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1.
Eneroth  E.  Bender Koch  C. 《Hyperfine Interactions》2004,158(1-4):423-427
The perturbed angular correlation method (PAC) was applied to investigate the lattice location of implanted 111In probe ions in Hf2Ni and Zr2Ni intermetallic compounds. It is concluded that the 111In/111Cd probe nuclei experiencing the highly asymmetric electric field gradient (EFG) occupy the unique hafnium or zirconium 8(h) sites in the investigated phases. Above room temperature, the EFGs decrease linearly with temperature. The results are compared with that of previous PAC measurements with 181Ta probes.  相似文献   

2.
The internal electric fieldgradient (EFG) at the Zr-site in antiferroelectric polycristalline PbZrO3 has been investigated as a function of temperature by time differential angular correlation measurements. For the application of this technique PbZrO3 was doped with small amounts of radioactive181Hf. In the antiferroelectric phase of PbZrO3 the EFG decreases continously with increasing temperature and exhibits a sharp discontinuity at 230 °C, the Curie point of the compound. In the cubic phase a small remaining EFG is observed which is probably caused by lattice imperfections in the source. The antiferroelectric distortion of the lattice decreases slightly when the temperature approaches the Curie point, as shown by the temperature dependence of the asymmetry parameter of the EFG. The measured EFG at room temperature is compared with the result of a lattice sum calculation.  相似文献   

3.
We report perturbed-angular-correlation (PAC) experiments on 181Hf(→181Ta)-implanted corundum Cr2O3 powder samples in order to determine the magnitude and symmetry of the electric-field gradient (EFG) tensor at Ta donor impurity sites of this semiconductor. These results are analyzed in the framework of ab initio full-potential augmented-plane wave plus local orbitals (FP−APW+lo) calculations. The results are also compared with EFG results coming from PAC experiments in isomorphous α-Al2O3 and α-Fe2O3 doped with 111In→111Cd and 181Hf→181Ta tracers. This combined analysis enables us to quantify the magnitude of the lattice relaxations induced by the presence of the impurity and to determine the charge state of the impurity donor level introduced by Ta in the band gap of the semiconductor.  相似文献   

4.
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.  相似文献   

5.
Junqueira  A. C.  Dogra  R.  Carbonari  A. W.  Saxena  R. N.  Mestnik-Filho  J.  Moralles  M. 《Hyperfine Interactions》2001,136(3-8):509-513
The perturbed angular correlation (PAC) technique has been used to study the electric field gradient (EFG) in LaCoO3 perovskite. The results are compared with those for LaCrO3, LaFeO3 measured earlier. The PAC probe, 111In → 111Cd, was introduced in the oxide lattice by means of chemical reaction during sample preparation. In the present work, the temperature dependence of the electric quadrupole interaction parameters, for LaCoO3 was investigated. The resulting systematics of EFG at 111Cd, in La(Cr,Fe,Co)O3 perovskites, reveals a linear dependence with temperature. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

6.
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.Work partially supported by U.S. Department of Energy.  相似文献   

7.
The pressure dependence of the electric field gradient (EFG) in the AgIn2 intermetallic compound was measured from zero up to 35 kbar using the time differential perturbed angular correlation technique in111Cd. The unit cell volume and thec/a ratio variations with pressure were measured up to 80 kbar. The temperature dependence of the cell parameters was also measured, in a range varying from 300 K up to 458 K. The relationship of these results showed that the contribution of the lattice thermal expansion to the EFG variations is about 1/3, a small but not negligible part. The estimated EFG volumetric dependence is at variance with the systematic results found in pure metals. Work supported in part by FINEP and CNPq (Brasil).  相似文献   

8.
2H NMR spectra and spin-lattice relaxation rates, R1 are measured as functions of temperature (4.2-310 K) for YD1.99 and YD1.99+0.04. In YD1.99 the measured temperature dependence of R1 is explained by the interaction of deuteron spins with conduction electrons and the value s-1 K-1 is obtained for the inverse Korringa product . Most of the spectral features in the “rigid” lattice regime are described by the dipolar interaction. No evidence of the deuterium self-diffusion was observed in the measured temperature range. The low-temperature spectra in YD1.99+0.04 consisted of two components: a central line and a doublet characteristic for the quadrupole interaction. The central line corresponds to zero electric field gradients (EFG) at the sites of the deuterons which have a surrounding with cubic symmetry like in YD1.99. The doublet indicates a non-zero EFG which can be explained in terms of the presence of short-range ordered domains of the hypothetical YD2.25 stoichiometry. Other possible interpretations of the spectra are also discussed. In the vicinity of 250 K both spectra and R1 reflect the onset of the rapid motion of deuterium atoms. The linear dependence of R1 at low temperatures yields s-1 K-1. Received 8 September 1998  相似文献   

9.
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.  相似文献   

10.
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.  相似文献   

11.
The anisotropy of the 133–182 keV γ-γ cascade in181Ta measured as a function of temperature between 70K and 298K shows lattice instability, the phenomenon of hysteresis in the vicinity of the lattice transformation temperature (TL) and a temperature dependent electric field gradient (EFG) at the cubic Hf sites in the superconducting cubic Laves phase (C15) compounds Hf1?xZrxV2 (x=0.0, 0.25 and 0.75). These observations are qualitatively understood by assuming the presence of γ-recoil effects and microdomains fluctuating between the high temperature (T>TL) and the low temperature (T < TL) phases of these compounds.  相似文献   

12.
Hill  G.J.  Haas  H.  Keartland  J.M.  Hoch  M.J.R. 《Hyperfine Interactions》1999,120(1-8):141-144
The variation of the electric field gradient (EFG) at nuclear sites with pressure in semimetallic arsenic and antimony has been calculated using the full-potential linearized augmented plane wave (FLAPW) method. The results are compared with NQR data, which are available up to a pressure of 2 GPa. The calculated EFG depends sensitively on the values of the lattice parameters used. A theoretical investigation of the relationship between c/a and the internal parameter z, using FLAPW methods, has been carried out and compared with available X-ray diffraction data for arsenic and antimony. For antimony, the FLAPW calculations give results which are consistent with the experimental pressure variation of both the lattice parameters and the EFG. In the As case, the use of theoretical values of z leads to good agreement with the measured pressure variation of the EFG. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

13.
The hyperfine field at 181Ta lattice sites in nanostructured HfO2 thin films was studied by the Perturbed Angular Correlation (PAC) technique. Thin oxide films were deposited by Electron Beam Evaporation on a silicon substrate. The thickness of the films was ~100 nm and ~250 nm. Radioactive 181Hf nuclei were produced by neutron activation of the film samples in the Brazilian Research Reactor (IPEN IEA-R1) by the reaction 180Hf(n,γ)181Hf. PAC measurements were carried out after annealing at 1473 K. The PAC technique allows the determination of the electric field gradient (EFG) at the probe sites.  相似文献   

14.
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.  相似文献   

15.
The quadrupole interaction (QI) in hexagonal close packed zinc lattice was measured using the 482 keV, 10.6 ns probe state of181Ta employing the time-differential perturbed angular correlation technique. The electric field gradient (EFG) at181Ta in Zn was derived from the measured quadrupole interaction frequency at room temperature asV zz =12.202×1017 V/cm2. The quadrupole interaction measured at various temperatures displayed normal temperature dependence similar to that seen by this probe in many non-cubic hosts.  相似文献   

16.
17.
The 121Sb NQR spectra of CdSb single crystal were measured in the presence of weak (up to 500 Oe) external magnetic fields. As the analysis of the 121Sb EFG symmetry showed, two magnetically nonequivalent Sb sites exist in the CdSb crystal lattice. An upper limit of the local magnetic field, which may exist in CdSb characterized with the observed 121Sb NQR spectra, was estimated by modeling the Zeeman perturbed patterns. This amounted to H loc ∼10 G, hence being much less than earlier found local fields H loc ∼30–200 G in bismuth-based compounds.  相似文献   

18.
In this work we present an ab initio study of Ta-doped Sc2O3 semiconductor. Calculations were performed at dilute Ta impurities located at both cationic sites of the host structure, using the Augmented Plane Wave plus Local Orbitals (APW+lo) method. The structural atomic relaxations and the electric-field gradients (EFG) were studied for different charge states of the cell in order to simulate different ionization states of the double-donor Ta impurity. From the results for the EFG tensor at Ta impurity sites and the comparison with experimental results obtained using the Time-Differential γ–γ Perturbed-Angular-Correlations technique we could determined the structural distortions induced by the Ta impurity and the electronic structure of the doped-semiconductor.  相似文献   

19.
The temperature dependences of the M2 and the M1 ferrous quadrupole splittings in two orthopyroxenes have been studied. The valence contributions to the EFG were calculated from the Boltzmann averages of the expectation values of the EFG components over the corresponding5D levels, while the lattice contributions were obtained from lattice summations. The proper5D level schemes were each determined by diagonalization of the full crystal-field Hamiltonian, for which the operator coefficients were calculated so as to take into account the real C1 symmetry of the FeO 6 10– clusters. The calculations were based on the lattice and the charge data related to ferrosilite.  相似文献   

20.
The orientations and asymmetry parameters of the EFG tensors at the different lattice sites of I127 in (NH4)2H3IO6 have been determined by proton-iodine crossover relaxation at room temperature and at 94°K. Based on these results it was possible to prove the existence of superstructure for the hexagonala- andc-directions and to reduce the number of allowed space groups of the low temperature phase to two: \(R\overline 3 c\) andR32. In both space groups half of the hydrogen bonds remain symmetric and only half of them become asymmetric (ordered) on lowering the temperature through the phase transition, but only inR32 there are antipolar sublattices. The transition entropies calculated for this model of the hydrogen arrangement agree fairly well with the measured value. With the evidence available it is not possible to rule out one of the two space groups.  相似文献   

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