Time Differential Perturbed Angular Correlation Research Articles

Comprehensive study on the origin of orthorhombic phase stabilization in Gd-doped HfO2 and DFT calculations.

In recent times, ultra-thin films of hafnium oxide (HfO2) have shown ferroelectricity (FE) attributed to the orthorhombic (o) phase of HfO2 with space group Pca21. This polar o-phase could be stabilized in the doped thin film of the oxide. In the present work, both polar and non-polar o-phases of HfO2 could be stabilized in Gd-doped bulk polycrystalline HfO2. Rietveld analysis of XRD data shows that the relative population of o-phases in the presence of the monoclinic (m) phase of HfO2 increases with increasing Gd-content. The local environment around the host atom has been investigated by time differential perturbed angular correlation (TDPAC) spectroscopy, synchrotron based X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) measurements showed a reduction in grain size with increasing Gd-dopant indicating a solute drag effect. It could be established that the segregation of the Gd-dopant in the grain boundary is a thermodynamically favorable process and the solute drag effect plays an important role in nucleation of the o-phase in bulk HfO2. Stabilization of Gd in both Pbca and Pca21 phases of HfO2 was supported by defect formation energy calculations using density functional theory (DFT). The present study has important implications in future applications of HfO2 in ferroelectric devices and in understanding the role of dopants in stabilizing the o-phase of HfO2 in the bulk.

Temperature dependence of the local electromagnetic field at the Fe site in multiferroic bismuth ferrite

In this paper, we present a study of the temperature-dependent characteristics of electromagnetic fields at the atomic scale in multiferroic bismuth ferrite (${\mathrm{BiFeO}}_{3}$ or BFO). The study was performed using time differential perturbed angular correlation (TDPAC) spectroscopy on implanted $^{111}\mathrm{In}$ ($^{111}\mathrm{Cd}$) probes over a wide temperature range. The TDPAC spectra show that substitutional $^{111}\mathrm{In}$ on the ${\mathrm{Fe}}^{3+}$ site experiences local electric polarization, which is otherwise expected to essentially stem from the ${\mathrm{Bi}}^{3+}$ lone pair electrons. Moreover, the TDPAC spectra show combined electric and magnetic interactions below the N\'eel temperature ${T}_{N}$. This is consistent with simulated spectra. X-ray diffraction (XRD) was employed to investigate how high-temperature TDPAC measurements influence the macroscopic structure and secondary phases. With the support of ab initio DFT simulations, we can discuss the probe nucleus site assignment and can conclude that the $^{111}\mathrm{In}$ ($^{111}\mathrm{Cd}$) probe substitutes the Fe atom at the B site of the perovskite structure.

Room-Temperature 181Ta(TiO2): An e-γ TDPAC Study

In this work, we report on the hyperfine parameters of the foreign 181Ta probe in the rutile structure of the single crystal TiO2 using the e−γ and γ−γ time differential perturbed angular correlation (TDPAC) technique. We implanted 181Hf ions into a sample of single crystal rutile TiO2 in the Bonn Isotope Separator. The implanted sample was then thermally annealed at a temperature of 873 K for 315 min in a vacuum. The 181Hf radioisotopes decayed by β− emission, followed by a cascade to the ground of γ rays or conversion electrons into a stable state 181Ta. The 181Ta probe substitutes the Ti lattice site with a unique nuclear quadrupole interaction, allowing for the precise measurement of the largest electric field gradient (Vzz) and asymmetry parameter (η). The hyperfine parameters obtained from the e−γ TDPAC spectroscopy agree with those of the γ−γ TDPAC spectroscopy at room temperature, apart from a calibration factor, both from our experiments and the literature. This suggests that the electronic recombination following the internal conversion of the L shell electron takes less time (ps) than the intermediate lifetime of the metastable 181Ta state (ns).

The Local Exploration of Magnetic Field Effects in Semiconductors

This study reports on the local exploration of magnetic field effects in semiconductors, including silicon (Si), germanium (Ge), gallium arsenide (GaAs), and indium phosphide (InP) using the time differential perturbed angular correlation (TDPAC) technique. TDPAC measurements were carried out under external magnetic fields with strengths of 0.48 T and 2.1 T at room temperature, and 77 K following the implantation of 111In (111Cd) probes. Defects caused by ion implantation could be easily removed by thermal annealing at an appropriate temperature. The agreement between the measured Larmor frequencies and the theoretical values confirms that almost no intrinsic point defects are present in the semiconductors. At low temperatures, an electric interaction sets in. It stems from the electron capture after-effect. In the case of germanium and silicon, this effect is well visible. It is associated with a double charge state of the defect ion. No such effects arise in GaAs and InP where Cd contributes only a single electronic defect state. The Larmor frequencies correspond to the external magnetic field also at low temperatures.

Probing electronic environment in gamma irradiated sodium borosilicate glass and simulated waste glass: a perturbed angular correlation spectroscopy study

Time Differential Perturbed Angular Correlation (TDPAC) has been used to investigate the local structural changes (if any) in Sodium borosilicate (NBS) Glass and Simulated Waste Oxide loaded sodium borosilicate glass (SNBS) used for immobilization of high-level liquid waste at Trombay, Mumbai. The glasses were doped with 181Hf tracer as TDPAC probe and irradiated with varying dose (1–50 MGy) of gamma radiation using 60Co (average γ energy 1.25 MeV) source. Hyperfine interaction parameters (quadrupole interaction frequency, its distribution and asymmetry of electric field gradient) were obtained for irradiated NBS and SNBS glasses from fitting of TDPAC data using DPAC software. The data was compared with the corresponding unirradiated glasses. Results do not show any micro-crystallinity upon irradiation in both the NBS and SNBS glass even at the highest gamma dose of 50 MGy even though small changes were observed in quadrupole interaction frequency at doses beyond 30 MGy. This suggests that the sodium borosilicate glass used for immobilization of high-level waste at Trombay is stable towards gamma radiation at least up to a gamma dose of 50 MGy.

DFT-based calculations of the magnetic hyperfine interactions at Cd sites in RCd (R = rare earth) compounds with the FP-LAPW ELK code

In the work here reported, we have calculated magnetic hyperfine interactions in rare-earth (R) intermetallic compounds by using the free open-source all-electron ELK code. The RCd (R = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) series was chosen as a test system because an almost complete set of experimental data on the hyperfine parameters at Cd sites was acquired through the time differential perturbed angular correlation (TDPAC) spectroscopy as previously reported. Moreover, results on magnetic hyperfine field (Bhf) from WIEN2k code were also reported allowing a qualitative comparison analysis. We emphasize that the utilized version of ELK accounted for the contact field only. Yet, as it is the only contribution expected for Cd site in RCd compounds, the calculated Bhf values are in reasonable agreement with the experimental results. The Spin-orbit coupling when taken into account led to a decrease in deviation from experimental data. Addition, the Hubbard-like term was revealed crucial in order to make Bhf predictions for CeCd, suggesting that this behavior may be associated with a weaker 4f electron localization in Ce.

Electron beam irradiation induced changes in local structure of simulated waste bearing sodium borosilicate glasses

Time differential perturbed angular correlation (TDPAC) and Extended X-ray absorption fine structure (EXAFS) studies have been carried out on electron beam (EB) irradiated sodium borosilicate (NBS) and simulated waste oxide loaded sodium borosilicate (SNBS) glasses with a view to investigate the effect of self β-irradiation in vitrified high level waste. NBS and SNBS glasses were synthesized and doped with 1.5% HfO2. For TDPAC studies the glasses were doped with 181Hf probe, while for EXAFS studies measurements were carried out at and around Hf LIII edge. TDPAC and EXAFS acquisition were performed before and after electron beam irradiation doses up to 10 MGy for both NBS and SNBS glasses. Fourier transforms of the TDPAC spectra show broad frequency distribution reminiscent of the disordered glass matrix, with increase in hyperfine interaction frequency in irradiated glasses with respect to unirradiated ones as well as with increasing EB dose. On the other hand, the width of the frequency distribution was found to decrease with EB dose. No change in asymmetry of the electric field gradient was observed in irradiated glasses with respect to unirradiated ones. These observations indicate that while there is no indication of appearance of micro-crystallinity in the glasses up to EB dose of 10 MGy, local structural changes around Hf do take place with EB irradiation in both the glasses. No change in metric parameters (Hf-O bond distance and coordination number of Hf) was observed from EXAFS measurements on both NBS and SNBS glasses up to EB dose of 10 MGy.

Measurement of electric quadrupole moment in neutron rich $$^{131,132}$$I

The quadrupole moments of the excited levels in neutron rich iodine isotopes, viz., $$^{131}$$ I (5/2 $$_1^+$$ ) and $$^{132}$$ I (3 $$_1^+$$ ), have been measured with LaBr $$_3$$ (Ce) detectors using Time Differential Perturbed Angular Correlation (TDPAC) spectroscopy. The excited levels were populated from $$\beta ^-$$ decay of the radiochemically separated tellurium (Te) fission products produced in $$^{nat}$$ U( $$^4$$ He,f) reaction at E $$_{\alpha }$$ (lab) = 40 MeV from K-130 cyclotron at VECC, Kolkata. The active tellurium fission products were radiochemically doped in metallic tellurium matrix to provide the necessary Electric Field Gradient (EFG) required for TDPAC measurement. The values of quadrupole moments for the 5/2 $$_1^+$$ level of $$^{131}$$ I and 3 $$_1^+$$ level of $$^{132}$$ I were determined to be $$(-)$$ 0.30(1) eb and (−)0.25(2) eb, respectively. The present measurement provides the first experimental determination on the electric quadrupole moment of 5/2 $$_1^+$$ level of $$^{131}$$ I.

Hyperfine field studies of the high-pressure phase of noncentrosymmetric superconductor RhGe (B20) doped with hafnium

We have used the time differential perturbed angular correlation (TDPAC) spectroscopy to measure the electric and magnetic hyperfine fields in RhGe crystallized in the B20 cubic lattice structure and weakly doped with Hf (0.5–2 atomic %) in the temperature range from 5 K to 295 K. Two most commonly used nuclei probes, 111In→111Cd and 181Hf→181Ta, have been used. The experimental results combined with theoretical density functional calculations indicate that the In/Cd impurities substitute into the Ge-site whereas the Ta/Hf probes substitute into the Rh-site. It has also been found that the Ta/Hf impurity strongly distorts the local crystal environment, whereas the effect from the In/Cd probe is weak. There are no reliable evidences of the magnetic order in the studied alloys at low temperatures.

TDPAC Studies of Local Defects and Phenomena in Ferroics and Multiferroics

We provide an overview of time-differential perturbed angular correlation (TDPAC) measurements of ferroic and multiferroic materials. Here, we explore chalcogenide spinels, lead titanate, lead zirconate, and bismuth ferrite, describing the use of TDPAC experiments to probe the physics of localized defects and the various mechanisms that govern electronic and magnetic interactions, the coupling of the associated degrees of freedom, and the structural, charge, and orbital correlations for these materials.

Dynamic motion and freezing of polaronic local structures in a colossal-magnetoresistive perovskite manganite La0.7Ca0.3MnO3 detected with radioactive nuclei

The magnetic hyperfine field and electric field gradients at a radioactive impurity probe of $^{111}\mathrm{Cd}(\ensuremath{\leftarrow}^{111m}\mathrm{Cd})$ occupying the La/Ca $A$ site in a perovskite manganaite ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ (${T}_{C}\ensuremath{\sim}250\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) were measured by means of time-differential perturbed angular correlation (TDPAC) spectroscopy. In the paramagnetic-insulator phase at room temperature, the $^{111}\mathrm{Cd}(\ensuremath{\leftarrow}^{111m}\mathrm{Cd})$ probes are distributed in two different environments: distorted and less distorted sites; whereas in the ferromagnetic-metal phase below ${T}_{C}$, the oscillatory structure of the distorted component vanishes from the TDPAC function. The vanishing of the oscillation is ascribable to dynamic motion of polarons dragged by conduction electrons induced by the double exchange interaction. In liquid helium, the dynamic motion freezes, leaving averaged local distortion. The dynamic motion and freezing of local structures associated with the colossal-magnetoresistance phase transition to the ferromagnetic phase is discussed on the basis of temperature-dependent electromagnetic fields at the probe nuclei of the $A$ site ions.

Mechanical alloying of Si and Fe: Quantum-mechanical calculations applied to Mössbauer and X-ray diffraction studies

Hyperfine parameters of iron nuclei such as isomer shift, quadrupole splitting, and asymmetry parameter are calculated for β-FeSi2 with and without vacancies, using density functional theory. They are applied, in combination with parameters of α-FeSi2 obtained earlier, for analyzing Mössbauer and X-ray diffraction experiments on the mechanical alloying of silicon with iron in the range of 1–33 at. % of Fe. Such an approach allows more detailed and precise information to be obtained on the structure of Fe–Si samples. In particular, the fraction of vacancies in α- and β-FeSi2 is estimated, and the mechanism of formation of these phases and their transformation into each other is discussed. A new model is developed for analyzing experiments on hyperfine interaction, such as Mössbauer spectroscopy, time-differential perturbed-angular correlation, and the like, for Fe–Si systems with a high Si content.

A hyperfine look at titanium dioxide

Titanium dioxide is a commonly used material in a wide range of applications, due to its low price, and the increasing demand for it in the food- and pharmaceutical industries, and for low- and high-tech applications. Time-differential perturbed angular correlation (TDPAC) and Mössbauer spectroscopy measurements have a local character and can provide important and new information on the hyperfine interactions in titanium dioxide. With the application of characterization techniques and radioactive beams, these methods have become very powerful, especially for the determination of temperature dependence of hyperfine parameters, even at elevated temperatures. Such measurements lead to a better understanding of lattice defects and irregularities, including local environments with low fractions of particular defect configurations that affect electric quadrupole interactions. At ISOLDE-CERN, physicists benefit from the many beams available for the investigation of new doping configurations in titanium dioxide. We report the annealing study of titanium dioxide by means of the time differential perturbed γ-γ angular correlation of 111mCd/111Cd in order to study the possible effects of vacancies in hyperfine parameters. This paper also provides an overview of TDPAC measurements and gives future perspectives.

Using radioactive beams to unravel local phenomena in ferroic and multiferroic materials

The increasing interest in using ferroic and multiferroic materials in high-tech applications requires that the underlying physical phenomena are studied at the atomic scale. Time-differential perturbed angular correlation (TDPAC) measurements have a local character and can provide important information concerning combined magnetic dipole and electric quadrupole interactions in multiferroic systems. With the application of characterization techniques and radioactive beams, this method has become very powerful, especially for the determination of the temperature dependence of hyperfine parameters, even at high temperatures. Such measurements lead to a better understanding of phase transitions, including observations of local environments in low fractions of different phases. Several facilities are in use at ISOLDE-CERN that benefit from the multitude of available ion beams that exist for the use and development of the TDPAC technique. An overview of the prior literature, combined with a discussion of measurement conditions and isotopes, is presented. Particular emphasis is given to the important case of measurements carried out at ISOLDE-CERN employing 111mCd as a probe.

Unraveling doping induced anatase-rutile phase transition in TiO2 using electron, X-ray and gamma-ray as spectroscopic probes.

The present work reports the microscopic details of anatase (A) to rutile (R) phase transformation in a Mn-doped TiO2 system. Titanium dioxide (TiO2) powder was synthesized at three different dopant percentages, namely 1, 5, and 10 atom% of Mn, by a coprecipitation technique. Time differential perturbed angular correlation (TDPAC) spectroscopy was used to identify the formation of the rutile-like phase (R*) during the phase-transition process and revealed interface nucleation to be promoted by the Mn dopant. Electron paramagnetic resonance (EPR) spectroscopy, synchrotron-based X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) studies showed that Mn exhibited a mixed valence states of 2+ and 4+ at different stages of the annealing process. The rutile onset temperature gradually decreased with the increase in the Mn content. The present report proposes the mechanism for the phase transformation and details the effect of Mn on the A to R phase-transformation process. This can assist in gaining a fundamental understanding of the A to R phase-transformation process and the role of the dopant in stabilizing one phase over the other.

TDPAC study of Fe-implanted titanium dioxide thin films

Fe-doping in TiO2 has been proven to improve several of its properties, including the photocatalytic activity. Time-differential perturbed angular correlation (TDPAC) as the applied spectroscopy method is particularly interesting because it can probe the electric and magnetic interactions on a local atomic scale. In this work the hyperfine interactions on 111Cd atoms substituting Ti atoms in TiO2 due to nearby Fe atoms also diluted within the TiO2 lattice were measured as a function of temperature. The results review two fractions with distinct quadrupole interaction parameters. One site, occupied by the 111Cd probes, presents the smaller quadrupole interaction frequency, namely υq1 = 45 MHz, and can be ascribed to sites that are more distant from the Fe substitutional site whereas the second site characterized with υq2 = 62 MHz is related to Cd probe atoms that are closer to the Fe defect. Additionally, the system has been characterized using electron dispersive spectroscopy.

Hyperfine interaction study of pressure induced phase transformations in Hafnia

Time Differential Perturbed Angular correlation (TDPAC) study of pressure dependent phase transition in HfO2 has been performed for the first time to reveal the change in local structure around Hf atom. The results show that the nuclear hyperfine interaction parameters namely, quadrupole interaction frequency and asymmetry parameter at atmospheric pressure are in agreement with literature while that at 45 GPa are significantly different from that at atmospheric pressure.

Identification of phase components in Zr-Ni and Hf-Ni intermetallic compounds; investigations by perturbed angular correlation spectroscopy and first principles calculations

Time-differential perturbed angular correlation (TDPAC) measurements have been carried out in stoichiometric ZrNi3 and HfNi3 intermetallic compounds using 181Ta probe in the temperature range 77–1073 K considering the immense technological applications of Zr-Ni and Hf-Ni intermetallic compounds. In ZrNi3, four components due to the production of Zr2Ni7, Zr8Ni21, Zr7Ni10 and ZrNi3 have been found at room temperature. The HfNi3 sample produces five electric quadrupole interaction frequencies at room temperature. The phase HfNi3 is strongly produced in stoichiometric sample of HfNi3 where two non-equivalent Hf sites are found to be present. Besides this phase, two other phases due to Hf2Ni7 and Hf8Ni21 have been found but, we do not observe any phase due to Hf7Ni10. X-ray diffraction, TEM/energy dispersive X-ray spectroscopy (EDX) and TEM-selected area electron diffraction (SAED) measurements were used to further characterize the investigated materials and it was found that these results agree with the TDPAC results. In order to confirm findings from TDPAC measurements, density functional theory (DFT) based calculations of electric field gradients (EFG) and asymmetry parameters at the sites of 181 Ta probe nucleus were performed. Our calculated results are found to be in excellent agreement with the experimental results.

Simultaneous measurement of electric field gradient both at 111Cd and 181Ta sites in a single perturbed γ–γ angular correlation measurement

Present work demonstrates, for the first time, the use of multiple probes in a single time differential perturbed angular correlation (TDPAC) measurement. Two different probes, viz., 111In/111Cd and 181Hf/181Ta, simultaneously have been incorporated in both pure rutile and Hafnium (Hf)-doped rutile Titanium Dioxide (TiO2) systems following a soft chemical method. The rutile structure was identified by the hyperfine parameters, viz., quadrupole frequency (ω Q) and asymmetry (η). Due to the doping of Hf at 5 and 10 at.%, the sample partially lost the purity of the crystal and the fact has been reflected at both the probe sites.

Implantation of cobalt in SnO2 thin films studied by TDPAC

Here we report time differential perturbed angular correlation (TDPAC) results of Co-doped SnO2 thin films. Making use of stable Co and radioactive 111In implanted at the Bonn Radioisotope Separator with energies of 80 keV and 160 keV, respectively, it was possible to study the dopant incorporation and its lattice location during annealing. The hyperfine parameters have been probed as a function of temperature in vacuum. Two quadrupole interactions were observed. At high temperatures the dominant fraction for the probe nuclei can be assigned to the Cd-incorporation at the cation substitutional site in a highly disordered structure, obtained after implantation, to high crystallinity for the measurements at 873 K and 923 K. The similarity in TDPAC spectra obtained in undoped SnO gives indirect evidence that In and Co diffuse to different depths during the annealing process. Other interpretations will be discussed.