TDPAC) technique, using the 191Ta probe.

Calculation of muon level-crossing resonance spectra for Si∶B and Si∶Al

The spin dynamics of muons interacting with acceptor impurity nuclei in silicon is investigated. Using values of electric field gradients obtained from ab-initio calculations, the field dependence of the longitudinal muon polarization is calculated for Si∶B and Si∶Al and compared to the contributions from paramagnetic bond-center muonium.

NMR and NQR study of the electronic and structural properties of Al-Cu-Fe and Al-Cu-Ru quasicrystals.

[sup 27]Al and [sup 63,65]Cu NMR is reported for powdered stable Al-Cu-Fe and Al-Cu-Ru icosahedral quasicrystals and crystalline approximants, and for an Al-Pd-Mn single-grain quasicrystal. [sup 27]Al NQR spectra at 4.2 K were observed in Al-Cu-Fe and Al-Cu-Ru samples. From quadrupole-perturbed NMR spectra at different magnetic fields, and from zero-field NQR spectra, a wide distribution of local electric-field gradient (EFG) tensor components and principal-axis-system orientations was found at the Al site. A model EFG calculation based on a 1/1 Al-Cu-Fe approximant successfully explained the observed NQR spectra. The average local gradient is largely determined by the [ital p]-electron wave function at the Al site, while the width of the distribution is due to EFG lattice contribution. Comparison of [sup 63]Cu and [sup 27]Al NMR shows the EFG distribution at the two sites is similar, but the electronic contribution to the EFG is considerably smaller at the Cu site, in agreement with a more [ital s]-type wave function of the conduction electrons. Overall spread of EFG values is well reproduced by calculation based on the approximant. However, the experimental spectra indicate a much larger number of nonequivalent sites when compared with the simulated NQR spectra based on the 1/1 approximant. Themore » short-range, local chemical order is well represented by the approximant, but differences in coordination must be included at intermediate range in the quasicrystal. Measured [sup 27]Al Knight shift, magnetic susceptibility, and nuclear spin-lattice relaxation time as a function of temperature indicate reduced density of states at the Fermi level by a factor of 7 or 8 from the value in Al metal, consistent with the notion of a pseudogap for these quasicrystals. No differences in measured parameters were detected as a function of composition of the quasicrystalline alloys.« less

O+ phase bunching and auroral arc structure

The equations of motion are solved for ions moving in a model electric field that corresponds to the nightside equatorial region of the magnetosphere. The model represents the poleward region of the Harang discontinuity mapped to the magnetosphere. Within this region the model electric field has a constant earthward gradient superimposed on a constant dawn‐to‐dusk electric field. In combination with the earthward drift motion due to the dawn‐to‐dusk field, the electric field gradient introduces an earthward inertia drift, which is proportional to the ion mass and therefore faster for O+ ions than for H+ ions or electrons. It is also found that the entry of the ions into the gradient region causes phase bunching and as a result ion density striations form. The striations are enhanced for more abrupt changes in the electric field gradient, a weaker magnetic field, a stronger cross‐tail electric field and colder O+ ions. The first two conditions apply during the growth phase of a substorm. Using the Tsyganenko (1987) model a minimum electric field gradient value of 1 × 10−9 V/m2 ((1 mV/m)/1000 km) at L = 6‐7 is found. Charge neutrality requires coupling with the ionosphere through electrons moving along magnetic field lines, and such electrons may be the cause of multiple auroral arcs.

Quantum chemical study of relative stability, electric field gradients at nitrogen atoms, rotational constants and electric dipole moments of all uracil, thymine and cytosine isomers

Abstract For four uracil and five cytosine isomeric forms, fully optimized quantum chemical structural data computed at the HF/6-31** level of approximation are reported. Effects of electron correlation (based on pointwise MP2/HF/6-31G** calculations) on the relative energies of all 13 isomers of uracil and thymine and all 14 isomers of cytosine are investigated. Principal values of electric field gradients at N atoms in the 1 and 3 positions of the ring and of the amino/imino substituent in the 6 position of cytosine, rotational constants and electric dipole moments were determined for all isomers at the same level of theory. An attempt to characterize attractive interactions between hydroxy (imino) substituents and lone pair electrons centered at adjacent N atoms (a hitherto unrecognized “intramolecular H-bond”) by means of electron density, difference electron density, field lines of the gradient and a contour map of the Laplacian of the electron density is presented. Electric field gradient tensors and their asymmetry parameters are shown to be correlated in a characteristic manner with repulsive and attractive interaction. These interactions were recently found to be connected in a simple manner with conversion energies of geometric isomers of tautomeric forms of the three pyrimidine nucleic acid bases.

Theoretical calculations of the nuclear quadrupole coupling in the spectra of D 3+, H 2D +, and HD 2+

The present paper reports a purely theoretical calculation of the quadrupole hyperfine structure in the rotation-vibration spectra of D 3 +, H 2D +, and HD 2 +. As the initial step in the calculation we have computed ab initio extensive sets of electric field gradient values for these molecules. These ab initio values are used, together with rotation-vibration wavefunctions obtained using the rotation-vibration theory developed previously for X 3 and Y 2 X equilateral triangular molecules [V. Špirko, P. Jensen, P. R. Bunker, and A. Čejchan, J. Mol. Spectrosc. 112, 183–202 (1985); P. Jensen, V. Špirko, and P. R. Bunker, J. Mol. Spectrosc. 115, 269–293 (1986)] to obtain the quadrupole splitting parameters for the individual rotation-vibration states of the molecules in question. We have calculated actual quadrupole splittings for selected transitions of H 2D + and HD 2 + and find these to be less than 100 kHz. Laboratory measurements of these small splittings would require sub-Doppler resolution and do not seem feasible at the present time. Even in cold interstellar clouds H 2D + lines certainly have widths larger than the calculated splittings so that interstellar detection of the splittings is impossible. Clearly in the present study theory is very far ahead of experiment. One might hope that in a not-too-distant future experimental techniques will be developed which allow the observation of the small splittings.

The electric field gradient at188Os nuclei in Re metal

The electric quadrupole interaction of the first excited 2+ state of188Os in hexagonal rhenium metal was investigated by means of the time-differential perturbed angular correlation technique. From the observed quadrupole frequencyVQ=170(7) MHz, we deduce an electric field gradient value of |Vzz|=4.77(23)·1021V/m2 for the system OsRe. The half-life of the 2+ state was measured to be 641(4)ps.

Lattice sum calculations and electric field gradients for orthorhombic and tetragonal phases of YBa 2Cu 3O x

Electric field gradient (EFG) and asymmetry parameter (ν) values have been calculated for all the cationic sites of orthorhombic and tetragonal YBa 2Cu 3O x . Different types of ordering of the oxygen vacancies around the Cul site and different copper charges in this site were considered, as well as the effect of an electron hole. A comparison of experimental and calculated EFG values is proposed.

The electric field gradients on Nd, Co and B in Nd/sub 2/Co/sub 14/B compound

The quadrupole splits and oscillations of the Nd, Co, and B NMR (nuclear magnetic resonance) lines make it possible to obtain the values of electric field gradients on those atoms in Nd/sub 2/Co/sub 14/B. To explain observed electric field gradients, the calculated lattice contributions and intraatomic contributions are considered to give reasonable agreement with the experimental values. From these analyses it is concluded that the effective boron charge is negative.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Grain size dependence of the electric field gradient

Precise measurements of the electric field gradient (EFG) with the perturbed angular correlation method (PAC) in Cd and Zn show significant differences between single-crystalline and poly-crystalline samples. These differences (up to four percent at low temperatures) decrease with increasing temperature and vanish at about 2 60 K. In all samples the lowest EFG values are obtained for single crystals. In poly-crystalline samples the EFG as well as the width of the EFG distribution increase with decreasing grain size. This effect is also observed in semiconducting ZnO.

161Dy Mossbauer effects study on DyRu 2Ge 2

161Dy Mössbauer spectra in DyRu 2Ge 2 were measured between 4.2K and 63K. This tetragonal compound has a Neel temperature T N = 22 K. Above T N, relaxation spectra are observed. B hf value at 4.2K gives a magnetic moment around 10 μ B. The lattice contribution deduced from e 2qQ experimental value, is in a good agreement with calculated electric field gradient values for a Ru 2− charge.

Crystal chemistry and 237Np mössbauer investigations of neptunyl(vi) carbonate NpO 2CO 3

Neptunyl(VI) carbonate was precipitated at pH 5 from a neptunyl(VI) nitrate solution by the addition of solid lithium carbonate. The precipitate was characterived by thermogravimetric analysis, v-ray diffraction and 237Np Mössbauer measurements. NpO 2CO 3 decomposes at 350–400 °C and was found to be isostructural with UO 2CO 3. The lattice parameters are a = 4.82(1)Å, b = 9.17(5) A ̊ and c = 4.24(2) A ̊ . The 4.2 K Mössbauer absorption spectrum can be described as a vuadrupole interaction with the hyperfine parameters δ = −35.5(2) mm s −1 (relative to NpAl 2), ¦e 2vv¦= 231(1) mm s −1 and η = 0.21(2). Both isomer shift and electric field gradient values are consistent with a neptunyl NpO 2+ 2 ion and confirm the crystal structure isotypic with UO 2CO 3.

Electric field gradient for simple metal impurities in Ag and Au

An attempt is made to explain the electric field gradient (EFG) and asymmetry parameter (η) measured at the 100Rh probe atom in dilute alloys of Ag and Au. The impurity effective valency is decided according to the Friedel criterion of bound states. The valence effect EFG is evaluated using the impurity induced charge density calculated self-consistently in the density functional formalism. The size effect EFG is evaluated in the elastic continuum model for the lattice, considering the dressed point ions interacting through the screened Coulomb potential which is determined with the help of impurity-probe binding energy. The size effect EFG is found to be larger than the valence effect EFG by an order of magnitude. The calculated values of EFG and η are found to be in fairly good agreement with the experimental values for all the alloys except Au Sn.

Quadrupole interactions at57Fe in titanium metal

The electric field gradient (EFG) at57Fe in titanium is measured using Mossbauer spectroscopy in the temperature range 80°–380°K. The value of EFG obtained at room temperature is 0.53(4)×1017 V/cm2. The value of EFG obtained is compared with the conduction electron charge shift model.

Electric field gradient for muonium and monovacancies in Cu and Al

The electric field gradient (EFG) and asymmetry parameter (η) at the first nearest neighbour (1NN) site of muonium (μ +) and at the first four NN sites of vacancies in Cu and Al are calculated. The defect induced charge density, used to evaluate the valence effect EFG, is calculated self-consistently in the density functional formalism. The formalism for the size effect EFG for μ + at the octahedral site in the fcc lattice is developed in the elastic continuum model. For both the μ + and the vacancy the components of the strain field tensor are evaluated assuming the lattice of dressed point ions interacting through the screened Coulomb potential. The size effect EFG for μ + is found cylindrically symmetric at the 1NN and 2NN sites. The size effect EFG is found larger than the valence effect EFG at all the NN sites for Cu- and Al-vacancies and the same is obtained at the 1NN site for μ +. The valence effect EFG for an Al-vacancy is found to be relatively larger than for a Cu-vacancy while the contrary is concluded for the μ +. It is noted that η depends upon the relative phases of components of EFG and not on their absolute values independently. The calculated and the experimental values of EFG and η agree reasonably.

Electric field gradient in nontransitional dilute alloys of aluminum

The electric field gradient (EFG) and asymmetry parameter (η) for nontransitional dilute alloys of Al are calculated upto second nearest neighbour (2NN) site. The valence effect contribution to EFG is evaluated using the perturbed electronic charge density, calculated self-consistently in the density functional formalism. The size effect contribution is evaluated in the elastic continuum model. The calculated values of EFG and η are found to be in reasonable agreement with the experimental values. The size effect contribution is found to be larger than the valence effect contribution at the 1NN site for most of the alloys and it is vice versa at the 2NN site.

Electric field gradients in nonmagnetic dilute alloys of aluminum and copper

The electric field gradient (EFG) and the asymmetry parameter ($\ensuremath{\eta}$) are investigated at the first-, second, third-, and fourth-nearest-neighbor (NN) sites for nonmagnetic dilute alloys of Al and Cu. The impurity effective valency is calculated according to the Friedel criterion of bound states. The impurity-induced electronic charge density is calculated self-consistently in the Hohenberg-Kohn-Sham density-functional formalism and is used to evaluate the valence-effect contribution to the EFG. The size-effect contribution to the EFG is estimated in the elastic continuum model. Reliable values of the core-enhancement and Sternheimer antishielding factors are used. No size-strength parameter is introduced to explain the EFG's at the 2NN, 3NN, and 4NN sites. However, the EFG's at the 1NN site are explained with the help of a size-strength parameter ${C}^{\ensuremath{'}}$ which accounts for the deficiency of the elastic continuum model. The calculated values of EFG and are $\ensuremath{\eta}$ found to be in reasonable agreement with the experimental values for most of the alloys. In Al alloys the size-effect EFG is found to be dominant at the 1NN site and the valence-effect EFG is found to be dominant at the 2NN, 3NN, and 4NN sites. In Cu alloys the size-effect EFG is dominant at all the NN sites.

Calculation of electric field gradients in some linear molecules using semi-empirical scfmo formalisms

The electric field gradients (EFG) at the D. Li. N and O sites in the linear molecules LID, DF, DCN, DCCD. OCCF, N 2, CO and NCCN have been rigorously evaluated with the inclusion of all integrals using four different semi-empirical SCFMO methods with a view to assess their suitability for EFG calculations. The methods chosen are the CNDO/2 and INDO methods of Pople, a method using explicitly orthogonalised AO's and distinguishing s and p orbitals in the valence shell due to Nanda and Narasimhan (NN-INDO) and a reparametrisation of the same using Clementi-Raimondi exponents. It is found that orbital exponents play a crucial role in semi-empirical EFG calculations. Use of explicitly orthogonalised basis sets as in the NN-INDO schemes is seen to improve the EFG values for the first-row atoms. A few comments are made on population-based methods for EFG calculations.

Electric-field-gradient tensor in cubic crystals with dilute concentration of vacancies: Application to Al

The distribution of electric field gradient (EFG) in Al crystal with dilute concentration of vacancies has been calculated in the approximations of free-electron-screening charge for the valence effect and elastic-continuum model for the size effect. The results, especially the asymmetry parameter, $\ensuremath{\eta}$, at the nearest-neighbor (1NN) sites and the magnitudes of EFG at the third- (3NN) and fourth- (4NN) neighbor sites, are in very good agreement with those from experiment. The somewhat large value of EFG at 1NN sites is ascribed to the failure of the asymptotic free-electron screening charge distribution around these neighbors. Nonzero values of $\ensuremath{\eta}$ for the 3NN and 4NN sites are predicted in conformity with symmetry prediction. These are equal to 0.93 and 0.42, respectively, for the 3NN and 4NN sites. The large value of $\ensuremath{\eta}$ at 3NN sites is mainly due to the greater asymmetry of the EFG from size effect. Unfortunately, experimental values of $\ensuremath{\eta}$ for the 3NN and 4NN sites are not available to compare with the theoretical prediction.