Perturbed Angular Correlation Research Articles

C-terminal Cysteines of CueR Act as Auxiliary Metal Site Ligands upon HgII Binding-A Mechanism To Prevent Transcriptional Activation by Divalent Metal Ions?

Intracellular CuI is controlled by the transcriptional regulator CueR, which effectively discriminates between monovalent and divalent metal ions. It is intriguing that HgII does not activate transcription, as bis‐thiolate metal sites exhibit high affinity for HgII. Here the binding of HgII to CueR and a truncated variant, ΔC7‐CueR, without the last 7 amino acids at the C‐terminus including a conserved CCHH motif is explored. ESI‐MS demonstrates that up to two HgII bind to CueR, while ΔC7‐CueR accommodates only one HgII. 199mHg PAC and UV absorption spectroscopy indicate HgS2 structure at both the functional and the CCHH metal site. However, at sub‐equimolar concentrations of HgII at pH 8.0, the metal binding site displays an equilibrium between HgS2 and HgS3, involving cysteines from both sites. We hypothesize that the C‐terminal CCHH motif provides auxiliary ligands that coordinate to HgII and thereby prevents activation of transcription.

Martensitic phase transformation in TiNi

From temperature dependent perturbed angular correlation (PAC) measurements (77–873 K) in equiatomic TiNi intermetallic alloy, martensitic phase transformations have been observed. Three frequency components corresponding to three different phases of TiNi have been found in the temperature range 298–873 K. The results of quadrupole frequency and asymmetry parameters at room temperature are found to be: ωQ = 14(1) Mrad/s, η = 0 (33%), ωQ = 40.0(5) Mrad/s, η = 0.66(3) (52%) and ωQ = 56.7(3)Mrad/s, η = 0.39(2) (15%). The frequency component with η = 0 and which enhances to ~52% at 373 K can be attributed to the cubic austenite phase. The predominant component (~52%) found at room temperature has been attributed to monoclinic martensitic phase of TiNi and the third component with values of ωQ and η similar to those for the martensitic phase is attributed to the intermediate orthorhombic phase. At 77 K, no intermediate and austenite phases have been found but only the martensite phase is observed at this temperature. From XRD measurements at room temperature also, three phases of TiNi have been observed.

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.

Martensitic to austenite phase transition in ZrPd

The equiatomic alloy ZrPd has been studied by perturbed angular correlation (PAC) spectroscopy in the temperature range 298-973 K to observe the martensitic to austenite phase transformation. In ZrPd, a predominant component (∼56%) found at room temperature with values of quadrupole frequency ωQ = 39.5(5) Mrad/s, asymmetry parameter η = 0.44(2) and frequency distribution width δ = 10(2)% is tentatively assigned to the monoclinic martensitic phase of ZrPd. At 373 K, a small component (∼7%) with values of ωQ = 26(3) Mrad/s, η = 0 appears which enhances with temperature at the expense of the martensitic phase and is found to be ∼35% at 973 K. This new component, probably, arises due to structural transition from monoclinic martensitic to cubic austenite phase, a phase transformation reported to occur around 830 K in previous work.

Rotational correlation times of proteins determined by 111mCd PAC spectroscopy

Molecules in solution experience rotational diffusion. For large biomolecules such as proteins, the rotational correlation times, τc, are often on the nanosecond time scale. PAC isotopes exhibit lifetimes of the intermediate nuclear level on the time scale of nanoseconds, and the hyperfine interactions measured by PAC spectroscopy are therefore sensitive to dynamics on this time scale. With this work we have compiled experimentally determined rotational correlation times, τcexp, reported in the literature in the slow reorientation limit, and compared to calculated rotational correlations times, τccalc, using the simple Stoke-Einstein-Debye approximation. In a number of cases τcexp agrees with the calculated value within the experimental error, including proteins which are expected to be rigid. In several cases, however, τcexp is smaller than the theoretical value, presumably due to local dynamics at the PAC probe site. In support of this interpretation, τcexp < τccalc is observed for many enzymes, which are expected to display dynamics at the PAC probe site due to water coordination to the active site metal.

Local structure study of 181Hf dopants in Zr7Ni10 by perturbed angular correlation spectroscopy and first principles calculations

Intermetallic compound Zr7Ni10 has been studied by perturbed angular correlation (PAC) spectroscopy to determine the electric field gradient and site occupation of Hf dopant in the compound. A mixture of phase components were found in this sample. The component with Vzz = 6.6(1)× 1021 V/m2 and η = 0.71(1) at room temperature has been assigned to Zr7Ni10 phase. In this sample, Zr2Ni7 and Zr8Ni21 phases were also found to be present along with Zr7Ni10. No phase transition is observed in the temperature range 77-1073 K in this sample. The electric field gradient (Vzz) for Zr7Ni10 phase was found to decrease linearly with temperature. The phase components in Zr7Ni10 have been determined also from X-ray powder diffraction (XRD) measurement. Theoretical calculations of EFG have been performed in Ta-doped Zr7Ni10 using all electron full potential (linearized) augmented plane wave [FP-(L)APW] method, within the framework of the density functional theory (DFT) to compare the results with the values obtained from PAC measurements and to know the site preference of 181Ta probe in the host matrix.

Magnetic field at Ce impurities in La sites of La0.5Ba0.5MnO3 double perovskites

Due to its rich variety of electromagnetic properties, such as a colossal magnetoresistance, charge and orbital ordering, and metal-insulator transition, the magnetic behavior in La0.5Ba0.5MnO3 double perovskite compounds has been investigated by several techniques, however more experimental data, especially from atomic resolution techniques, are still necessary to understand such complex behavior. In this paper, we have used a nuclear and short-range technique, the Perturbed Angular Correlation (PAC) spectroscopy, to investigate the magnetic hyperfine interaction at the 140Ce and 111Cd probe nuclei as impurities occupying La sites. This double perovskite samples were synthesized by Sol-Gel route. The crystal structure was determined by X-ray diffraction and the analyses showed that this method produced perovskite oxides with cubic structure in Pm-3m space group and the homogeneity was determined by Transmission Electron Microscopy (TEM). The local properties investigated by PAC spectroscopy revealed a ferromagnetic transition temperature above 300 K and a pure antiferromagnetic interaction below 100 K. Moreover, it also indicates an anomalous behavior of the temperature dependence of magnetic hyperfine field at La sites measured with 140Ce probe nuclei, which can be ascribed to the contribution of 4f band of Ce to Magnetic Hyperfine Field (Bhf) at low temperatures due to the increase in its localized character.

Local structure study of Hf dopants in Zr2Pd and ZrPd2 intermetallics

The intermetallic compounds Zr2Pd and ZrPd2 have been investigated by perturbed angular correlation (PAC) spectroscopy considering anomalies in their hydrogen absorption properties. It is known that both these compounds have same crystal structure, but Zr2Pd forms an excellent hydride while ZrPd2 does not, even at high pressure. From PAC measurement at room temperature in annealed ZrPd2 sample, this phase is found as a minor component (∼27%) while the dominating phase is identified to be the ZrPd3 (∼73%). In Zr2Pd, this phase has been found to be predominant (∼86%) along with a minor phase (∼14%) due to ZrPd. In ZrPd2, unlike Zr2Pd, a weak temperature dependence of electric field gradient (EFG) has been found from present PAC measurements. Also, site occupancies for both Zr and Pd atoms by the Ta-probe atoms have been found in ZrPd2 and ZrPd3 (formed in ZrPd2). In ZrPd3, Pd site occupancy by the probe is found to be maximum while a small fraction for Zr site occupancy has been found. But, in case of Zr2Pd and ZrPd, no Pd site occupancy of the probe is observed. Calculation of electric field gradients at 181Ta impurity atom for different phases have been performed by density functional theory based on the all electron full potential linearized augmented plane wave plus local orbitals (FP-(L)APW + lo) method to assign the different components observed from PAC measurements. Measurements by XRD have also been carried out to confirm the presence of component phases in the samples.

Crystalline phases in Zr9Ni11 and Hf9Ni11 intermetallics; Investigations by perturbed angular correlation spectroscopy and ab initio calculations

Crystalline phases formed in stoichiometric Zr9Ni11 and Hf9Ni11 have been studied by perturbed angular correlation (PAC) spectroscopy, XRD and TEM/SAED measurements. In Zr9Ni11, the phases Zr9Ni11 (∼89%) and Zr8Ni21 (∼11%) have been found at room temperature from PAC measurements. At 773 K, Zr9Ni11 partially decomposes to Zr7Ni10 and at 973 K, it is completely decomposed to ZrNi and Zr7Ni10. In Hf9Ni11, a predominant phase (∼81%) due to HfNi is found at room temperature while the phase Hf9Ni11 is produced as a minor phase (∼19%). No compositional phase change at higher temperature is found in Hf9Ni11. Phase components found from XRD and TEM/SAED measurements are similar to those observed from PAC measurements. Electric field gradients in Zr9Ni11 and Hf9Ni11 have been calculated by density functional theory (DFT) using all electron full potential (linearized) augmented plane wave plus local orbitals [FP-(L)APW+lo] method in order to assign the phase components.

Anomalous behavior of the magnetic hyperfine field at 140Ce impurities at La sites in LaMnSi2

Magnetic hyperfine field has been measured in the orthorhombic intermetallic compound LaMnSi2 with perturbed angular correlation (PAC) spectroscopy using radioactive 140La(140Ce) nuclear probes. Magnetization measurements were also carried out in this compound with MPSM-SQUID magnetometer. Samples of LaMnSi2 compound were prepared by arc melting the component metals with high purity under argon atmosphere followed by annealing at 1000°C for 60 h under helium atmosphere and quenching in water. X-ray analysis confirmed the samples to be in a single phase with correct crystal structure expected for LaMnSi2 compound. The radioactive 140La (T1/2 = 40 h) nuclei were produced by direct irradiation of the sample with neutrons in the IEA-R1 nuclear research reactor at IPEN with a flux of ∼ 1013 n cm−2s−1 for about 3 - 4 min. The PAC measurements were carried out with a six BaF2 detector spectrometer at several temperatures between 10 K and 400 K. Temperature dependence of the hyperfine field, Bhf was found to be anomalous. A modified two-state model explained this anomalous behavior where the effective magnetic hyperfine field at 140Ce is believed to have two contributions, one from the unstable localized spins at Ce impurities and another from the magnetic Mn atoms of the host. The competition of these two contributions explains the anomalous behavior observed for the temperature dependence of the magnetic hyperfine field at 140Ce. The ferromagnetic transition temperature (TC) of LaMnSi2 was determined to be 400(1) K confirming the magnetic measurements.

Nanosecond Dynamics at Protein Metal Sites: An Application of Perturbed Angular Correlation (PAC) of γ-Rays Spectroscopy.

Metalloproteins are essential to numerous reactions in nature, and constitute approximately one-third of all known proteins. Molecular dynamics of proteins has been elucidated with great success both by experimental and theoretical methods, revealing atomic level details of function involving the organic constituents on a broad spectrum of time scales. However, the characterization of dynamics at biomolecular metal sites on nanosecond time scales is scarce in the literature. The aqua ions of many biologically relevant metal ions exhibit exchange of water molecules on the nanosecond time scale or faster, often defining their reactivity in aqueous solution, and this is presumably also a relevant time scale for the making and breaking of coordination bonds between metal ions and ligands at protein metal sites. Ligand exchange dynamics is critical for a variety of elementary steps of reactions in metallobiochemistry, for example, association and dissociation of metal bound water, association of substrate and dissociation of product in the catalytic cycle of metalloenzymes, at regulatory metal sites which require binding and dissociation of metal ions, as well as in the transport of metal ions across cell membranes or between proteins involved in metal ion homeostasis. In Perturbed Angular Correlation of γ-rays (PAC) spectroscopy, the correlation in time and space of two γ-rays emitted successively in a nuclear decay is recorded, reflecting the hyperfine interactions of the PAC probe nucleus with the surroundings. This allows for characterization of molecular and electronic structure as well as nanosecond dynamics at the PAC probe binding site. Herein, selected examples describing the application of PAC spectroscopy in probing the dynamics at protein metal sites are presented, including (1) exchange of Cd2+ bound water in de novo designed synthetic proteins, and the effect of remote mutations on metal site dynamics; (2) dynamics at the β-lactamase active site, where the metal ion appears to jump between the two adjacent sites; (3) structural relaxation in small blue copper proteins upon 111Ag+ to 111Cd2+ transformation in radioactive nuclear decay; (4) metal ion transfer between two HAH1 proteins with change in coordination number; and (5) metal ion sensor proteins with two coexisting metal site structures. With this Account, we hope to make our modest contribution to the field and perhaps spur additional interest in dynamics at protein metal sites, which we consider to be severely underexplored. Relatively little is known about detailed atomic motions at metal sites, for example, how ligand exchange processes affect protein function, and how the amino acid composition of the protein may control this facet of metal site characteristics. We also aim to provide the reader with a qualitative impression of the possibilities offered by PAC spectroscopy in bioinorganic chemistry, especially when elucidating dynamics at protein metal sites, and finally present data that may serve as benchmarks on a relevant time scale for development and tests of theoretical molecular dynamics methods applied to biomolecular metal sites.

Fine zircon (ZrSiO4) powder mechanical activation, a Perturbed Angular Correlation (PAC) analysis

Abstract By means of Perturbed Angular Correlation (PAC) analysis the resulting phases during mechano-chemical activation process on zircon (ZrSiO 4 ) commercial fine powder (D 50 ≅ 0.8 µm) were accurately identified and characterized. A high energy planetary mill was employed with 850 rpm up to 120 min. The phenomenological macroscopic confirmation of the structural change and mechanical activation consisted in an important enhancement of the sintering behavior of the treated fine zircon powders. Three different well known zircon phases were identified and quantified as a function of the milling time: a fully crystalline phase, an aperiodic phase and a distorted phase. A decrease in the first two phases was accompanied by the appearance of the third one; finally, at long term treatments, a partial dissociation was observed. Particularly the resulting zirconium oxide is a highly distorted one. The results were discussed together with those obtained using XRD, SEM and laser scattering. The XRD only showed the partial dissociation of zircon and failed in the differentiation of its nanoconfigurations observed by PAC. The milling of this hard material can be optimized through the performed characterization strategy.

Electric field gradients in Zr8Ni21 and Hf8Ni21 intermetallic compounds; results from perturbed angular correlation measurements and first-principles density functional theory

Numerous technological applications of Ni-based Zr and Hf intermetallic alloys promoted comprehensive studies in Zr8Ni21 and Hf8Ni21 by perturbed angular correlation (PAC) spectroscopy, which were not studied earlier until this report. The different phases produced in the samples have been identified by PAC and X-ray diffraction (XRD) measurements. Using 181Hf probe, two non-equivalent Zr/Hf sites have been observed in both Zr8Ni21 and Hf8Ni21 compounds. From present PAC measurements in Zr8Ni21, a component due to the production of Zr7Ni10 by eutectic reaction from the liquid metals is also observed. The phase Zr7Ni10, however, is not found from the XRD measurement. In Zr8Ni21, while the results do not change appreciably up to 973 K exhibit drastic changes at 1073 K. In Hf8Ni21, similar results for the two non-equivalent sites have been found but site fractions are in reverse order. In this alloy, a different contaminating phase, possibly due to HfNi3, has been found from PAC measurements but is not found from XRD measurement. Density functional theory (DFT) based calculations of electric field gradient (EFG) and asymmetry parameter (η) at the sites of 181Ta probe nucleus allowed us to assign the observed EFG fractions to the various lattice sites in (Zr/Hf)8Ni21 compounds.

Structural Characterization of Natural and Processed Zircons with X-Rays and Nuclear Techniques

In ceramic industry, zircon sand is widely used in different applications because zirconia plays a role as common opacifying constituent. In particular, it is used as a basic component of glazes applied to ceramic tiles and sanitary ware as well as an opacifier in unglazed bulk porcelain stoneware. Natural zircon sands are the major source of zirconium minerals for industrial applications. In this paper, long, medium, and short range studies were conducted on zirconium minerals originated from Australia, South Africa, and United States of America using conventional and less conventional techniques (i.e., X-Ray Diffraction (XRD), Positron Annihilation Lifetime Spectroscopy (PALS), and Perturbed Angular Correlations (PAC)) in order to reveal the type and the extension of the regions that constitute the metamict state of zircon sands and the modifications therein produced as a consequence of the industrial milling process and the thermal treatment in the production line. Additionally, HPGe gamma-ray spectroscopy confirms the occurrence of significant levels of natural radioactivity responsible for metamictization in the investigated zircon samples. Results from XRD, PALS, and PAC analysis confirm that the metamict state of zircon is a dispersion of submicron disordered domains in a crystalline matrix of zircon.

Direct Observation of Nanosecond Water Exchange Dynamics at a Protein Metal Site

Nanosecond ligand exchange dynamics at metal sites within proteins is essential in catalysis, metal ion transport, and regulatory metallobiochemistry. Herein we present direct observation of the exchange dynamics of water at a Cd2+ binding site within two de novo designed metalloprotein constructs using 111mCd perturbed angular correlation (PAC) of γ-rays and 113Cd NMR spectroscopy. The residence time of the Cd2+-bound water molecule is tens of nanoseconds at 20 °C in both proteins. This constitutes the first direct experimental observation of the residence time of Cd2+ coordinated water in any system, including the simple aqua ion. A Leu to Ala amino acid substitution ∼10 Å from the Cd2+ site affects both the equilibrium constant and the residence time of water, while, surprisingly, the metal site structure, as probed by PAC spectroscopy, remains essentially unaltered. This implies that remote mutations may affect metal site dynamics, even when structure is conserved.

Low energy γ- γ and e −- γ PAC measurements using APDs and the probe nuclei 83Rb(83Kr) and 83m Kr(83Kr)

In the field of Perturbed Angular Correlation (PAC) measurements Avalanche Photo Diodes (APD) are rarely used, despite their favourable properties for fast counting purposes at low energies. This work demonstrates their application in combination with a simple and cheap custom build voltage sensitive preamplifier module. Using the PAC nuclei 83Rb(83Kr) and 83mKr(83Kr), the time resolution of the set-up is analysed and the feasibility of precise timing measurements is shown.

Site occupation of indium and jump frequencies of cadmium in FeGa 3

Perturbed angular correlation (PAC) measurements using the In-111 probe were carried out on FeGa3 as part of a broader investigation of indium site occupation and cadmium diffusion in intermetallic compounds. One PAC signal was observed with hyperfine parameters w1 = 513.8(1) Mrad/s and eta = 0.939(2) at room temperature. By comparison with quadrupole frequencies observed in PAC measurements on isostructural RuIn3, it was determined that indium occupies only the 8j site in the FeGa3 structure, denoted Ga(2) below because two out of the three Ga sites have this point symmetry. PAC spectra at elevated temperature exhibited damping characteristic of electric field gradients (EFGs) that fluctuate as Cd probes jump among Ga(2) sites within the lifetime of the excited PAC level. A stochastic model for the EFG fluctuations based on four conceivable, single-step jump-pathways connecting one Ga(2) site to neighboring Ga(2) sites was developed and used to fit PAC spectra. The four pathways lead to two observable EFG reorientation rates, and these reorientation rates were found to be strongly dependent on EFG orientation. Calculations using density functional theory were used to reduce the number of unknowns in the model with respect to EFG orientation. This made it possible to determine with reasonable precision the total jump rate of Cd among Ga(2) sites that correspond to a change in mirror plane orientation of site-symmetry. This total jump rate was found to be thermally activated with an activation enthalpy of 1.8(1) eV.

The influence of coordinated defects on inhomogeneous broadening in cubic lattices

The joint probability distribution function (JPDF) of electric field gradient (EFG) tensor components in cubic materials is dominated by coordinated pairings of defects in shells near probe nuclei. The contributions from these inner shell combinations and their surrounding structures contain the essential physics that determine the PAC-relevant quantities derived from them. The JPDF can be used to predict the nature of inhomogeneous broadening (IHB) in perturbed angular correlation (PAC) experiments by modeling the G2 spectrum and finding expectation values for Vzz and η. The ease with which this can be done depends upon the representation of the JPDF. Expanding on an earlier work by Czjzek et al. (Hyperfine Interact. 14, 189–194, 1983), Evenson et al. (Hyperfine Interact. 237, 119, 2016) provide a set of coordinates constructed from the EFG tensor invariants they named W1 and W2. Using this parameterization, the JPDF in cubic structures was constructed using a point charge model in which a single trapped defect (TD) is the nearest neighbor to a probe nucleus. Individual defects on nearby lattice sites pair with the TD to provide a locus of points in the W1−W2 plane around which an amorphous-like distribution of probability density grows. Interestingly, however, marginal, separable PDFs appear adequate to model IHB relevant cases. We present cases from simulations in cubic materials illustrating the importance of these near-shell coordinations.

Magnetic hyperfine interactions on Cd sites of the rare-earth cadmium compoundsRCd(R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er)

This paper reports the investigation of the magnetic hyperfine field ${B}_{hf}$ in a series of rare-earth ($R$) cadmium intermetallic compounds $R\mathrm{Cd}$ and ${\mathrm{GdCd}}_{2}$ measured by perturbed angular correlation (PAC) spectroscopy using $^{111}\mathrm{In}/^{111}\mathrm{Cd}$ as probe nuclei at Cd sites as well as first-principles calculations of ${B}_{hf}$ at Cd sites in the studied compounds. Vapor--solid state reaction of $R$ metals with Cd vapor and the $^{111}\mathrm{In}$ radioisotope was found to be an appropriate route of doping rare-earth cadmium compounds with the PAC probe $^{111}\mathrm{In}/^{111}\mathrm{Cd}$. The observation that the hyperfine parameters depend on details of the sample preparation provides information on the phase preference of diffusing $^{111}\mathrm{In}$ in the rare-earth cadmium phase system. The $^{111}\mathrm{Cd}$ hyperfine field has been determined in the compounds $R\mathrm{Cd}$ for the $R$ constituents Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er, in several cases as a function of temperature. For most $R$ constituents, the temperature dependence ${B}_{hf}(T)$ of $^{111}\mathrm{Cd}$:$R\mathrm{Cd}$ is consistent with ferromagnetic order of the compound. DyCd, however, presents a remarkable anomaly: a finite magnetic hyperfine field is observed only in the temperature interval 35 K $\ensuremath{\le} T \ensuremath{\le}$ 80 K which indicates a transition from ferromagnetic order to a spin arrangement where all $4f$-induced contributions to the magnetic hyperfine field at the Cd site cancel. First-principles calculation results for DyCd show that the ($\ensuremath{\pi}, \ensuremath{\pi}$, 0) antiferromagnetic configuration is energetically more favorable than the ferromagnetic. The approach used in the calculations to simulate the $R\mathrm{Cd}$ system successfully reproduces the experimental values of ${B}_{hf}$ at Cd sites and shows that the main contribution to ${B}_{hf}$ comes from the valence electron polarization. The de Gennes plot of the hyperfine field ${B}_{hf}$ of $^{111}\mathrm{Cd}$:$R\mathrm{Cd}$ vs the $4f$-spin projection $(g\ensuremath{-}1)J$ reflects a decrease of the strength of indirect $4f\ensuremath{-}4f$ exchange across the $R$ series. Possible mechanisms are discussed and the experimental results indicate that the indirect coupling is provided by the intra-atomic $4f\ensuremath{-}5d$ exchange and interatomic $5d\ensuremath{-}5d$ interaction between the spin-polarized $5d$ electrons of neighboring $R$ atoms. The ratio of the hyperfine fields of GdCd and ${\mathrm{GdCd}}_{2}$ scales with the number of nearest Gd neighbors. In the paramagnetic phases of the $R\mathrm{Cd}$ compounds, the PAC spectra indicate the presence of a broad distribution of weak quadrupole interactions suggesting a perturbation of the cubic CsCl symmetry of the Cd site, most probably due to chemical disorder of the $R$ and Cd sublattices. A substantial interchange of $R$ and Cd atoms is also reflected in the temperature dependence of the linewidth of the magnetic hyperfine interaction in the magnetically ordered phase of $R\mathrm{Cd}$ and ${\mathrm{GdCd}}_{2}$. Its critical increase towards the order temperature is evidence for a distribution of the order temperature with a width of about 10 K.

Growth of Ga2O3 by furnace oxidation of GaN studied by perturbed angular correlations

Ga2O3 is a promising material for use in “solar-blind” UV-detectors which can be produced efficiently by oxidation of GaN. In this study we focus on the evolution of the oxide layer when GaN is heated in air. The experimental method applied is the perturbed angular correlation (PAC) spectroscopy of γ-rays emitted by radioactive nuclides, here 111Cd and 181Ta, whose parent nuclei are ion implanted into films of GaN grown on sapphire. As the emission pattern for nuclei in GaN is clearly distinct from that of nuclei in Ga2O3, the fraction of probe nuclei in the oxide layer can be directly measured and allows to follow the time dependent growth of the oxide on a scale of less than 100 nm. Additional measurements were carried out with the oxidized sample held at fixed temperatures in the temperature range from 19 K to 973 K showing transitions between the hyperfine interactions of 111Cd in the oxide matrix both at high and low temperatures. A model for these transitions is proposed.