Lattice Electric Field Gradient Research Articles

Electronic defects in lattices of YBa2Cu3O7 and La2 − xSrxCuO4

Using emission Mössbauer spectroscopy data for the 67Zn isotope and nuclear quadrupole resonance data for the 17O isotope, as well as calculations of the lattice electric field gradient, the effective charges of all atoms in superconducting copper metal oxide YBa2Cu3O7 and La2 – xSrxCuO4 crystal lattices were determined. The effective charges of metal atoms and most oxygen atoms correspond to their standard oxidation states (Y3+, La3+, Ba2+, Sr2+, Cu2+ and O2−). However, the atoms of chain oxygen (in YBa2Cu3O7) and planar oxygen (in YBa2Cu3O7 and La2 – xSrxCuO4) show a reduced charge, which is explained by the localisation of holes in the corresponding sublattices.

61Ni and 63Cu Hyperfine Coupling in Copper Sites of Superconducting Cupric Ceramic Metallic Oxides and Their Dielectric Analogues

The 61Cu (61Ni) emission Mossbauer spectra of high-temperature superconducting ceramics are measured, as well as a number of dielectric and antiferromagnetic cupric metal oxides, which are structural analogues of high-temperature superconductors. The 61Ni nuclei quadrupole coupling constants are determined and their linear correlations are established with the calculated values of the lattice electric field gradient tensor (EFG) created in the copper sites. Linear correlations are found between the 61Ni and 63Cu nuclei quadrupole coupling constants, which make it possible to determine the valence contribution to the EFG for the both probes.

Hyperfine Interactions in Copper Lattice Sites of Antiferromagnetic Compounds Representing the Analogues of Superconducting Copper Metal-Oxide Compounds

Emission 61Cu(61Ni) Mossbauer spectra of dielectric metal-oxide compounds of divalent copper with molecular formulas Ca1 – xSrxCuO2, Ca2CuO2Cl2, SrCuO2, Sr2CuO2Cl2, YBa2Cu3O7 – x, La2 – xSrxCuO4, and Nd2 – xCexCuO4 correspond to the quadrupole and Zeeman interactions of the 61Ni nuclei with local fields at the copper lattice sites, whereas the spectra of superconducting metal-oxide complexes correspond to the interaction of the quadrupole moment of the 61Ni nuclei with the tensor of the electric field gradient (EFG). For both groups of metal oxides, linear dependences of the quadrupole interaction constants on the calculated values of the principal component of the lattice EFG tensor in the copper lattice sites are observed both on the 61Ni nuclei (data of the emission Mossbauer spectroscopy on the 61Cu(61Ni) isotopes) and on the 63Cu nuclei (data of the nuclear magnetic resonance on the 63Cu isotope). This fact is explained by the constant values of the valence component of the EFG for both the 61Ni2+ and 63Cu2+ probes in all metal-oxide complexes of divalent copper.

Сверхтонкие взаимодействия в узлах меди антиферромагнитных соединений, аналогов сверхпроводящих металлооксидов меди

Emission Mössbauer spectra of 61Cu(61Ni) in dielectric metal oxides of divalent copper Ca1−xSrxCuO2, Ca2CuO2Cl2, Sr2CuO2Cl2, YBa2Cu3O7−x, La2−xSrxCuO4 and Nd2−xCexCuO4 correspond to the combined quadrupole and Zeeman interaction of 61Ni nuclei with local fields at the sites of copper. For the superconducting metal oxides, on the other hand, spectra correspond to the interaction of the quadrupole moment of 61Ni nuclei with the electric field gradient. The both groups of metal oxides show linear dependences of the quadrupole interaction constants on the calculated values of the principal components of the lattice electric field gradient tensor at the copper sites for 61Ni nuclei (emission 61Cu(61Ni) Mössbauer spectroscopy data) as well as for 63Cu nuclei (63Cu nuclear magnetic resonance data). This fact is explained by invariable values of the valence component of electric field gradient for both 61Ni2+ and 63Cu2+ probes in all divalent copper metal oxides under investigation.

Absolute atomic charges in YBa2Cu3O7 lattice determined by analysis of the nuclear quadrupole interaction parameters

The absolute charges of atoms in all sites of the YBa2Cu3O7 lattice were determined using Mossbauer data for 67Cu(67Zn) and 67Ga(67Zn) isotopes, NMR/NQR data for 17O and 137Ba isotopes, and calculations of the lattice electric-field gradient. The obtained charge distributions correspond to a hole occurring predominantly in the sublattice of chain oxygen.

Angular correlation studies on 172Lu(172Yb) in GaN and measurements at low temperatures

For optoelectronic devices semiconductors with large band gap doped with rare earth are used. Doping is generally performed during growth but for more structured doping the ion implantation technique is preferable. The perturbed γγ-angular correlation technique is an ideal tool to study the behavior of semiconductors after implantation. An adequate rare earth isotope for such investigations of semiconductors is 172Yb. The temperature dependence of the hyperfine fields for 172Lu(172Yb) in GaN has been analysed. The total electric field gradient (EFG) at the site of this probe is a superposition of the lattice EFG due to the GaN wurtzite structure and the EFG due to the 4f shell of the rare earth probe itself. The latter is strongly temperature dependent and opposed to the lattice EFG which in contrast is nearly constant since the lattice parameters change only negligibly with temperature. At elevated temperatures the sublevels of the 4f shell, split by the crystal electric field, are equally populated. But at low temperatures the lowest level is occupied preferentially. Sign and magnitude of the EFG caused by the 4f shell can be calculated (Tomala et al, J Magn Magn Mater 89(1–2):143, 1990) and the result compared to the experimental data.

NMR study of Sm 2Co 17H x hydrides

An NMR study of Sm 2Co 17H x ( x = 0, 1.5, 4.6) hydrides is reported. The 147Sm and 149Sm spin echo spectra were measured on powder samples at 4.2 K with no applied magnetic field. The quadrupole septets (nuclear spin I of both Sm isotopes is 7/2) observed in the spectra are attributed to Sm sites with different numbers of hydrogen nearest neighbours located at the 9(e) and 18(g) sites in the crystal structure. Hydrogen neighbours at 9(e) sites cause a decrease of Sm hyperfine field, whereas hydrogen at 18(g) sites causes its increase, which is attributed to the changes of an anisotropic contribution of spin dipolar origin. The 9(e) neighbours cause a decrease of the quadrupole splitting, which corresponds to an increase of the lattice contribution to the electric field gradient. Hydrogen at 18(g) sites causes a slight decrease of the lattice electric field gradient. The results are analysed in terms of electron transfer between hydrogen and samarium sites and the difference between the effect of hydrogen at the 9(e) and 18(g) sites on the magnetocrystalline anisotropy is discussed.

Hydrogen-induced reduction of electric field gradient in La 2Fe 14BH x studied by NMR on 139La

The 139La NMR spin-echo spectra of La 2Fe 14BH x are reported. Since lanthanum has no 4f electrons, the observed quadrupole splittings are directly proportional to the lattice part of the electric field gradient, V zz lat. This quantity decreases as the content of interstitial hydrogen x grows. To describe the fall-off of V zz lat, a dimensionless function f 2( x) is introduced, normalised so that f 2(0)=1. Analysis of the new and earlier published data obtained by several experimental techniques demonstrates that the reduction of both the second-order crystal field parameter A 20 and the lattice electric field gradient V zz lat is described by the same function f 2( x) in all R 2Fe 14BH x . The only distinction that needs to be made is between R being a light and a heavy rare earth.

Correlations of the 63Cu NQR/NMR Data with the 67Cu(67Zn) Emission Mössbauer Data for HTSC Lattices as a Tool for the Determination of Atomic Charges

A linear correlation between the quadrupole coupling constants C (Cu) measured by the 63Cu NQR/NMR technique on the one hand and C (Zn) measured by the 67Cu/67Zn emission Mössbauer spectroscopy on the other hand has been found. The deviation from the linear dependence for a particular substance indicates a considerable difference of the copper charge state from 2+ or different orientations of the z-axes for the total and lattice electric field gradient (EFG) tensors on 63Cu. A similar deviation from the linear dependence of C (Cu) on the principal component of the lattice EFG tensor calculated for the copper sites permits to find a difference of the accepted model from the actual charge distribution in the HTSC lattice sites.

Simulated Quadrupolar Broadening in Condensed Matter PAC Spectroscopy

Perturbed angular correlation (PAC) spectroscopy of materials containing static defects must often deal with quadrupolar broadening due to randomly distributed defects. The electric field gradient (EFG) due to defects adds to the crystal lattice EFG causing quadrupolar frequencies to differ from site to site in a crystal. We report a simulation study of the static defect EFG using a uniform random distribution of point charges. In this simulation, the distribution of random EFG's is found to depend on the ratio γ of the most probable values of the lattice and random EFG magnitudes, γ=V zz lat/V zz ran. We present fits from the results of the simulations to Hf-doped cubic niobium and Nb-doped tetragonal ZrO2 PAC data. The single scaling parameter fitting for γ=0 provides an unprecedented excellent description for the cubic PAC data.

Atomic charges in RBa2Cu4O8 lattices, determined by comparison of experimental and calculated nuclear quadrupole interaction parameters

Nuclear quadrupole interaction parameters have been determined for the rare-earth and copper sites of the RBa2Cu4O8 lattices (R = Sm, Y, Er) by 67Cu(67Zn) and 67Ga(67Zn) emission Mössbauer spectroscopy. The lattice electric field gradient tensors have been calculated for all sites of the above lattices using the point charge approximation. A comparison of the measured and calculated parameters has shown that in the RBa2Cu4O8 lattices holes are predominantly confined to the oxygen chain sublattices.

Electric Field Gradients at 57Fe Nuclei in the Sr3Fe2Ge4O14 Trigonal Germanate

A computation of the lattice electric field gradient tensor at the octahedral and tetrahedral Fe 3+ sites in the Sr 3 Fe 2 Ge 4 O 14 trigonal germanate is performed, taking into account the fractional ionic charges provided by the valence summation procedure. The correlation of the calculated Mossbauer quadrupole splittings with those found experimentally shows the role of the local environment distortion in the quadrupole interaction of iron nuclei in this structure.

Nuclear magnetic resonance study of the Nd2Co17Ax(A=N,H) compounds

The 145Nd, 143Nd, and 59Co nuclear magnetic resonance (NMR) spin-echo spectra and echo decays for Nd2Co17 and its interstitial nitride, Nd2Co17N3 and hydride Nd2Co17H3.6 at 4.2 and 77 K at zero field and in applied fields of up to 5 T are reported. The Nd quadrupole splitting of 3.88 MHz for 145Nd in Nd2Co17 is reduced to 0.37 MHz for Nd2Co17N3. The value for Nd2Co17H3.6 is 3.66 MHz. The reduction of quadrupole splitting for Nd2Co17N3 as compared to the host Nd2Co17 is 50% larger than for the isostructural Nd2Fe17N3 and is assigned to a huge increase in the magnitude of the lattice electric field gradient at the Nd site and, thus to a huge rise of magnetocrystalline anisotropy caused by neighboring N atoms, whereas H atoms have only a slight influence on both quantities. The 59Co spectrum of the nitride is much different from that of the host compound indicating a large influence of N neighbors on Co hyperfine fields, whereas only minor changes are observed after hydriding.

Atomic charges in RBa2Cu3O7 superconductor lattices. Electronic structure of the copper atoms

AbstractThe effective atomic charges for all sites of the RBa2Cu3O7 lattices have been found using our Mössbauer data withthe 67Cu(67Zn),133Ba(133Cs), 155Eu(155Gd) isotopes, our calculations of the lattice electric field gradient (EFG) at the lattice sites, as well as the 17O NMR data available in literature. The charges show holes placed in the chain oxygen sublattice mainly. The Cu2+ Sternheimer factor has been estimated and the Cu2+ valence EFG in YBa2Cu37 has been found. The z‐axes of the total, lattice, and valence EFG tensors for Cu(2) coincide with the crystal axis c. For Cu(1) they coincide with the axes b, a, and a, respectively.

139La NMR study of RE2Fe14B compounds (RE = La,Nd,Y)

The 139La NMR spin-echo spectra of La2Fe14B, La0.2Nd1.8Fe14B, La0.2Y1.8Fe14B and La0.1Y1.9Fe14B at 4.2 K are reported. The values of the lattice electric field gradient at the rare earth sites 4f and 4g correspond to a difference of the crystal electric field coefficients A20 by a factor of 2 between those sites. In La0.1Y1.9Fe14B the occupancy of lanthanum on the 4g sites is found to exceed that on the 4f sites by a factor of 4.

Nuclear magnetic resonance study of expanded lattice permanent magnet nitrides and carbides at high pressure

The nuclear magnetic resonance spin-echo spectra of 89Y and 147Sm for Y2Fe17, Y2Fe17C0.4, Y2Fe17C1.5, Y2Fe17N2.5, Y2Fe17CN1.5, Y2Fe17H1.6, and Sm2Fe17 have been measured at pressures up to 10 kbar at 4.2 K. A reduction of the magnitude of the valence electron polarization contribution to the effective field Be at Y and Sm sites with pressure is observed. The effect is strongly related to the nearest neighbor environment of the site. A significant reduction of ∂B/∂p is observed for the sites with nitrogen neighbors. Carbon increases (decreases) ∂B/∂p in the rhombohedral (hexagonal) structure of the compound. A decrease of the lattice electric field gradient at the Sm site in Sm2Fe17 with pressure is observed. A corresponding decrease of the crystal electric field coefficient A02 at the Sm site with pressure of 0.7% per kbar is derived, indicating an anisotropic compressibility of the material.

Nuclear magnetic resonance study of the Sm 2Fe 17 carbides

The 147Sm and 149Sm NMR spin echo spectra and the spin echo decays of Sm 2Fe 17C x for x = 0, 0.5, 1 and about 3 at 4.2 K are reported. The quadrupole septets in the spectra are assigned to the samarium 6c site with different numbers of carbon atoms occupying the neighbouring 9e interstitial sites. A significant influence of carbon neighbours on the Sm quadrupole splitting and hyperfine field is observed and is attributed to a strong change of the Sm valence electron distribution and polarisation caused by carbon neighbours. From the quadrupole splittings the values of the lattice electric field gradient and the corresponding crystal electric field coefficient A 2 0 are derived. For the Sm sites with 0 (3) carbon nearest neighbours the A 2 0 value of −66 K a 0 −2 (−350 K a 0 −2 is obtained.

61Cu(61Ni) emission Mossbauer study of hyperfine interactions in copper-based oxides

Simple (CuO2, CuO) and complex superconducting (YBa2Cu3O7-x, La2-xSrxCuO4) copper oxides have been investigated by 61Cu(61Ni) emission Mossbauer spectroscopy. A linear relationship between the quadrupole coupling constant for 61Ni and the calculated lattice electric field gradients is established, and the contribution of the Ni2+ valence electrons to the coupling constant is determined. Magnetic fields are observed at 61Ni nuclei in those cases when the copper sublattice shows magnetic ordering.

Lattice EFG tensors at the rare-earth metal sites in RBa2Cu3O7 and La2-xSrxCuO4

Parameters of the tensors of the electric field gradient (EFG) created by lattice ions at the rare-earth metal (REM) sites in RBa2Cu3O7 (R is a REM or Y) and La2-xSrxCuO4 (0<x<0.3) have been determined by means of 155Eu(155Gd) emission Mossbauer spectroscopy. The EFG tensors at the REM sites have been calculated in the point charge approximation. The experimental and calculated EFGs are shown to be in good agreement when holes are supposed to be mainly in sublattices of the chain and Cu-O plane oxygen for RBa2Cu3O7 and La2-xSrxCuO4, respectively.

Annealing behaviour of In impurities in SiC after ion implantation

The annealing behaviour of indium impurities in SiC after ion implantation, and the formation and stability of complexes at In in SiC have been studied by perturbed angular correlation (PAC) spectroscopy for the first time. Samples of 6H-SiC and 3C-SiC single crystals were doped with radioactive 111In by ion implantation. The implantation-induced radiation damage was annealed by furnace treatment up to 1250K. In the 6H-modification two different electric field gradients (EFG) characterized by the quadrupole coupling constants v Q1 = 36(1) MHz, v Q2 = 63(2) MHz and asymmetry parameters η 1 = 0.00 (10), η 2 = 0.65 (10), respectively, were observed. The EFG 1 is assigned to the lattice EFG experienced by the probe atoms at carbon sublattice sites. The EFG 2 indicates the formation of a distinct In-defect complex, which vanishes at temperatures above 320 K. In the 3C-modification no distinct EFG could be detected.