Time Differential Perturbed Angular Correlation Research Articles

A first-principle study of the site preference of Cd impurity in Hf–Ag and Zr–Ag compounds

A detailed theoretical study of the structure and electric field gradients (EFG) of the Hf2Ag, HfAg, Zr2Ag and ZrAg compounds is presented. Using all-electron augmented plane waves plus local orbitals (APW+lo) formalism, the equilibrium volumes and EFGs for the mentioned compounds are calculated. We have also performed supercell calculations with Cd and Ta impurities. Through the comparison of theoretical and experimental EFGs in these cases, we elucidate the role played by the Cd probe in the time-differential perturbed angular correlation (TDPAC) measurements of Hf–Ag and Zr–Ag compounds.

Nuclear quadrupole interaction atS44cin the anatase and rutile modifications ofTiO2: Time-differential perturbed-angular-correlation measurements andab initiocalculations

The nuclear quadrupole interaction of the first excited $I=1$ state of $^{44}\text{S}\text{c}$ in the anatase and rutile modifications of bulk ${\text{TiO}}_{2}$ was determined by time-differential perturbed-angular correlation of $\ensuremath{\gamma}$ rays. New ${\text{LaBr}}_{3}(\text{Ce})$ scintillators with excellent energy resolution allowed us to separate the 68 and 78 keV lines of the $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ cascade. For anatase, an almost axially symmetric electric field gradient (EFG) with an asymmetry parameter of $\ensuremath{\eta}=0.10(1)$ and a nuclear quadrupole frequency of ${\ensuremath{\omega}}_{Q}=9.29(3)\text{ }\text{Mrad}/\text{s}$ was obtained. For rutile, an almost antiaxial EFG with $\ensuremath{\eta}=0.94(1)$ and ${\ensuremath{\omega}}_{Q}=16.14(6)\text{ }\text{Mrad}/\text{s}$ was obtained, which contradicts literature values. Ab initio calculations of the EFG using the WIEN2K code were performed for anatase and rutile for pure systems as well as for relevant impurities, such as Sc and Cd on Ti sites. The nuclear quadrupole moment of the $I=1$ state in $^{44}\text{S}\text{c}$ is redetermined with a sevenfold improved accuracy to be $Q=\ifmmode\pm\else\textpm\fi{}0.214(3)\text{ }\text{b}$.

Hyperfine magnetic field at Ta impurities in nickel: Perturbed angular correlation and first principle calculation study

The hyperfine magnetic field ( H h f ) in 0.2 at.% Hf–Ni alloy is measured at the 181Ta probe using the time-differential perturbed angular correlation (TDPAC) method, in the temperature range 78–675 K. The obtained value of 8.6 (3) T at room temperature is in good agreement with the previously reported measurements for similar Hf concentrations in Ni. X-ray powder diffraction (XRPD) experiments confirmed that small atomic concentrations of Hf atoms (<1 at.%) mainly substitute on Ni lattice sites in the fcc crystal lattice without forming any intermetallic phase. In addition, ab-initio calculation using all-electron augmented plane waves plus local orbitals (APW+lo) formalism is performed and the obtained result for the hyperfine magnetic field at Ta site is in accordance with the measurement.

Online time-differential perturbed angular correlation study with an 19O beam – Residence sites of oxygen atoms in highly oriented pyrolytic graphite

The online time-differential perturbed angular correlation (TDPAC) method was applied to a study of the physical states of a probe 19F, the β− decay product of 19O (t1/2=26.9s), implanted in highly oriented pyrolytic graphite. The observed magnitude of the electric field gradient at the probe nucleus, ∣Vzz∣=2.91(17)×1022Vm−2, suggests that the incident 19O atoms are stabilized at an interlayer position with point group C3v. Exhibiting observed TDPAC spectra having a clear sample-to-detector configuration dependence, we demonstrate the applicability of the present online method with a short-lived radioactive 19O beam.

Time-differential perturbed angular correlation study of the electric field gradient in Ti2Rh MoSi2-type compound

The electric field gradient (EFG) at the 181Hf→181Ta site in Ti2Rh C11 b -type compound was measured as a function of temperature using time-differential perturbed angular correlation (TDPAC) technique. The room temperature results show one EFG with the parameters of: ν Q =336(1) MHz→V zz =5.9×1017 V cm−2, η=0.1. Very week linear temperature dependence of this EFG was measured with the slope of 3.6 (2)×10−5 K−1. The results are compared with those for other isostructural compounds.

Time-dependent quadrupole interactions for 140Ce ions implanted in highly oriented pyrolytic graphite

The time-differential perturbed angular correlation (TDPAC) method was applied to a study on the behavior of 140Ce atoms implanted in highly oriented pyrolytic graphite (HOPG). Exponential-type gradual attenuation of the directional anisotropy of the relevant γ-ray cascade was observed in the perturbation patterns. From temperature dependence of the relaxation rate, a thermally activated dynamic motion of the probe atoms was suggested.

Magnetic Nanostructures: Radioactive Probes and Recent Developments

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Hyperfine interaction measurements on ceramics: PZT revisited

The solid solution of PbZr 1− x Ti x O 3, known as lead-zirconate titanate (PZT), was probably one of the most studied ferroelectric materials, especially due to its excellent dielectric, ferroelectric and piezoelectric properties. The highest piezoelectric coefficients of the PZT are found near the morphotropic phase boundary (MPB) (0.46⩽ x⩽0.49), between the tetragonal and rhombohedral regions of the composition-temperature phase diagram. Recently, a new monoclinic phase near the MPB was observed, which can be considered as a “bridge” between PZT's tetragonal and rhombohedral phases. This work is concerned with the study of the structural properties of the ferroelectric PZT (Zr/Ti=52/48, 53/47) by hyperfine interaction (HI) measurements obtained from experiments performed by using the nuclear spectroscopy time differential perturbed angular correlation (TDPAC) in a wide temperature range.

The nuclear quadrupole interaction at inequivalent lattice sites in ammonium paramolybdate: A TDPAC study

A nuclear quadrupole interaction (NQI) study using the time differential perturbed angular correlation (TDPAC) technique on ammonium paramolybdate (APM) has shown three inequivalent molybdenum sites in this compound which consists of seven MoO 6 polyhedra connected through edges. In this study the nuclear probe 99Mo was used to measure the γ– γ perturbed angular correlation of 99Tc on Mo-sites to obtain the quadrupole interaction parameters. The quadrupole interaction frequencies ( ω Q ) for the three sites are 0.0224, 0.0386 and 0.0434 rad/ns and the asymmetry parameters ( η) of the electric field gradient (EFG) are 0.45, 0.18, and 0.58, respectively. The site assignment is based on the population ratios 4:2:1. The Mo atoms with the highest population show the lowest ω Q indicating that this set of polyhedra is “least” distorted or condensed. Besides the least squares fit, a cross-correlation algorithm has been used to analyze the experimental data to corroborate the fitted parameters and quoted errors. The derived NQI-parameters can be used for site assignments in other compounds built from condensed Mo–O octahedra.

&lt;SUP&gt;111&lt;/SUP&gt;Cd(&amp;larr;&lt;SUP&gt;111&lt;/SUP&gt;In) Time Differential Perturbed Angular Correlation (TDPAC) Spectroscopy in Fe/Ag Films

We have investigated the magnetic properties of Cd atoms decayed from 111 In at the interfaces of Fe/Ag evaporated films. In order to determine the magnetic properties of the films, we have measured 111 Cd( 111 In) time differential perturbed angular correlation (TDPAC) spectra at room temperature as a function of distance from the interface. Fe and Ag were evaporated using electron beam evaporation, and small amounts of radioactive 111 In were evaporated by resistance-heating. The evaporated films had the following stacking orders on kapton polyimide film substrate: (1) polyimide/Fe 50 nm/ 111 In/Fe x nm/Ag 20 nm (x¼ 0:2; 0:3; 1; 3; 5; 10), (2) polyimide/Fe 50 nm/Ag x nm/ 111 In/Ag 20 nm (x¼ 0; 0:2; 1). We used an Ag layer as a cap layer in order to prevent oxidation in air. TDPAC spectra were measured using a four-detector arrangement. The hyperfine magnetic field tends to be slightly larger when 111 In is close to the interface of the Fe/Ag layer. The intensity of perturbation damps rapidly as 111 In is close to the interface of Fe and Ag. These behaviors are thought to depend on electric field gradients and the influence of the surface roughness.

Temperature dependence of electric field gradient inLaCoO3 perovskite investigated by perturbed angular correlation spectroscopy

The time differential perturbed angular correlation (TDPAC) technique wasused to study the temperature dependence of electric field gradient (EFG) inLaCoO3 perovskite using and nuclear probes. The radioactive parent nuclei111In and 181Hf were introduced into the oxide lattice through a chemical process duringsample preparation and were found to occupy only the Co sites inLaCoO3. The PACmeasurements with 111Cd and 181Ta probes were made in the temperature range of 4.2–1146 K and 4.2–1004 K,respectively. No long-range magnetic order was observed up to 4.2 K. The EFGs at111Cd and 181Ta show very similar temperature dependences. They increase slowly between 4.2 and about77 K and then decrease almost linearly with increasing temperature until about 500–600 K,where a broad peak-like structure is observed, followed by linear decrease at stillhigher temperatures. These discontinuities at about 77 K and 500–600 K havebeen interpreted as thermally activated spin state transitions from the low-spin(t2g6eg0) ground state configuration to the intermediate-spin(t2g5eg1) state and from the intermediate-spin to the high-spin(t2g4eg2) stateof the Co3+ ion, confirming previous observation in other recent studies. An indication of a Jahn–Tellerdistortion, which stabilizes the intermediate-spin state with orbital ordering, is also pointedout.

TDPAC Study of the Hydrogen Uptake Process in HfNi

The electronic properties of the HfNi-hydrogen system has been investigated, as a function of the hydrogen composition ratio (x), using combined Time Differential Perturbed Angular Correlation (TDPAC) technique and standard full-potential Linearized-Augmented-Plane-Wave method. The experimental TDPAC data confirm the presence of a two-step hydrogenation process in this system, with the octahedral holes filled first. The major part of the electric field gradient at the hafnium site is from p-p contribution, shifted down in energy by hydrogen s-states contribution.

Mössbauer effect and first principle calculations of the electronic structure and hyperfine interaction parameters of Hf 2Fe

A detailed theoretical study of the structure, electronic properties and the electric field gradients of the Hf 2Fe intermetallic compound is presented. Using all-electron full-potential linearized augmented plane wave (FP-LAPW) formalism the equilibrium volume, bulk modulus and electric field gradients are calculated. The obtained results are compared with EFG values inferred from measurements performed using Mössbauer spectroscopy and the earlier reported time differential perturbed angular correlation (TDPAC) measurements. The lattice relaxation and the supercell calculations are found to be essential for the correct interpretation of the experimental results.

PAC Studies on Zr-Based Intermetallic Compounds

The Zr2Al, Zr3Al2 and Zr6NiAl2 intermetallic compounds were characterized by means of time differential perturbed angular correlation (TDPAC) and X-ray diffraction. Our interest in these Zr(Hf) aluminides comes from crystallization studies of Zr(Hf)-based bulk metallic glasses which have a wide supercooled liquid region.

The low-temperature magnetism of cerium atoms inCeMn2Si2 and CeMn2Ge2 compounds

The low-temperature magnetic properties of the Ce atoms in the intermetallic compoundsCeMn2Ge2 and CeMn2Si2 were studied. Previous neutron scattering measurements did not detectan ordered moment at Ce atoms in either compound despite the fact thatthey are surrounded by the Mn moments ordered ferromagnetically in theCeMn2Ge2 and antiferromagneticallyin the CeMn2Si2. Contrasting with this result, a recent measurement performed with the timedifferential perturbed angular correlation (TDPAC) technique showed thepresence of a pronounced magnetic hyperfine field (MHF) at Ce sites in theCeMn2Ge2 compound andno MHF in CeMn2Si2. The absence of the Ce magnetic moment and MHF in the silicide can be understood interms of too weak a Ce–Ce magnetic interaction while in the germanide the TDPACresult suggests that some magnetic ordering of Ce atoms may occur. Aiming tounderstand the effects which result in the quenching of the Ce 4f moment inboth cases, we performed first-principles band-structure calculations for bothsystems, using the full potential linear augmented plane wave method. It is shownthat the magnetism of the Ce sublattice has fundamentally different nature inCeMn2Si2 and CeMn2Ge2. While the Ce atoms are intrinsically non-magnetic in the silicide, having a zero magnetic momentwith both spin and orbital contributions identically zero, they display magnetic properties in theCeMn2Ge2 since their very small total moment is composed of finite spin and orbital componentswhich almost cancel each other accidentally.

Interaction between HfC precipitates and vacancies in quenched Cu:Hf as studied byTDPAC and positron lifetime measurements

A Cu:Hf sample with 1 wt% Hf as prepared by arc melting is characterized by TEM andmicrodiffraction analysis to contain HfC precipitates. HfC precipitates in a Cu matrix bindvacancies and divacancies strongly in the quenched Cu:Hf sample as deducedby time differential perturbed angular correlation (TDPAC) studies. Isochronalannealing studies using TDPAC and positron lifetime measurements indicatethe stability of these vacancy complexes in the quenched sample for annealingtreatments up to 1200 K, beyond which the de-trapping of the vacancies fromHfC precipitates is observed to occur. This shows that HfC precipitates presentin Cu inhibit the formation of voids by strongly binding quenched vacancies.

Magnetic interaction in NdScGe: a local investigation by perturbed angular correlationspectroscopy

The magnetic and electric hyperfine interactions for the111Cd probe nucleus in the equi-atomic ferromagnetic compound NdScGe ( K) have been investigated by the time differential perturbed angular correlation (TDPAC) technique.The Cd probe occupying the Sc site experiences a large magnetic hyperfine field with saturation valueBhf(0) = −8.5 T. By comparing the results with the hyperfine field data in Nd metal and estimates madewith the RKKY interaction, we find an indication for sizeable spin polarization of theconduction electrons in NdScGe. In addition, we find evidence of lattice softening near theCurie temperature reflected by an abrupt decrease in the quadrupole interaction frequencyνQ(T).

&lt;SUP&gt;181&lt;/SUP&gt;Ta (&amp;larr; &lt;SUP&gt;181&lt;/SUP&gt;Hf) Time-Differential Perturbed Angular Correlation Spectroscopy of Hf/Fe Multilayers

Hf/Fe multilayers are soft magnetic materials and are a good candidate for magnetic heads of storage devices. We have investigated Hf/Fe multilayers using time-differential perturbed angular correlation (TDPAC) spectroscopy of 1 8 1 Ta (← 1 8 1 Hf), examining specifically the magnetic nature of the non-magnetic Hf layers sandwiched between two ferromagnetic Fe layers. We prepared multilayer samples with Hf layers of various thicknesses, [Hf(x nm)/Fe] n (x = 0.5, 1.0, 2.0, 4.8, 10.0). Three different hyperfine magnetic fields 53(1) T, 43(1) T and 2(1) T were detected in the samples of [Hf (2 nm, 4.8 nm, 10 nm)/Fe] n . The hyperfine magnetic field of 53(1) T corresponds to Hf atoms in a turbulent interface between the Hf and Fe layers. The value of 43(1) T corresponds to Hf atoms inside Hf layers within 0.5 nm from the interface. The value of 2(1) T corresponds to Hf atoms inside the Hf layers within 3.5 nm of the interface.

Study of hafnium complexes by time differential perturbed angular correlation technique

Abstract Complexation of hafnium with two ligands viz. diethylene triamine pentaacetic acid (DTPA) and its dimethyl ester derivative (DMDTPA) has been studied by using time differential perturbed angular correlation (TDPAC) technique. Nuclear quadrupole coupling constant (ν Q), asymmetry of the EFG (η) and the frequency distribution (δ) for these complexes have been obtained. Square antiprism structures have been suggested for these complexes using these parameters. A systematic trend of the ν Q has been observed in these and other hafnium complexes with similar ligands and has been explained in terms of the electronegativity of the donor atoms in the ligands.

Hyperfine interactions studies on Pt–In/Nb 2O 5 catalysts

Pt–In/Nb 2O 5 catalysts were investigated using temperature-programmed reduction (TPR) and time differential perturbed angular correlation (TDPAC) experiments. The results indicated the presence of In–O surface complexes for low loading In/Nb 2O 5 and Pt–In/Nb 2O 5 catalysts after calcination. These complexes did not form a In 2O 3 crystalline phase. After reduction of the Pt–In/Al 2O 3 catalyst, In is present in different states. A fraction of In atoms is bonded to niobia surface, as a surface complex that does not show crystalline structure similar to bulk In 2O 3. Other fraction of In atoms interacts with platinum, in the form of an alloy, in locations that present trigonal symmetry.