The Electric Field Gradients in (Zr/Hf)Al3 and (Zr/Hf)2Al3 Intermetallic Compounds Studied by 181Ta- and 111Cd-PAC Spectroscopy

The electric quadrupole hyperfine interactions for ¹⁸¹Hf/¹⁸¹Ta and for ¹¹¹In/¹¹¹Cd probes in polycrystalline ZrAl₃ and Zr₂Al₃ compounds were measured in the temperature range 30–1100 K and compared with the results for isostructural hafnium aluminides. On the basis of the similarities of the numbers, sizes and asymmetries of electric field gradients, lattice site allocations were made. In all matrices, ¹⁸¹Hf/¹⁸¹Ta was found to substitute the Hf/Zr site. The ¹¹¹In/¹¹¹Cd impurities were also assigned to the Hf/Zr site in the compounds (Zr/Hf)Al₃, but appear to substitute the two non-equivalent Al sites in the Zr₂Al₃ and Hf₂Al₃ phases. This revised version was published online in September 2006 with corrections to the Cover Date.     

Investigation of hyperfine interactions in GdCrO3 perovskite oxide using PAC spectroscopy

Perturbed angular correlation (PAC) measurements have been carried out in the antiferromagnetic GdCrO₃ perovskite oxide using ¹¹¹In (¹¹¹Cd) and ¹⁸¹Hf(¹⁸¹Ta) nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced in the compound through a chemical process during sample preparation. The PAC measurements were carried out in the temperature range 20–300 K. Measurements with the ¹⁸¹Ta indicated a unique quadrupole interaction above 170 K and a combined electric quadrupole and magnetic dipole interactions below this temperature. The observed interaction was assigned to the probe nuclei substituting Cr sites. Measurements with ¹¹¹Cd showed two quadrupole interactions. Only one of the fractions however, showed a combined electric and magnetic interaction in the temperature rage 20–170 K which was assigned to ¹¹¹Cd probe substituting Cr site. The other fraction was attributed to the Gd site. The present results are compared with those of LaCrO₃ and NdCrO₃.     

Solution of 111In impurities in the Hf(Zr)3Al2 minority phases of Hf(Zr)4Al3 samples

The hyperfine quadrupole interactions of ¹¹¹In/¹¹¹Cd probes in the polycrystalline isostructural Zr₄Al₃ and Hf₄Al₃ compounds were studied. Strong preference of ¹¹¹In solute for the Zr₃Al₂ and Hf₃Al₂ phases occurring as contaminant fractions in the measured samples was observed.     

Hyperfine interactions in Zr(Fe1−x Al x )2 measured forx≤0.20 by TDPAC and Mössbauer spectroscopy

Mössbauer (⁵⁷Fe) and TDPAC spectroscopy (¹⁸¹Hf) have been used to study quasibinary compounds Zr(Fe_1?x Al _x )₂ forx≤0.20. It has been found that the dependence of the mean values of the hyperfine magnetic field, quadrupole splitting and isomer shift on the Al concentrationx is strong. The dependence of the hyperfine magnetic field on the number of Al atoms as nearest and next-nearest neighbours of⁵⁷Fe has been established. The TDPAC results also indicate a dependence of the hyperfine field on¹⁸¹Ta on Al concentration.     

Lattice Location of 181Ta and 111Cd Probes in Hafnium and Zirconium Aluminides Studied by Perturbed Angular Correlation

The perturbed angular correlation technique was applied to study the lattice location of the ¹¹¹In/¹¹¹Cd and ¹⁸¹Hf/¹⁸¹Ta probe atoms in hafnium and zirconium aluminides. Compounds of different stoichiometries and crystallographic structures were the subject of investigation. According to the expectation, in all investigated compounds ¹⁸¹Hf/¹⁸¹Ta probes occupy the Hf(Zr) crystallographic sites. The ¹¹¹In/¹¹¹Cd probes are placed at the sites of all constituent metals—aluminum, hafnium and zirconium, depending on the crystallographic structure of compound, concentration of the constituent metals and temperature of the sample.     

Temperature dependence of electric field gradient in TbCoO3

The temperature dependence of the electric field gradient (efg) in TbCoO₃ perovskite was measured by perturbed angular correlation (PAC) technique using ¹¹¹Cd and¹⁸¹Ta nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced into the compound through a chemical process during sample preparation. The electric quadrupole interactions at ¹¹¹Cd show two different sites, assigned to probe substituting Tb and Co atoms. The temperature dependence of quadrupole frequencies show sharp discontinuities which have been interpreted as thermally activated spin state transitions from low-spin ground state configuration to the intermediate-spin state and from intermediate-spin to high-spin state of Co^3?+? ion. For ¹⁸¹Ta only one interaction was observed, which was assigned to probe at Co site. Indication of a Jahn–Teller distortion, which stabilizes the intermediate-spin state with orbital ordering, is also pointed out. No magnetic order was observed till 77 K.     

Phase transitions in the SrHfO3 perovskite measured with 111In/111Cd PAC

The temperature dependence of the hyperfine parameters in SrHfO₃ powder samples has been investigated by means of Perturbed Angular Correlation spectroscopy using implanted ¹¹¹In probes. Three quadrupole interactions have been established, with the largest fraction showing a pronounced dynamic interaction. We assign this fraction to ¹¹¹In / ¹¹¹Cd probe atoms on substitutional Hf sites. The temperature dependence of the dynamic interaction has been associated to the Pnma↔Imma phase transition at ∼700 K. We discuss the results in relation to those obtained for ¹⁸¹Hf / ¹¹¹Ta-probes in AHfO₃ (A=Ba, Sr, Ca) and for ¹¹¹In / ¹¹¹Cd-probes in PbZrO₃ and BaTiO₃. This revised version was published online in August 2006 with corrections to the Cover Date.     

Decoupling experiments for the study of electric and magnetic hyperfine interactions in ferromagnetic compounds

The decoupling experiments involving the time-differential perturbed angular correlation (TDPAC) of the 133–482 keV γ-γ cascade in the presence of an external magnetic field applied along the quantization axis have been performed to measure the electric quadrupole and the magnetic hyperfine interactions experienced by the¹⁸¹Ta nuclei at Hf sites in the pseudobinary compounds HfFe_2-xSi_x, withx=0.1 andx=0.3. The hyperfine magnetic fields measured at 298 K areH _hf=133.1±12.0 kG in the cubic (C15) Laves phase compound HfFe_1.9Si_0.1 andH _hf=76.8±7.0 kG in the hexagonal (C14) Laves phase compound HfFe_1.7Si_0.3. The decoupling technique has also been used to obtain a —ve sign for the hyperfine field experienced by¹⁸¹Ta nuclei at the Ti or Hf sites in the Heusler compound Co₂Ti_0.8Hf_0.2Sn and a+ve sign for the hyperfine field at Zr sites in the cubic (C15) Laves phase compound ZrFe₂.     

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.     

The electric field gradient of111Cd in an α-Al2O3 single crystal

Radioactive¹¹¹In⁺ ions were implanted into an α-Al₂O₃ single crystal. The hyperfine parameters of¹¹¹Cd at substitutional Al lattice sites were identified by measuring the perturbed angular correlation for different sample orientations. The electric field gradientV _zz =1.04(17)·10²²V/m² was obtained from the quadrupole coupling constant. This result is compared with the efg values of²⁷Al in α-alumina and¹¹¹Cd in α-Fe₂O₃, which also has the corundum structure. Two additional fractions with broad frequency distributions were observed, one of which is attributed to¹¹¹Cd atoms in a strongly distorted Al₂O₃-lattice.     

Investigation of spin transition in GdCoO3 by measuring the electric field gradient at Co sites

In the present work, the time differential perturbed angular correlation (PAC) technique was used to study the temperature dependence of electric field gradient (efg) in GdCoO₃ perovskite using ¹¹¹Cd and ¹⁸¹Ta nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced in the oxide lattice through chemical process during sample preparation and were found to occupy only the Co sites in GdCoO₃. The efg's at ¹¹¹Cd and ¹⁸¹Ta show temperature dependence with two different fractions each that change with temperature. In the case of ¹¹¹Cd the quadrupole frequency slowly decreases, with corresponding increase of the temperature and shows a peak like structure at around 200 K and a discontinuity at 700 K. These changes have been interpreted as thermally activated spin-state transitions from low-spin ground state configuration to the intermediate-spin state and from intermediate-spin to high-spin state of Co³⁺${\mathrm{Co}}^{3+}$ ion similar to LaCoO₃ compound. Indication of a Jahn–Teller distortion, which stabilizes the intermediate-spin state with orbital ordering, is also pointed out.     

Hyperfine Interactions in the Sr0.88Ba0.12HfO3 Compound

The temperature dependence of the hyperfine parameters in a powder sample of the Sr_0.88Ba_0.12HfO₃ compound has been investigated for the first time using perturbed angular correlation spectroscopy. The time spectra were measured as a function of the temperature from 293 to 1273 K. Activated ¹⁸¹Ta nuclei were used as hyperfine probes at the Hf sites. The analysis of the time spectra indicates the presence of two different surroundings for the Ta probes. The most populated site (f ₁≈75% at laboratory temperature) was fitted with the usual static quadrupole hyperfine interactions found in most perovskite-type compounds. The resulting quadrupole frequency is ω _Q ≈24 Mrad/s at this temperature. This frequency continuously decreases to nearly 9 Mrad/s at 1273 K. Its line width temperature dependence displays three regions and the changes observed probably correspond to phase transitions. The other site has a large and temperature independent quadrupole frequency ω _Q ≈120 Mrad/s that reversibly transforms into the first at high temperatures and probably originates from some defect in the structure. This revised version was published online in September 2006 with corrections to the Cover Date.     

Influence of radiation defects on the hyperfine magnetic field at181Ta in (Zr0.9Hf0.1)Fe2

TDPAC measurements for the 133–482 keV -ray cascade in¹⁸¹Ta were performed with¹⁸¹Hf sources in the ferromagnetic Laves intermetallic compound (Zr_0.9Hf_0.1)Fe₂. Two samples, annealed and not annealed after activation of¹⁸¹Hf in a reactor, were investigated. It was shown that in the annealed sample practically all the¹⁸¹Ta daughters occupied regular sites in the cubic Zr(Hf) sublattice and experienced a unique magnetic hyperfine fieldH _h.f.(Ta)=–6.38(14) T. In the unannealed source the regular precession was observed only for 50% of¹⁸¹Ta nuclei, another half of them were located at some unobservable sites due, obviously, to a recoil after the (n, ) reaction. Possible reasons for discrepancies in the results onH _h.f.(Ta) reported in the literature are discussed.     

Electric-field gradients at Ta donor impurities in Cr2O3(Ta) semiconductor

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.     

TDPAC characterization of tin oxides using181Ta

In connection with a general study of the evolution of tin-oxygen thin films, we report here on the hyperfine interactions of¹⁸¹Ta substitutionally replacting tin in the isolated phases SnO and SnO₂. For this purpose, pure SnO pressed powder and a thin SnO₂ film were implanted with¹⁸¹Hf. In both cases, unique quadrupole frequencies were found after thermal annealing treatments. The results indicate that the following hyperfine parameters: υ _Q = 740.6(2.1) MHz, η=0.07(2) and υ _Q = 971.5(1.9) MHz, η=0.72(1) characterize¹⁸¹Ta in SnO and SnO₂, respectively.     

PAC detection of oxide formation in zirconium and hafnium hydrides

The formation of HfO₂ and ZrO₂ oxides in Hf and Zr hydrides has been detected by¹⁸¹Ta PAC spectroscopy. The oxides were identified by their¹⁸¹Ta quadrupole frequency and asymmetry parameter. The growth of the oxide contamination in a H₂ atmosphere has been investigated as a function of temperature and time.     

PAC studies of hyperfine interactions of181Ta in YBa2Cu3O7-δ and of140Ce in La2CuO4 and La1.85Sr0.15CuO4

Results of TDPAC studies of hyperfine interaction in high temperature superconducting ceramics are reported. The γ-ray cascade of 329–487 keV in¹⁴⁰Ce excited in the decay of¹⁴⁰La in La_2-xSr_xCuO₄ samples (x=0 and 0.15), and 133–482 keV cascade in¹⁸¹Ta excited in the decay of¹⁸¹Hf in YBa₂Cu₃O_7-δ samples were used. The procedure of introducing radioactive¹⁸¹Hf into the ceramics is described and indirect evidence for the occupation of Cu sites by the¹⁸¹Hf-¹⁸¹Ta probe is presented.     

Perturbed angular correlation study of a nanostructured HfO2 film

The hyperfine field at ¹⁸¹Ta lattice sites in a nanostructured HfO₂ thin film was studied by the perturbed angular correlation (PAC) technique. The thin oxide film was deposited by pulsed laser ablation on a silicon substrate kept at 673 K. The thickness was about 25 nm. The radioactive ¹⁸¹Hf ions were produced by neutron activation of the very thin film in the Portuguese research reactor by the reaction ¹⁸⁰Hf(n,γ)¹⁸¹Hf. PAC measurements were carried out at room temperature after annealing at different temperatures up to 1,473 K in air. The PAC technique allows determining the electric field gradient at the ¹⁸¹Ta probe sites. The ¹⁸¹Ta isotopes appear in the sample as disintegration product of ¹⁸¹Hf.     

Hyperfine Fields of 181Ta in UFe4Al8

The γ–γ Perturbed Angular Correlation technique was used to study the hyperfine interaction of ¹⁸¹Ta at the Hf site(s) in UFe₄Al₈ at room temperature and 12 K. The data at room temperature are well described by two electric field gradients, while at low temperature two combined hyperfine interactions have to be considered, one with the magnetic hyperfine field collinear with the c-axis and another with the magnetic hyperfine field in the basal plane. The results are compared with previous Mössbauer and neutron diffraction experiments and the lattice site of Hf is discussed.

     

Ab initio calculations of electric field gradients detected by impurities in TiO2, Al2O3 and CaCO3

We present ab initio calculations of electric field gradients (EFGs) at impurity sites in ionic crystals TiO₂, Al₂O₃ and CaCO₃. The electronic structure was calculated self-consistently by the KKR method in the framework of the local spin density approximation of the density functional theory. The system with a single impurity was simulated by the super cell method. It was found that EFGs for the transition metal impurities (Sc, Nb, Cd and Ta) in TiO₂ were well reproduced by the calculations if the charge state of them in TiO₂ was taken into account. The present method was applied to the determination of the implantation sites of N and O nuclei in TiO₂. The calculation of EFGs at a Si impurity in Al₂O₃ and at Ca site in CaCO₃ were used to derive the quadrupole moments of ²⁷Si and ³⁹Ca from their quadrupole coupling constants. This revised version was published online in August 2006 with corrections to the Cover Date.