Time Differential Perturbed Angular Correlation Research Articles

Cold-worked and hydrogen chargedNbHf as studied by TDPAC and positron lifetime measurements

Time differential perturbed angular correlation (TDPAC) and positron life-time measurements on cold-worked and hydrogen chargedNbHf indicate a strong binding of hydrogen decorated vacancy clusters with Hf impurities. Both TDPAC and positron lifetime results are mutually consistent on this aspect. Transformation of H-vacancy clusters into H-bubbles is indicated by the positron lifetime behaviour at annealing temperatures beyond 900 K. Impurity effects like oxygen pickup and coating of voids cannot be ruled out at elevated temperatures.

Investigation of the compound : the electronic structure and the electric field gradient

The electronic structure and electric field gradient (EFG) of the intermetallic compound are calculated on the basis of a recently developed first-principles band-structure full-potential (FP) linear-muffin-tin-orbital (LMTO) scheme within the atomic sphere approximation (ASA). The calculated value of the EFG at the Hf sites is in very good agreement with the results obtained by the time-differential perturbed angular correlation (TDPAC) technique used for the experimental study of the EFG at impurity sites in the compound . The asymmetry parameter and the individual contributions to the EFG are discussed. It is shown that various treatments of 4f hafnium electrons have a large influence on the EFG. The formulae for all of the components of the EFG tensor (the contributions from the valence electrons as well as the lattice contribution) are derived and presented also.

Hf<sub>2</sub>Ni Compound Investigation: Electronic Structure and Electric Field Gradient

The electronic structure of the Hf2Ni intermetallic compound has been obtained using the first-principle full-potential (FP) linear-muffin- tin-orbital (LMTO) method in the atomicsphere approximation (ASA). The electric field gradient (EFG) tensor is calculated for both HI' and Ni sites and compared with the experiment. The theoretical results show that the EFG is determined dominantly by the p electrons. 1. Introd uction The interaction between the nuclear quadrupole moment and the electric field gradient (EFG) at the atomic site with noncubic point symmetry can be determined by various hyperfine interaction measurements, such as: perturbed angular correlation (PAC), perturbed angular distribution, and nuclear magnetic resonance (NMR). These methods were used widely to get information about surfaces, impurities, and vacancy related defects. A lot of experimental investigations of EFG in complex compounds with many atoms per primitive cell have appeared in recent years. Some of them have been devoted to the timedifferential perturbed angular correlation (TDPAC) measurements of nuclear quadrupole interactions in the ordered intermetalic compounds of transition metal Hf. It was shown that EFG reveals interesting properties in these compounds. These compounds represent a challenge for the theory due to their complexity. The explanation of the origin and the magnitude of EFG in one of them, e.g. Hf2Ni, is the subject of the present theoretical investigation.

TDPAC study of the Hf-doped LaNi 5-hydrogen system

The dependence of the electric field gradients (EFGs) in the Hf-doped LaNi 5-hydrogen system, as a function of hydrogen composition ratio ( X), has been investigated in the range 0≤ X≤5.7 using the Time Differential Perturbed Angular Correlation (TDPAC) technique. The TDPAC spectrum of LaNi 5 was fitted with a single EFG, while two EFGs were needed to fit the LaNi 5H x spectra. The first EFG was similar to the one found in metallic LaNi 5 and the second one was interpreted as related to those probe atoms located in the basal plane next to hydrogen interstitial sites located in the same plane. The relative fraction of the probe atoms experiencing these EFGs is dependent on the relative occupation of hydrogen in these sites.

181Ta TDPAC studies in proton implanted Nb

Time differential perturbed angular correlation (TDPAC) measurement performed on the 482 keV level of181Ta probe nuclei inp-implantedNbHf sample shows that hydrogen decorated vacancy complexes are trapped by 6% of probe nuclei experiencing a Lorentzian distribution of quadrupole frequencies with a width of 32 MHz having a mean value of 545 ± 10 MHz with an asymmetry parameter 0·32. Measurements at RT following isochronal annealing treatment of the sample shows that the onset of detrapping of H-V complexes from probe nuclei occurs at 373 K and gets completed at 573 K. Hydrogen atoms stabilized vacancy clusters are found to be trapped by probe nuclei. The role played by other gaseous impurities in the sample in stabilizing the lattice defects bound to probe nuclei is discussed, to have a complete understanding of the recovery of defects in the sample.

Laser Raman spectroscopy (LRS) and time differential perturbed angular correlation (TDPAC) study of surface species on Mo/SiO2 and Mo,Na/SiO2. Their role in the partial oxidation of methane

Silica-supported molybdenum (1.6 and 5.0 wt%) and molybdenum (5 wt%)-sodium (0.4 wt%) catalysts have been characterized by laser Raman spectroscopy (LRS), time differential perturbed angular correlation (TDPAC), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The presence of different molybdenum species was correlated with activity and selectivity to formaldehyde during the methane partial oxidation reaction. The main species identified on the Mo(5.0 wt%) /SiO2 surface were MoO3 and monomeric species with a single Mo=O terminal bond. The pre-impregnation of the silica support with sodium strongly diminishes the Mo=O concentration due to the formation of Na2Mo2O7 species and tetrahedral monomers with a high degree of symmetry. As a result of these modifications, both methane conversion and formaldehyde formation are strongly inhibited. The combination of LRS and TDPAC techniques resulted in a powerful tool for the identification and quantification of the molybdenum species present on the surface of a silica support.

Hf impurity and vacancy interaction in α-Zr

The interaction between Hf impurities and vacancies in α-Zr metal containing 1 at.% Hf has been investigated by time differential perturbed angular correlation (TDPAC) measurements on the 482 keV level of 181Ta probe nuclei in the sample. All 181 Ta nuclei were found to experience a mean quadrupole frequency 〈 ν Q〉 = 310 ± 5 MHz with a Lorentzian distribution of fractional width δ = 0.002 and an asymmetry parameter η = 0.04, characteristic of substitutional sites in the α-Zr matrix. Measurements carried out at room temperature on the sample quenched from 1123 K did not reveal the presence of any defect related quadrupole frequency. These observations show that Hf impurity atoms occupy substitutional sites and they do not bind vacancies in α-Zr.

Study of the EFG Trends in Zr2T (T = Fe, Co, Ni) Intermetallic Compounds

Abstract We use the linear muffin-tin orbital formalism, in the atomic sphere approximation, to investigate the trends of the electric field gradient (EFG) at the nucleus for the non-equivalent sites in Zr2T (T = Fe, Co and Ni) intermetallic compounds. As all those compounds crystallize in the same C16 crystallographic structure, they offer a rare opportunity to investigate electronic structure effects coming from transition metals on the EFG at Zr site. Those results are compared with EFG values obtained from quadrupole coupling constant measurements performed with the time differential perturbed angular correlation (TDPAC) technique, using the 191Ta probe.

PAC Studies of Ag-Hf Intermetallic Compounds

Abstract The hyperfine quadrupole interaction of 181Ta and 111Cd probes in HfAg and HfiAg com-pounds was studied at 24 to 1100 K with the time -differential perturbed angular correlation (TDPAC) technique. The EFGs measured in HfAg on 181Ta and 111Cd are 1.388(5)x1018 Vcm-2 and 0.340(5)x1018 Vcm -2 , respectively. For both probe atoms a linear decrease of the EFG with temperature was observed with slopes 1.11(2)x10-4 K-1 and 1.73(5)x10-4 K-1 for 181Ta and 111Cd, respectively. In Hf2 Ag, 0.469(2)x1018 Vcm-2 was determined on 181Ta nuclei and 0.428(5)x1018 Vcm-2 on 111Cd. Also a linear EFG temperature dependence with the large slope of 2.39(6)x10-4 K-1 was observed for 111Cd probes in Hf2Ag, while the EFG measured on 181Ta nuclei in this compound exhibits a very weak temperature variation which can be fitted neither with a linear T nor a T 3/2 dependence. In all cases the electronic contributions to the EFG are very small indicating that these transition element systems do not obey the universal correlation between the electronic and the ionic EFG observed in numerous other systems not belonging to the transition metal series.

Nonstoichiometric Zinc Oxide and Indium-Doped Zinc Oxide: Electrical Conductivity and111In-TDPAC Studies

Indium-doped zinc oxide powders have been prepared which show room-temperature electrical conductivities as high as 30 Ω−1cm−1. The indium doping apparently occurs as Zn1−xInxO, Zn1−yInyO1+y/2, or a combination of these. Optimum conductivity occurs for Zn1−xInxO where the maximum value ofxobtained was about 0.5 at%. This substitution results in a lattice volume expansion of 0.4%. The degrees of sample reduction were determined by iodimetric titration. Time differential perturbed angular correlation (TDPAC) spectroscopy on indium doped zinc oxide is consistent with indium substituting at normal zinc sites in the ZnO lattice. TDPAC studies on zinc oxide annealed under zinc vapors show a second environment for the111In probe. In this case, there is an unusually high temperature dependence of the electric field gradient which may be caused by a nearby zinc interstitial. An important conclusion of this work is that zinc interstitials are not ionized and do not therefore contribute significantly to the increased conductivity of reduced zinc oxide.

Experimental Hyperfine Characterization of Adsorbed Molybdates on Oxide Surfaces

The dispersion of metal oxides on supports is a problem currently being investigated in supported catalysts. On the other hand, a nuclear spectroscopy technique, namely time differential perturbed angular correlation (TDPAC), has been used in extensive investigations on Mo-based catalysts. This technique, through the measurement of the local electric field gradient (EFG) at radioactive probes sites, can give information about the characteristics (coordination, symmetry, distortions, etc.) of the different environments ofthe probes, their concentration and modifications related to in situ conditions (temperature, atmosphere, pressure, etc.) TDPAC technique was used in the present work to investigate the dispersion of molybdenum oxides on Al 2 O 3 , TiO2/Al 2 O 3 , and SiO 2 . The 99 Mo obtained by neutron irradiation of natural molybdenum present in the samples was used as TDPAC probe. Measurements at room temperature and higher ones as well as with different Mo concentrations in the samples were performed. The corresponding results allowed us through the fitted hyperfine parameters to find out the dispersion, type of adsorbed Mo-O species, and their relative abundances. For Mo concentrations below the theoretical monolayer we were able to find bulklike MoO 3 and two-dimensional-like Mo oxides. Previous results obtained with TDPAC and calculations based on a point charge model suggest different MoO 3 crystallites sites for each support. The structures of the monolayer type molybdena are probably different in each catalyst.

Structural and physical properties of the misfit intercalation compounds HgxTaS2 (x=0.58, 1.19, and 1.3).

Time-differential perturbed angular correlation (TDPAC) and x-ray powder diffraction (XPD) have been used to investigate the complex physical and structural properties of mercury intercalation compounds derived from 1T-${\mathrm{TaS}}_{2}$ and 2H-${\mathrm{TaS}}_{2}$, respectively. In situ TDPAC studies of the $^{181}\mathrm{Ta}$ nuclear quadrupole interaction during Hg intercalation and thermal deintercalation proved the complete reversibility of the intercalation process and revealed the formation of three distinct phases (\ensuremath{\alpha}-${\mathrm{Hg}}_{1.19}$${\mathrm{TaS}}_{2}$, \ensuremath{\beta}-${\mathrm{Hg}}_{1.3}$${\mathrm{TaS}}_{2}$, and a disordered second-stage phase ${\mathrm{Hg}}_{0.58}$${\mathrm{TaS}}_{2}$ derived from \ensuremath{\alpha}-${\mathrm{Hg}}_{1.19}$${\mathrm{TaS}}_{2}$). The stage-1 compounds \ensuremath{\alpha}-${\mathrm{Hg}}_{1.19}$${\mathrm{TaS}}_{2}$ and \ensuremath{\beta}-${\mathrm{Hg}}_{1.3}$${\mathrm{TaS}}_{2}$ are characterized by two different structural arrangements of the intercalated Hg that bear a strong resemblance to those realized in solid-state phases of \ensuremath{\beta}-Hg and \ensuremath{\alpha}-Hg, respectively. \ensuremath{\alpha}-${\mathrm{Hg}}_{1.19}$${\mathrm{TaS}}_{2}$ adopts an unusual (3+1)-dimensional composite crystal structure which can be described as two interpenetrating sublattices formed by the ${\mathrm{TaS}}_{2}$ host and the Hg guest.These C-face-centered orthorhombic sublattices share common a and c axes but are incommensurate along the b axis [a=5.765(1) \AA{}, b=3.309(1) \AA{}, c=17.842(1) \AA{}, and ${\mathit{b}}_{\mathrm{Hg}}$=2.782(1) \AA{}]. The in-plane Hg-Hg distances of 2.782 and 3.201 \AA{} indicate a chainlike Hg arrangement similar to that found in ${\mathrm{Hg}}_{1.24}$${\mathrm{TiS}}_{2}$. In the case of \ensuremath{\beta}-${\mathrm{Hg}}_{1.3}$${\mathrm{TaS}}_{2}$ the Hg arrangement is such that the ${\mathrm{TaS}}_{2}$ host lattice (a=3.318 \AA{}) can be considered as a 2\ifmmode\times\else\texttimes\fi{}2 superstructure formed upon hexagonal close-packed Hg layers (${\mathit{a}}_{\mathrm{Hg}}$=2.902 \AA{}). Hg intercalation into 1T-${\mathrm{TaS}}_{2}$ requires an elevated reaction temperature of 423 K, whereas 2H-${\mathrm{TaS}}_{2}$ intercalates Hg already at ambient temperature. Both the TDPAC and x-ray-diffraction data indicate that in the case of 1T-${\mathrm{TaS}}_{2}$ the Hg intercalation process is accompanied by an irreversible change of the ${\mathrm{TaS}}_{2}$ host layers from octahedral to a trigonal-prismatic arrangement.

X-ray and time differential perturbed angular correlation measurements in ZrCr 2 and ZrCr 2H 3 Laves phase compounds

The existence of stacking defects in ZrCr 2 has been detected by X-ray analysis and the time differential perturbed angular correlation (TDPAC) technique, using 181Ta probes occupying Zr sites. It was found that two types of Zr site are responsible for the large observed width of the distribution of electric field gradient (EFG) at the 181Ta probes. One type of Zr site is the normal P6 3/ mmc lattice position which possesses 3 m point symmetry. The other zirconium site comes from stacking defect positions which relax to a local configuration with 4 3 m point symmetry, thus generating strains around the defects. The mean EFG and its asymmetry parameter ν for ZrCr 2 are temperature independent in the range 77⩽ T⩽300 K, while for ZrCr 2H 3 a phase transition occurs at 230 K. This phase transition is quite wide and ranges from 200 to 260 K.

Local structure and valence of impurity ions in oxides by TDPAC and Mössbauer emission spectroscopy

The features of time-differential perturbed angular correlation (TDPAC) in studies on local chemical structures around impurity atoms are described in comparison with those of Mossbauer emission spectroscopy with four typical examples of recent applications to metal oxide systems, namely,99Rh→99Ru in α-Fe2O3 and YBa2Cu3O7−δ and117Cd→117In in MO (M=Mn, Co, Ni) and BaRu2/3Cd1/3O3.

TDPAC research in the border region of physics, chemistry, biology: Charge density waves, intercalation reactions, and metal proteins

Three examples for modern applications of time -differential perturbed angular correlation (TDPAC) to research in the border region of physics, chemistry, and biology are given: (i) charge density waves in layered transition-metal dichalcogenides; (ii) intercalation of Li, Ag, and In into 2H-TaS2 monitored in situ from both the guest and the host point of view; (iii) heavy metal derivatives of small blue Cu-proteins and multi-Cu enzymes. These studies require advanced spectrometers and suitable facilities for isotope production.

Magnetic behavior of dilute Rh impurities in Au1-xFex alloys: A perturbed-angular-correlation study.

We have studied the local magnetic behavior of isolated Rh impurities in ${\mathrm{Au}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$ alloys for compositions x=0.01, 0.03, 0.10, and 0.18, using the time-differential perturbed-angular-correlation (TDPAC) method and by electronic structure calculations within the local spin-density (LSD) approximation. From the measured Curie-Weiss local susceptibility of the $^{100}\mathrm{Rh}$ probe as well as the LSD calculations we find large stable magnetic moments 0.5-1.0${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ on Rh for compositions x\ensuremath{\ge}0.10. In low Fe concentration alloys the observed weak local susceptibility can be reconciled with the calculated large local moments, both positive and negative, on Rh in terms of a typical Ruderman-Kittel-Kasuya-Yosida (RKKY)-type mechanism between Fe and Rh. In the reentrant spin-glass regime, T\ensuremath{\le}60 K for x=0.18, the hyperfine field ${\mathit{B}}_{\mathrm{hf}}$ shows a two-step temperature dependence, increasing sharply below 60 K, such that its extrapolated value at 0 K is enhanced by 30% over its saturated value in the ferromagnetic region.

Reply to "Comment on 'Time-differential perturbed-angular-correlation studies of the amorphous Cu-Hf alloys prepared by mechanical alloying and melt spinning' "

We present some noteworthy facts for discussion of the short-range order in the amorphous alloys from the time-differential perturbed-angular-correlation (TDPAC) spectra in reply to the comment of Mendoza-Z\'elis, Damonte, and L\'opez Garc\'{\i}a.

A Study on Hyperfine Magnetic Field at117In (←117Cd) in Fe3O4by γ-γ Time-Differential Perturbed Angular Correlation

Hyperfine magnetic fields, H hf , at 117 In arising from 117 Cd were measured in Fe 3 O 4 by time-differential perturbed angular correlation (TDPAC) of 90–345 keV γ-rays through the intermediate 660 keV state (3/2+). The values of H hf were found to be 123 and 138 kOe at 295 and 77 K, respectively. The present work is the frrst one which measured H hf at 117 In in magnetically ordered materials, and demonstrates the applicability of TDPAC of this nuclide in the field of magnetism.

Ambient Temperature Intercalation of Na and In into TaS2

We report on ambient temperature intercalation of Na and In into TaS 2 (2H and 1T) unng liquid metal eutectics and amalgams (In 0.17 Ga 0.83 , In 0.5 Hg 0.5 , Sn 0.15 Ga 0.85 , and Na x Hg). The intercalation process was monitored in situ by measuring the 181 Ta nuclear-quadrupole-interaction (NQI) via time differential perturbed angular correlation (TDPAC). At ambient temperature, a single metal species is intercalated selectively. The final products were found to be comparable to those obtained by high temperature synthesis. In the case of 1T-TaS 2 , a soft chemistry induced Ta-coordination change from octohedral to trigonal-prismatic was observed

Cu-Hf multilayers studied by slow positron beam and time differential perturbed angular correlation

Time differential perturbed angular correlation (TDPAC) and monoenergetic positron beam measurements of the Cu-Hf multilayers have been performed. The structural disorder and the remarkable strain in Hf layers of the multilayer are discussed from the microscopic point of view.