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.     

Perturbed angular correlation study of order– disorder transition in HfW2O8

The perturbed angular correlation (PAC) technique has been used to characterize the degree of atomic order in the neighbourhood of radioactive ¹⁸¹Hf isotopes in HfW₂O₈. PAC measurements were carried out at temperatures between 14 and 723 K. The compound was synthesized starting with the oxides HfO₂ and WO₃, using a method involving ball milling, high temperature annealing and quenching in liquid nitrogen. Fast cooling allows to have the compound at temperatures below 1050 K. The compound has a high degree of stability below such temperature and around 430 K atomic ordering occurs. This transition order–disorder is reversible. ^aAlso at Comisión de Investigasiones Científicas de la Provincia de Buenos Aires, Argentina.     

Ga and Pt NMR study of UPtGa5

^69,71Ga and ¹⁹⁵Pt NMR/NQR measurements have been carried out for the 5f-antiferromagnet, UPtGa₅. From a NMR study using a single crystal sample, Knight shift measurements are reported for two different Ga sites and for Pt. The principal axes of the electrical field gradient tensor at both Ga sites have been determined and the values of the splitting parameter ν_Q and the asymmetry parameter η have been evaluated. The hyperfine coupling constants with the external field along various directions are also reported for the two Ga sites and Pt.     

Site selective spectroscopy of rare earth doped ThO2

The individual sites present in ThO₂ doped with Eu³⁺ and Er³⁺ have been studied by site selective spectroscopy. A cubic site corresponding to a rare earth charge compensated distantly and two sites having local charge compensation have been identified. One of the locally compensated sites is present only at moderate temperature while the other is formed at higher temperatures and is stable to high temperatures. The measurements are related to catalytically excited fluorescence of ThO₂, previous luminescence measurements, and magnetic resonance experiments that observe the presence of local F^_ charge compensation of rare earth dopants.     

Magnetic Properties of Iron Clusters in Silver

Time differential perturbed γ-γ angular correlation technique was used to measure the magnetic hyperfine field (MHF) at Tb sites in the intermetallic compound Tb₃In₅ using the ¹⁴⁰La → ¹⁴⁰Ce nuclear probe. The measurements were carried out in the temperature range of 8 to 295 K. Two different temperature dependent magnetic frequencies were observed below 30 K, which were assigned as ¹⁴⁰Ce substituting the two inequivalent Tb sites in the orthorhombic structure of Tb₃In₅. The temperature dependence of MHF also shows a possible deviation from an expected Brillouin-like behavior for temperatures below 18 K. A Néel transition at 27 K was observed from magnetization measurements in the samples. The magnetization as a function of the applied magnetic field was measured at two temperatures, 5 and 40 K, and the results show antiferromagnetic and a typical paramagnetic behavior, respectively. In both cases it was not observed saturation under high magnetic field.     

Hyperfine interactions at R and In sites in RNiIn (R = Gd, Tb, Dy, Ho) compounds measured by perturbed angular correlation spectroscopy

Perturbed gamma–gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (mhf) in RNiIn (R = Gd, Dy, Tb, Ho) intermetallic compounds using the ¹¹¹In→¹¹¹Cd and ¹⁴⁰La→¹⁴⁰Ce probe nuclei. The PAC spectra for ¹¹¹Cd measured above magnetic transition temperature show a major fraction with a well defined quadrupole interaction for all compounds except GdNiIn where a single frequency was observed. PAC measurements below T _C showed a combined electric quadrupole plus magnetic dipole interaction for ¹¹¹Cd probe at In sites, and a pure magnetic interaction for ¹⁴⁰Ce at R sites. The temperature dependence of mhf measured with ¹⁴⁰Ce at R sites shows that the values of fields drop to zero at temperatures around the expected T _C for each compound. However, in the measurements with ¹¹¹Cd at In sites, the mhf values become zero at temperatures which are smaller than T _C . The difference between the temperatures at which mhf is zero for ¹⁴⁰Ce and ¹¹¹Cd probes correlates with T _C . For each compound this difference decreases with T _C . The results are discussed in terms of the RKKY model for magnetic interactions and the existence of two magnetic systems, with distinct exchange interaction energies due to different types of atomic layers in these compounds.     

57Fe Mössbauer study of YBa4Cu2.95Fe0.05O8.5+y

⁵⁷Fe Mössbauer measurements were performed on the Fe-doped YBa₄Cu₃O_8.5+y compound as a function of annealing temperature in vacuum. Mössbauer spectra were analyzed using three different quadrupole split doublets. The resistance measurements showed that the compound is not superconducting. The possible identification of the Cu sites with their nearest neighbor oxygen sites in this compound is suggested. Comparison of the results with data of the presence of the superconductor YBa₂Cu₃O₇ allows us to suppose that the presence of the Cu-chain sites alone does not result in superconductivity.     

Lattice Location Studies of Indium in Cr2O3

The lattice sites of implanted In atoms in Cr₂O₃ were investigated by means of electron emission channeling (EC) measurements using ¹¹¹In probe atoms. EC spectra were recorded for several axes and compared to simulations. Indium atoms are most likely located at near-Cr sites. Small differences in the EC patterns for prompt and delayed electrons may be an indication for displacements of ¹¹¹Cd emitter atoms following the electron capture decay of In. This revised version was published online in September 2006 with corrections to the Cover Date.     

Characterization of nanostructured HfO2 films using Perturbed Angular Correlation (PAC) technique

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.     

High-temperature cation ordering in olivine: an in situ Mössbauer study of synthetic (Mg0.55Fe0.45)2 SiO4

High-temperature in situ Mössbauer spectroscopy measurements (300–950°C) were done on synthetic olivine of composition (Mg_0.55Fe_0.45)₂ SiO₄ (=Fa45) in order to study the distribution of Fe^2?+? over the M1 and M2 octahedral sites. The spectra are fit with two doublets, which are assigned to Fe^2?+? at the M1 (smaller splitting) and M2 sites. The Fe^2?+? site-occupancies at M1 and M2, obtained from the Mössbauer relative areas, suggest that Fe^2?+? has a slight preference for the M1 site at temperatures below ~500°C, with a tendency of disordering around this temperature. At higher temperatures, Fe^2?+? again prefers to occupy the M1 site, where it shows a considerable order at this site up to 750°C. At still higher temperatures, the spectra indicated partial reduction of the Fa-component into metallic iron and the resolution of the doublets was severely deteriorated.     

57Fe and119Sn Mössbauer investigations on some substituted barium hexaferrites

⁵⁷Fe and¹¹⁹Sn Mössbauer measurements have been carried out on powder samples of three differently substituted M-type barium hexaferrites. By⁵⁷Fe Mössbauer measurements in an external magnetic field applied parallel to the -ray direction, we found a canted spin structure for all samples. Furthermore, we detected a strong preference of the Sn⁴⁺ ions for the 4f₂ sites. From⁵⁷Fe Mössbauer measurements aboveT _N, we conclude that the substitution does not influence the 2b sites. The analysis of the magnetically split¹¹⁹Sn Mössbauer spectra at room temperature in the case of the Co-Sn and Zn-Sn substituted samples shows a strong difference between the two. The spectra were interpreted due to the different surroundings of the Sn⁴⁺ ions.     

From direct to absolute mass measurements: A study of the accuracy of ISOLTRAP

For a detailed study of the accuracy of the Penning trap mass spectrometer ISOLTRAP all expected sources of uncertainty were investigated with respect to their contributions to the uncertainty of the final result. In the course of these investigations, cross-reference measurements with singly charged carbon clusters ¹²C⁺ _n were carried out. The carbon cluster ions were produced by use of laser-induced desorption, fragmentation, and ionization of C₆₀ fullerenes and injected into and stored in the Penning trap system. The comparison of the cyclotron frequencies of different carbon clusters has provided detailed insight into the residual systematic uncertainty of ISOLTRAP and yielded a value of 8×10^-9. This also represents the current limit of mass accuracy of the apparatus. Since the unified atomic mass unit is defined as 1/12 of the mass of the ¹²C atom, it will be possible to carry out absolute mass measurements with ISOLTRAP in the future. Received 7 June 2002 Published online 6 November 2002 RID="a" ID="a"e-mail: a.kellerbauer@cern.ch RID="b" ID="b"Current address: Centre de Physique des Particules de Marseille, 13288 Marseille Cedex 9, France.     

Effects of nitrogen and vanadium on the57Fe hyperfine field of RFe12−x V x N y (R=Y,Nd)

Magnetization and⁵⁷Fe Mössbauer measurements were carried out on RFe_12–x V _x N _y compounds (R=Y and Nd,x=1.7 and 2.2) and the effects of nitrogen and vanadium atoms on the⁵⁷Fe hyperfine fields at the different iron crystallographic sites were investigated. The hyperfine field decreases with increasing number of vanadium neighbour atoms at all the iron sites. The hyperfine field is strongly enhanced in the nitrogen composition withy>1 where the compound tends to transform into an amorphous-like solid. The iron moment deduced from the hyperfine field increases more upon nitrogenation for the 8i-site than for the other sites, and exceeds the moment of bcc iron.     

Determination of the crystal structures of In70-Ni30 and In70-Pd30 using perturbed angular correlation

According to phase diagrams based on x-ray measurements, In70-Pt30 has the cubic Sn₇Ir₃ crystal structure (D8_f, cI40) but the alloys In70-Ni30 and In70-Pd30 have been variously reported to have either a cubic gamma-brass (D8_1???3, cI52) or the Sn₇Ir₃ structures. In this study, hyperfine interaction measurements are applied as an alternate method to identify phases. Perturbed angular correlation (PAC) measurements were made of characteristic nuclear quadrupole interactions of ¹¹¹In/Cd probe atoms, and demonstrated a common, characteristic “signature” of the Sn₇Ir₃ structure in all three alloys. The Sn₇Ir₃ structure has two inequivalent Sn-sites with a 3:4 ratio of atoms and point symmetries indicate that the electric-field gradients at both sites should be axially symmetric. Measured perturbation functions for all three alloys exhibited two axially symmetric quadrupole interaction signals having the expected 3:4 ratio of amplitudes, as expected for the structure. Furthermore, ratios of the two quadrupole interaction frequencies in each alloy were characteristically large, with frequencies for probe atoms on In(3) sites roughly five times greater than on In(4) sites. Taken together, these observations confirm that all three phases have the Sn₇Ir₃ structure. Quadrupole interaction frequencies are also reported for isostructural alloys of gallium with Pt, Pd and Ni. Negligible inhomogeneous broadening was observed in measurements near room temperature in all six phases, indicating excellent atomic ordering at the stoichiometric 70:30 compositions.     

Magnetic hyperfine fields at Gd and in sites in GdPdIn compound

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the hyperfine interactions in the intermetallic compound GdPdIn using ¹¹¹In→ ¹¹¹Cd and ¹⁴⁰La→ ¹⁴⁰Ce probe nuclei at the In and Gd sites, respectively. The PAC results for ¹¹¹Cd show two well-defined electric quadrupole frequencies above T _C assigned to probes occupying Gd and In sites, with ~50% of site occupation each. The fraction corresponding to In sites increases with temperature reaching 95% around 500 K. Below T _C the measurements for ¹¹¹Cd probe showed combined electric quadrupole plus magnetic dipole interaction with sharp increase in the magnetic field below around 80 K. A pure magnetic interaction with lower hyperfine field values was observed at the Gd sites occupied by ¹⁴⁰Ce below 100 K.     

Laser excitation of Pr3+ sites in ThCl4:Pr3+

Use of site selection spectroscopic techniques combined with low temperature absorption spectral studies has enabled the identification and characterization of ten optically important charge-compensated crystallographic sites for Pr³⁺ in the β-ThCl₄:Pr³⁺ system. Fluorescence decay time measurements, variation of site distribution with dopant concentration and fluorescence spectra were used to characterize the sites. Distant charge compensation is found to be one of the dominant modes of charge compensation of Pr³⁺ in ThCl₄, and its predominance over other sites increases as Pr³⁺ concentration decreases. The sinusoidal distortion of the crystal field symmetry around Pr³⁺ in the incommensurate phase of β-ThCl₄ is believed to be mainly responsible for the splitting and continuous shift in fluorescence spectral lines which occur when ³P₀→³F₂ fluorescence scans are made with excitation at different points on the ³P₀←³H₄ absorption line belonging to different sites.     

Magneto-transport of InAs-quantum wells using InP0.69Sb0.31as a barrier material

InAs/InP_0.69Sb_0.31quantum-well structures grown by metal organic vapor-phase epitaxy are studied by temperature-dependent Hall measurements and by quantum Hall and Shubnikov de Haas effect measurements. At temperatures below 0.3 K a two-dimensional electron gas without a conductive by-pass was demonstrated. For a two-dimensional electron gas with a sheet electron concentration of 2.2 × 10¹²cm⁻²mobilities as high as 118 000 cm²(Vs)⁻¹were observed. In contrast to samples doped on both sides of the quantum well, a beating pattern in the longitudinal resistance was observed for samples which were doped on only one side. This effect is explained by spin–orbit coupling of the electrons in the quantum well which leads to a separation in two spin-splitted subbands. A spin-split energy in the range from 6.9 meV to 8.4 meV was extracted from the Shubnikov de Haas measurements.     

Luminescence of La and Sc impurity centers in YAlO3 single-crystalline films

The luminescence of La_Y³⁺ and Sc_Y³⁺ and Sc_Al³⁺ centers created by lanthanum and scandium ions at Y³⁺ and Al³⁺ cation sites of YAlO₃ perovskite lattice was investigated. The features of emission of excitons localized at the mentioned centers in YAlO₃:La and YAlO₃:Sc single-crystalline films were analyzed by means of time-resolved emission spectroscopy and luminescence decay kinetics measurements under excitation by synchrotron radiation at 9 and 300 K.     

Partitioning of atmospheric carbon dioxide over Central Europe: insights from combined measurements of CO 2 mixing ratios and their carbon isotope composition

Regular measurements of atmospheric CO ₂ mixing ratios and their carbon isotope composition (¹³C/¹²C and ¹⁴C/¹²C ratios) performed between 2005 and 2009 at two sites of contrasting characteristics (Krakow and the remote mountain site Kasprowy Wierch) located in southern Poland were used to derive fossil fuel-related and biogenic contributions to the total CO ₂ load measured at both sites. Carbon dioxide present in the atmosphere, not coming from fossil fuel and biogenic sources, was considered ‘background’ CO ₂. In Krakow, the average contribution of fossil fuel CO ₂ was approximately 3.4%. The biogenic component was of the same magnitude. Both components revealed a distinct seasonality, with the fossil fuel component reaching maximum values during winter months and the biogenic component shifted in phase by approximately 6 months. The partitioning of the local CO ₂ budget for the Kasprowy Wierch site revealed large differences in the derived components: the fossil fuel component was approximately five times lower than that derived for Krakow, whereas the biogenic component was negative in summer, pointing to the importance of photosynthetic sink associated with extensive forests in the neighbourhood of the station. While the presented study has demonstrated the strength of combined measurements of CO ₂ mixing ratios and their carbon isotope signature as efficient tools for elucidating the partitioning of local atmospheric CO ₂ loads, it also showed the important role of the land cover and the presence of the soil in the footprint of the measurement location, which control the net biogenic surface CO ₂ fluxes.     

Lithium in In2S3

The electrochemical insertion of lithium into β-In₂S₃ at different concentrations has been investigated by means of γγ-Perturbed Angular Correlations measurements using implanted radioactive ¹¹¹In nuclei. The study of the hyperfine interactions of the ¹¹¹Cd nuclei during the decay ¹¹¹In(EC)¹¹¹Cd allowed to determine the evolution of the three observed electric field gradients at different temperatures and lithium contents. This study reveals a great difference of behavior between the three indium sites of the structure, and shows the mobility of indium ions and their partial reduction after lithium insertion. In₂S₃ is good model material to study the properties of more complex materials based on this structure, which may be used as electrodes for lithium-ion batteries. This revised version was published online in September 2006 with corrections to the Cover Date.