Lattice Electric Field Gradient Research Articles

Nuclear magnetic resonance and mössbauer effect studies of Sm 2Fe 17N x and Y 2Fe 17N x compounds

The 147Sm and 149Sm nuclear magnetic resonance spin-echo spectra of Sm 2Fe 17N x as well as the 57Fe Mössbauer spectra of Sm 2Fe 17N x and Y 2Fe 17N x series are presented. From the quadrupolar splittings at the samarium site in Sm 2Fe 17 the value of the lattice electric field gradient equal to 102 × 10 20 V m −2 was obtained. A threefold increase in this value for the three-nitrogen nearest-neighbour configuration in Sm 2Fe 17N 2.5is observed and the relation to the second-order crystal electric field parameter A 2 o is discussed. The observed increase in the average iron hyperfine fields with nitrogenation amounting to 4–5 T is explained by the magnetovolume effect. The larger increase observed in the samarium-containing series is due to the reorientation of the easy magnetization direction from basal plane to c axis upon nitrogenation.

Systematics of electric field gradients in metals

The intensive study of electric field gradients (EFG) at the site of atomic nuclei in noncubic metals has revealed several systematic trends, e.g. in the relation of the total EFG to the so called lattice EFG as well as in its variation with temperature. Numerous investigations have been carried out in order to test these systematics. The results will be reviewed and compared to the known trends. Progress in theoretical calculations of the EFG in pure and impurity — host systems will be discussed and compared to the latest available experimental data. Recent measurements of the EFG at metal surfaces and new calculations of the EFG at host sites in impure cubic metals may contribute to the understanding of the EFG in metals.

Electric-field gradients in ionic gadolinium compounds

The calculation of the electric-field gradients (EFGs) in a number of ionic gadolinium compounds is considered. The lattice EFGs at the sites of Gd3+ ions have been evaluated by means of a point-charge model. Comparison of the results with quadrupole-splitting data yields a value of -75+or-2 for the Sternheimer antishielding factor gamma infinity for the Gd3+ ion. The accuracy of the lattice EFGs is discussed in terms of uncertainties in the structural parameters, the assumption of complete ionicity and the omission of ionic polarisabilities.

Temperature dependence of the electric field gradient at Ta impurities in the heavy-rare-earth metals Gd, Dy, Ho, and Er

The time-differential perturbed-angular-correlation (TDPAC) technique has been used to study the temperature dependences of the electric field gradient (EFG) at /sup 181/Ta impurities in the heavy-rare-earth (RE) metals Gd, Dy, Ho, and Er. At room temperature the ratio ..cap alpha.. equivalent vertical-bar V/sub z/z/ (1-..gamma../sub infinity/) V/sup 1at//sub z/z vertical-bar of the measured EFG V/sub z/z and the calculated ionic EFG (1-..gamma../sub infinity/) V/sup 1at//sub z/z decreases linearly with increasing rare-earth atomic number. A linear but much stronger decrease of ..cap alpha.. has previously been reported for the impurity /sup 111/Cd. A simple model is proposed which explains the linear decrease of ..cap alpha.. and the different slopes for /sup 181/Ta and /sup 111/Cd in terms of the lanthanide contraction. This model assumes the conduction-electron contribution to the EFG to be mainly determined by the number of electrons in the Wigner-Seitz cell of the impurity. In all rare-earth hosts the EFG decreases with increasing temperature. This decrease, which is slightly stronger for Gd than for Er, is better described by a linear function of temperature than by a T/sup 3/2/ behavior, observed in many other impurity-host systems. The temperature dependence of the EFG is much stronger than expected from themore » lattice expansion. The difference between the temperature dependence of the measured EFG and of the calculated lattice EFG decreases across the rare-earth series. This can be attributed to a decrease of the amplitudes of the lattice vibrations between Gd and Er.« less

The quadrupole interaction of100Rh in various intermetallic compounds of palladium

The nuclear quadrupole interaction of the 75 keV excited state of100Rh in the ordered intermetallic compounds PdHg, PdPb2 PdSb and PdTe was measured. Using an estimate for the nuclear quadrupole moment of the 75 keV state in100Rh and point ion lattice sums for the lattice electric field gradient (EFG) at the Rh site, the electronic contributionV zz el to the total EFG was derived.V zz el was found to be correlated with the average valence electron concentration in the compounds studied. In the case of PdPb2, the temperature dependence of the quadrupole interaction was also studied and found not to follow theT 3/2 rule which is observed in virtually all pure noncubic metals.

Temperature Dependence of the Electric Field Gradient at Ytterbium Impurities in Thulium Metal

Time-differential perturbed angular correlations of nuclear $\ensuremath{\gamma}$ rays from the decay of $^{172}\mathrm{Lu}$ to $^{172}\mathrm{Yb}$ have been used to measure the temperature dependence of the electric quadrupole hyperfine interaction at dilute ytterbium impurities in polycrystalline thulium metal over the temperature range from 77 to 560 K. The dependence on temperature is found to be relatively weak and almost linear. Based on a value for the quadrupole moment of the 79-keV state obtained from Coulomb-excitation transition probabilities, the electric field gradient (EFG) is (6.15 \ifmmode\pm\else\textpm\fi{} 0.15) \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ V/${\mathrm{cm}}^{2}$ at 77 K, and decreases with increasing temperature by (6.6 \ifmmode\pm\else\textpm\fi{} 0.6) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{K}}^{\ensuremath{-}1}$ of the extrapolated value of ${V}_{\mathrm{zz}}(0 \mathrm{K})=6.33\ifmmode\times\else\texttimes\fi{}{10}^{17}$ V/${\mathrm{cm}}^{2}$, with some evidence of leveling off above 500 K. Such a linear temperature dependence is inconsistent with any appreciable contribution to the EFG from an open $4f$ shell, and thus the ionic state of the ytterbium ion is dipositive. The observed EFG is thus due to contributions from the crystal-field and conduction electrons. The temperature variation of the lattice EFG comes mainly from the anisotropy of thermal expansion between the $c$ axis and the basal plane of the thulium lattice. Estimates based on point-charge calculations of the crystal field combined with measured thermal-expansion coefficients yield a magnitude and temperature dependence that are both too small to explain the observed results on this basis. The remaining temperature dependence is ascribed to the effects of local conduction electrons near the Fermi surface.

Quadrupole Coupling and Knight Shift Anisotropy in "hcp" Lanthanum

The nuclear magnetic resonance of ${\mathrm{La}}^{139}$ has been observed in hcp polycrystalline lanthanum metal at 25.6 kOe and 300\ifmmode^\circ\else\textdegree\fi{}K. The recorded pattern at 15.5 Mc/sec displays a central line and one pair of satellite lines. The position and shape of the central line yields an isotropic Knight shift of (0.685\ifmmode\pm\else\textpm\fi{}0.020)% and an axial shift of (0.046\ifmmode\pm\else\textpm\fi{}0.020)%. The satellite spacing yields a lowest pure quadrupole frequency ${\ensuremath{\nu}}_{Q}=101\ifmmode\pm\else\textpm\fi{}4$ kc/sec or $\frac{{e}^{2}\mathrm{qQ}}{h}=(1.41\ifmmode\pm\else\textpm\fi{}0.06)$ Mc/sec. A calculation of the lattice electric field gradient is made for the $\mathrm{ABAC}$ stacking sequences, yielding 0.540\ifmmode\times\else\texttimes\fi{}${10}^{22}$ and 0.487\ifmmode\times\else\texttimes\fi{}${10}^{22}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ for the so-called $A$ and $B$ (or $C$) layer sites, respectively, by the method of plane-wise summation of de Wette. Improved values of the nuclear electric quadrupole moment, $Q({\mathrm{La}}^{139})=0.21$ b, and the Sternheimer antishielding factor, ${\ensuremath{\gamma}}_{\ensuremath{\infty}}({\mathrm{La}}^{3+})=\ensuremath{-}73.5$, are adopted and discussed. The sign of the conduction-electron electric field gradient is shown to be negative.