Hyperfine interactions in 111Cd-doped lutetium sesquioxide

We report here first Perturbed Angular Correlation (PAC) results of the electric field gradient (EFG) characterisation at ¹¹¹Cd impurities located at both non-equivalent cation sites of the bixbyite structure of Lutetium sesquioxide, between room temperature (RT) and 1273 K. The comparison with results coming from a systematic ¹¹¹Cd PAC study in bixbyites and with point-charge model (PCM) predictions shows the presence of a trapped defect at RT in the neighbourhood of the asymmetric cation site, which is completely removed at T > 623 K. The anomalous EFG temperature dependence in Lu₂O₃ can be described in the frame of a “two-state” model with fluctuating interactions, which enables the experimental determination of the acceptor energy level introduced by the Cd impurity in the band-gap of the semiconductor and the estimation of the oxygen vacancy density in the sample. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electric field gradients in Hf2Fe

Perturbed Angular Correlations (PAC) technique was applied to measure the electric field gradients (EFG) at ¹⁸¹Ta sites in Hf₂Fe. The compound has the cubic structure of the Ti₂Ni prototype with two non‐equivalent crystallographic sites for Hf atoms. EFGs for the two sites were measured as a function of the temperature. In addition, one more EFG was observed, which was assigned to the presence of defects in the lattice. The ratio of the measured EFG at the regular lattice sites has been used to assign charges to the ions through a simple calculation using the point charge model and it is shown that the lattice symmetry can explain the very different EFG at both sites. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Nonanuclear [Mo(IV){(CN)Fe(III)(3-ethoxy-saldptn)}8]Cl4 Complex Compound Exhibits Multiple Spin Transitions Observed by Mössbauer Spectroscopy

The hyperfine interactions at ¹⁸¹Ta ions on Fe³⁺ sites in α-Fe₂O₃ (hematite) were studied in the temperature range 11–1100 K by means of the perturbed angular correlation (PAC) technique. The ¹⁸¹Hf(β⁻)¹⁸¹Ta probe nuclei were introduced chemically into the sample during the preparation. The hyperfine interaction measurements allow to observe the magnetic phase transition and to characterize the supertransferred hyperfine magnetic field B_hf and the electric field gradient (EFG) at the impurity sites. The angles between B_hf and the principal axes of the EFG were determined. The Morin transition was also observed. The results are compared with those of similar experiments carried out using ¹¹¹Cd probe. ^aAlso at Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, Argentina.     

Ab initio study of the EFG tensor at Cd impurities in Sc2O3 semiconductor

We present an ab initio study of diluted Cd impurities localized at both cation sites of the semiconductor Sc₂O₃. The electric-field-gradient (EFG) tensor at Cd impurities located at both cationic sites of the host structure was determined from the calculation of the electronic structure of the doped system. Calculations were performed with the full-potential augmented-plane wave plus local orbitals (APW+lo) method within the framework of the density functional theory. We studied the atomic structural relaxations and the perturbation of the electronic charge density induced by the impurities in the host system in a fully self-consistent way. We showed that the Cd impurity introduces an increase of 8% in the nearest oxygen neighbors bond-lengths, changing the EFG sign for probes located at the asymmetric cation site. The APW+lo predictions for the charged state of the Cd impurity were compared with EFG results existent in the literature, coming from time-differential γ–γ perturbed-angular-correlations experiments performed on ¹¹¹Cd-implanted Sc₂O₃ powder samples. From the excellent agreement between theory and experiment, we can strongly suggest that the Cd acceptor impurities are ionized at room temperature. Finally, we showed that simple calculations like those performed within the point-charge model with antishielding factors do not correctly describe the problem of a Cd impurity in Sc₂O₃.     

Electric field gradient in bixbyite rare-earth oxides R2O3 (R=Tl,Eu, Lu,Tm) measured by perturbed angular correlation

The electric field gradient (EFG) at ¹¹¹Cd nuclei dilutely substituting the cation site in bixbyite rare-earth oxides Tl₂O₃, Eu₂O₃, Lu₂O₃ and Tm₂O₃ has been measured using perturbed angular correlation technique. The ¹¹¹In(EC) ¹¹¹Cd probe nuclei were introduced into the samples by thermal diffusion. The experimental EFG values are compared with those calculated using the point charge model (PCM). The results are discussed in terms of a correlation between the electric field gradient and cation–oxygen bond length in metal oxides. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dynamic hyperfine interactions in In(Cd)-doped ZnO semiconductor: PAC results supported by ab initio calculations

In this work, we present results of Time-Differential γ–γ Perturbed-Angular-Correlations (PAC) experiments performed in ¹¹¹Cd-doped ZnO semiconductor. The PAC technique has been applied in order to characterize the electric-field-gradient (EFG) tensor at (¹¹¹In (EC)→) ¹¹¹Cd nuclei located, as was later demonstrated, at defect-free cation sites of the ZnO host structure. The PAC experiments were performed in the temperature range of 77–1075 K. At first glance, the unexpected presence of low-intensity dynamic hyperfine interactions was observed, which were analyzed with a perturbation factor based on the Bäverstam and Othaz model. The experimental EFG results were compared with ab initio calculations performed with the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) method, in the framework of the Density Functional Theory (DFT), using the Wien2K code. The presence of the dynamic hyperfine interactions has been analyzed enlightened by the FP-APW+lo calculations of the EFG performed as a function of the charge state of the cell. We could correlate the large strength of the dynamic hyperfine interaction with the strong variation of the EFG due to changes in the electronic charge distribution in the Cd vicinity during the time-window of the PAC measurement. It was also revealed that the Cd impurity decays to a final stable neutral charge state (Cd²⁺) fast enough (in few ns) to produce the nearly undamped observed PAC spectra.     

Mössbauer Studies on Nanocrystalline Diol Capped γ-Fe2O3

Nanocrystalline indium (nano-In) was prepared with different particle sizes by electrochemical deposition. The temperature dependence of the local electric field gradient (EFG) of nano-In was investigated in a temperature range of 20–300 K using the probe ¹¹¹In for perturbed γγ angular correlation (PAC) experiments. The temperature dependence of the EFG of nano-In can be described by a (1−BT ^3/2) dependence as in bulk-In. It is shown that the low temperature value of the EFG and the proportionality constant B vary systematically with particle size.     

Calculated Electric Field Gradients and Electronic Properties of Acceptors in CdTe

Linearised augmented plane wave (LAPW) calculations are performed in order to determine the electric field gradients (EFG) induced by group V and group Ib acceptors in CdTe at the nearest neighbour (NN) atomic site. Experimentally, the EFG are measured by the perturbed γγ-angular correlation spectroscopy using the radioactive probe isotopes ¹¹¹In and ⁷⁷Br. Besides the identification of the experimentally observed defects, the LAPW calculations provide information about fundamental properties of the EFG. For the group Ib acceptors, the dependence of the EFG on the NN distance is opposite to the expectation from the simple point charge model. This revised version was published online in September 2006 with corrections to the Cover Date.     

Electric field gradients in PrBa2 Cu 3 O 7 : LSDA+U results and comparison with experiment

Electric-field gradient (EFG) and asymmetry parameter (η) at all oxygen and copper sites of nonsuperconducting PrBa₂Cu₃O₇ (Pr123) compound were calculated using the full-potential (linearized)-augmented-plane-wave plus local orbitals method. Exchange and correlation effects were treated by LSDA+U for Cu(3d) and Pr(4f) electrons. The effects of changing screened Coulomb parameters U_Pr, U_Cu1, and U_Cu2 on the results were individually studied. The calculated EFG of O2 site is close to the EFG of O3 site at variance with the experimental result. It was shown that by increasing superconducting holes in O2 and O3 sites the EFG at these sites increase and vice versa. The most famous theories which have been proposed to explain the suppression of superconductivity in perfect (without any mis-substitution or other defects) Pr123 compound are not consistent with the experimental EFG at O2 and O3 sites. By replacing one Pr atom at the Ba site on unit cell of Pr123 (Pr_Ba), it was shown that Pr_Ba mis-substitution reduces the superconducting holes in both O2 and O3 sites and could be responsible for the suppuration of superconductivity in Pr123 samples. It is very probable that the unusual behaviors of experimental EFG at O2 and O3 sites of Pr123 are related to oxygen defects which are produced with Pr_Ba mis-substitution.     

Effect of vacancy diffusion on NQR parameters in LaF3

A simple model calculation of the electric field gradient (EFG) at the La site in crystalline LaF₃ was carried out using Bertaut's method in order to elucidate a possible relation between the anomalous temperature dependence of the asymmetry parameter of the EFG found in the ¹³⁹La nuclear quadrupole resonance and the vacancy-mediated diffusion of the fluorine anions. Redistribution of some particular fluorine species among the four crystallographically inequivalent positions between their original sites and the Schottky-type vacancy sites via rapid diffusion contributes to the reduction of the asymmetry of the EFG, this being consistent with the experimental fact that the asymmetry parameter decreases sharply above room temperature.     

Temperature variation of electric field gradient in scandium metal

Electric field gradient (EFG) in scandium metal has been evaluated at temperatures 11 K and 293 K using band wave functions determined in the temperature dependent model potentials. The results of net EFG obtained are −3.756×10¹³ esu/cm³ and −8.009×10¹³esu/cm³ at 11 K and 293 K respectively. The agreement with available experimental result is reasonably good.     

Electric field gradient calculations in rare-earth iron garnets

The electric field gradient (EFG) tensor at the three cation sites in RE₃Fe₅O₁₂ garnets (RE = Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu) has been calculated using a monopole-point-dipole model. The monopole contribution to the EFG tensor can explain the signs and the changes with the RE radius of the experimental quadrupole coupling constants at the octahedral and the tetrahedral sites. The monopole and dipole contributions together cannot account for the magnitude of the experimental tensors at these sites for any value of oxygen dipole polarizability α. The dipole contribution is important in garnets even for very small values of α. Presently available calculations of the overlap contribution to the EFG tensor combined with the lattice contribution also do not agree with the experimental data for these two sites. A possible cause of the disagreement is given. It is suggested that accurate experimental data of the quadrupole coupling constants at the dodecahedral sites in a RE₃Fe₅O₁₂ series, especially in Gd₃Fe₅O₁₂, could resolve the importance of various contributions at all three sites.     

Study of highT c superconductivity in Y1Ba2Cu3O7−x by PAC

HighT _c superconductivity in the YBaCuO superconductor has been investigated through the quadrupole interaction of the probe nuclei⁹⁹Tc. The quadrupole interactions were measured by the TDPAC method from 77 to 296 K. The probe nuclei⁹⁹Tc were introduced into the YBaCuO superconducting specimen by diffusion. The derived electric quadrupole interaction parameters show that the probe nuclei are subject to a unique EFG interaction and occupy a substitutional lattice site in the YBaCuO superconductor. A strong EFG of 10¹⁹ V/cm² was observed. The temperature dependence of the EFG exhibits a linear decrease with temperature increase. Anomalies of both EFG and ν were found in the superconducting transition temperature region. The role of the oxygen vacancies in the Cu−O chains is discussed.     

Mixed hyperfine interactions and defect structure in magnesiowüstite Fe−Mg−O

An analysis of the 4.2 K spectra of Fe_xO (x∼0.91) and (Fe_0.4Mg_0.6)_xO is presented in which it is considered that because of the large electric field gradient at Fe²⁺ defect sites, the spectra cannot be described by Lorentzian sextets. It is assumed that the magnetic hyperfine field vector is oriented at random in the coordinate system defined by the EFG main axis, and that the EFG coordinate system is also distributed randomly. The simplifying assumption of the asymmetry parameter η=0 allows an analytical formula to be used to describe the complex spectra. Distributions of both magnetic hyperfine field and quadrupole interaction were progressively refined resulting in reasonable fits to the spectra with the main features being well reproduced. The magnetic hyperfine field distribution is rather broad with several features present while distinct values were obtained in the distribution of quadrupole interactions. These latter values are considered to correspond to the defect configurations around the Fe²⁺ sites. The distribution of hyperfine fields is considered to reflect the varying strengths of superexchange due to the high defect concentration as well as the effects of magnetic relaxation.     

Electric field gradient in transition metal: Scandium

Corrections to results of electric field gradient (EFG) already published [Pramana — J. Phys.41, 443 (1993)] are reported. The corrected net EFG is:q=−8.01×10¹³ esu/cm³ against the published valueq=16.06×10¹³ esu/cm³. The present result agrees reasonably well with the experimental result, |q _expt|=13×10¹³ esu/cm³. Recently, a computational error is detected, which modifies the results of EFG, we have already published [1]. The error was committed mainly in the part that evaluated thep-p contribution [1] to EFG by the conduction electrons. The corrected results are summarized in table 1 which must replace the table 1 of the published work [1]. In addition, the lattice parameters as well as the temperature were also misquoted in the previous work [1]. The right parameters are:a=6.25311 au andc=9.96509 au. The temperature at which EFG’s are calculated is 293 K instead of 11 K as reported before [1]. The discussions and conclusions made in the published work [1] remain almost unchanged except that they now refer to the corrected numbers. Although the corrected net EFG suffers a sign reversal from the one already published [1], the agreement with experiment is still considered reasonably good because the sign of experimental EFG is not determined. The computational error however does not affect the introduction and theory section of the published work [1].     

Anomalous Temperature Dependence of the EFG in AlN Measured with the PAC-Probes 181Hf and 111In

¹⁸¹Hf and ¹¹¹In ions were implanted into AlN-layers in order to investigate their immediate lattice site environment and its temperature dependence by means of the Perturbed Angular Correlation (PAC) technique. After rapid thermal annealing at 1273 K up to 50% of the probe atoms were incorporated on undisturbed lattice sites defined by an electric field gradient (EFG) of 33 MHz for In and 572 MHz for Hf for measurement at room temperature. PAC-spectra taken at temperatures between 25 and 1200 K show that the EFG measured at the site of the undisturbed probes changes with temperature. While for Hf it decreases by 3%, for In it increases by 25% within the measured temperature range. Thus, the change cannot be due only to the thermal lattice expansion. In the case of In the fraction of probe atoms on substitutional sites increases with temperature until it reaches nearly 100% at 973 K. These effects are fully reversible. For the Hf probe, an additional EFG was detected at temperatures above 300 K.     

On the “naive” model of EFG in hcp metals

In the “naive” but fairly successful model by Bodenstedt and Perscheid, the electric field gradient (EFG) in a hcp s-p metal is first evaluated by the lattice sum method, postulating a conduction electron charge shift and approximating the new conduction electron distribution by point charges. Adding to this EFG, a contribution due to the anisotropy of lattice vibrations, i.e. (<z ²>−<x ²>), was presumed to give the total temperature-dependent EFG. We find, however, from the EFG results of the present calculation for¹¹¹CdCd and the original result for⁶⁷ZnZn, that the experimental data are better represented without the addition of the anisotropic vibration term. This finding, as well as the outline of the present evaluation of <z ²> and <x ²> and hence the EFG for Cd, are discussed.     

Hyperfine interactions of 19O in TiO2 and quadrupole moments of 13,19O

The electric quadrupole interaction of ¹⁹O(I^π=(5/2)⁺, T_1/2=27.0 s) in TiO₂ single crystal was studied in detail by means of the β-NQR to determine the electric quadrupole moments Q of short-lived β-emitting nuclei ¹⁹O and ¹³O(I^π=(3/2)^-, T_1/2=8.6 ms). Two implantation sites were found for the implanted O nucleus and the quadrupole coupling constants of ¹⁹O at these sites were determined. We observed FT-NMR of the enriched stable isotope ¹⁷O in TiO₂ and obtained the electric field gradient (EFG) at the oxygen substitutional site. With this knowledge, we have determined Q(¹³O)=11.0 ± 1.3 mb and Q(¹⁹O)=3.7 ± 0.4 mb. The present results are compared with the theoretical values calculated by the shell model code, OXBASH and by the Hartree–Fock calculation with the realistic potential. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electric field gradient in Cu2O from band structure calculations

We have performed self-consistent full potential LAPW calculations of Cu₂O. Since the new results agree well with a previous muffin-tin calculation, it can be concluded that non-muffin-tin effects are relatively small in Cu₂O. By using the present self-consistent charge density, we, obtain the electric field gradient (EFG) at the Cu site without further approximations. Our theory yields a value of −0.85 e/au³, in good agreement with experiment but in contrast to a previous cluster calculation in which a positive sign of the EFG was obtained. The origin of the EFG and the discrepancies between our band structure and the cluster calculation is analyzed.