Search

We report the synthesis and characterization of Fe 0.36(4) Pd 0.64(4) Se 2 with a pyrite-type structure. Fe 0.36(4) Pd 0.64(4) Se 2 was synthesized using ambient pressure flux crystal growth methods even though the space group Pa3 is high-pressure polymorph for both FeSe 2 and PdSe 2 . Combined experimental and theoretical analysis reveal magnetic spin glass state below 23 K in 1000 Oe that stems from random Fe/Pd occupancies on the same atomic site. The frozen-in magnetic randomness contributes significantly to electronic transport. Electronic structure calculations confirm dominant d-electron character of hybridized bands and large density of states near the Fermi level. Flux-grown single crystal alloys in Pd-Fe-Se atomic system therefore open new pathway for exploring different polymorphs in crystal structures and their novel properties. © 2019 American Chemical Society.

In this paper we investigated the effects of Fe -doping of the anatase TiO 2 (1 0 1) surface on the crystal structure, electronic and optical properties, and impurity formation energy by means of density functional theory (DFT). The calculations were performed by the SIESTA DFT code and were carried out by using Troullier-Martins pseudopotentials for the 12-electron valence configuration ( 3 s 2 3 p 6 3 d 2 4 s 2 ) of Ti atom, 6-electron valence configuration ( 2 s 2 2 p 4 ) of O atom and 8-electron valence configuration ( 3 d 6 4 s 2 ) of Fe atom. We used a double- ζ basis set including polarization functions. All calculations were spin-polarized. The mechanism of narrowing the band gap and increasing the photocatalytic activity in the visible light region, of the doped TiO 2 is discussed by investigating the density of state. The band gap decreases as the concentration of the dopant increases. The Partial Density of States (PDOS) is not the same in the case of spin-up state or spin-down state. Enhanced optical absorption, for light polarized in the z direction (parallel to the surface normal) is clearly observed for Fe doped as compared to the pure anatase TiO 2 and the optical absorption is found to increase with the increase in the Fe concentration. The DFT results indicate that the source of the increasing photocatalytic activity in the visible light region of the Fe doped material is due to the introduction of additional electronic states within the band gap. Since the Fe atoms are more stable in Ti substitutional lattice positions for the entire range of Fermi energy E F over the band gap, only this substitutional position is considered. We hope that our results will highlight a route to improved electronic and optical properties of anatase TiO 2 for industrial applications.

X-ray absorption spectroscopy is employed to investigate site preference and lattice relaxation around Mo, Ru, Hf, W and Re dopants in Ni3Al. The site occupation preference and the measured distances between the refractory elements as dopants and the nearest host atoms are compared with the results of ab initio calculations within the density functional theory. Combined experimental and theoretical results indicate that Mo, Hf, W and Re atoms reside on the Al sublattice in Ni3Al, while Ru atoms occupy the Ni sublattice. A more pronounced lattice relaxation was detected in the case of Hf and Ru doping, with a strong outward relaxation of the nearest Ni and Al atoms. 2nd Workshop on FEL Photon Diagnostics, Instrumentation and Beamlines Design (PhotonDiag), Jun 08-10, 2015, Int Ctr Theoretical Phys, Trieste, Italy

The local electronic and magnetic structure, hyperfine interactions, and phase composition of polycrystalline Ni-deficient Ni3-xFexAl (x = 0.18 and 0.36) were investigated by means of Fe-57 Mossbauer spectroscopy. The samples were characterized by X-ray diffraction and magnetization measurements. The ab initio calculations performed with the projector augmented wave method and the calculations of the energies of iron point defects were done to elucidate the electronic structure and site preference of Fe doped Ni3Al. The value of calculated electric field gradient tensor V-zz = 1.6 10(21) Vm(-2) matches well with the results of Mossbauer spectroscopy and indicates that the Fe atoms occupy Ni sites. (C) 2015 Elsevier B.V. All rights reserved.

Incorporation of suitable dopants in MgH2 is widely investigated as the way of improving hydrogen storage characteristics of this material. The catalytic role of transition metal dopants on hydrogen desorption from MgH2 is very promising, but further attention is required in order to optimize the experimental methods and design materials with desired properties. In this paper we investigate the role of Ti, Fe, and Mn on the transport properties of hydrogen in MgH2, which are marked as limiting factor in the effort to lower the hydrogen desorption temperature. Taking into account the lattice relaxation around the impurities, we consider a number of different diffusion paths in the pure and doped system. Using PAW DFT calculations in combination with the NEB method, we demonstrate that the diffusion of the most relevant positively charged hydrogen vacancy and negatively charged interstitial hydrogen atom is locally hindered by the presence of the impurity atoms.

We present first principles calculations of the electronic structure, enthalpies of formation and electric field gradients in C15 HfV2Hx (x = 1, 2, 3, 4). In C15 Laves phases, hydrogen can occupy three possible interstitial positions: 96g, 32e, and 8b. To determine which one of these interstitials is the most favorable for storing hydrogen, enthalpies of formation were calculated for every site, with different concentrations of hydrogen. In order to investigate the change in electronic structure before and after hydrogenation, we calculated the electric field gradients induced by hydrogen on the vanadium, and compared them with the existing nuclear magnetic resonance measurements. This comparison enabled us to study the distribution of hydrogen atoms in the crystal lattice, as well as the occupation of possible interstitials. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

The hydrogenation properties of HfNi and Hf(2)Ni(7) intermetallics were investigated at the constant pressure of 1 bar and in the temperature ranges 373-573 K for HfNi and 323-473 K for Hf(2)Ni(7). The kinetic parameters, rate constants and activation energies of the absorption processes were determined. Maximal hydrogen absorption, i.e., number of hydrogen atoms absorbed per metal atom, HIM, are 1.05 and 0.04 achieved at 373 K for HfNi and Hf(2)Ni(7), respectively. Multiple hydriding/dehydriding was found to influence the improvement of the kinetic parameters. XRD and SEM methods were used to investigate the structural and morphological changes of the samples due to hydrogen absorption. The thermodynamic parameters of hydriding together with the structural properties of the intermetallics and their hydrides, calculated using the full-potential linearized augmented plane waves (FP-LAPW) code based on the density functional theory (DFT), were utilized for the sake of explaining the experimental investigations. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Ab initio calculations of the structural, electronic and thermodynamic properties of Hf2Ni, HfNi, HfNi2 and HfNi5 are presented. Using all-electron augmented plane waves plus local orbitals (APW+lo) method the relative stability of the selected compounds was determined by calculating their enthalpies of formation and cohesive energies. In addition, their electronic structure and bonding properties were determined. The results are discussed in view of the potential application of these intermetallics as materials for hydrogen storage. (C) 2010 Elsevier B.V. All rights reserved.

The perturbed angular correlation method has been employed to study the temperature dependence of the 181Ta hyperfine interaction parameters in the polycrystalline intermetallic compound αHfNi. At ambient temperature the frequency of the electric quadrupole interaction was ωQ = 26.0(2) Mrad/s and the asymmetry parameter η = 0.22(1). The magnitude of the observed electric field gradient decreases with increasing temperature from 78 to 900 K. The calculations were done using the augmented plane wave plus local orbitals method as implemented in the WIEN2k code, using the generalized gradient approximation. In addition, a supercell calculation with Ta impurity located at the hafnium site was performed. The obtained result is in a good agreement with the experiment.

Hafnium-based interriletallics Hf(2)Fe, Hf(2)Co and Hf(2)Ni are investigated as hydrogen absorbers at atmospheric pressure and their electrocatalytic activities are compared. The absorption process is carried out in typical volumetric apparatus. Their crystal structures and morphological characteristics are investigated by X-ray difractometry (XRD) and scanning electron microscopy (SEM). According to the absorption abilities, the investigated intermetallics can be ordered as follows: Hf(2)Fe GT Hf(2)Co GT Hf(2)Ni. Ab initio calculations, within the framework of the density functional theory (DIFT), as implemented in the full-potential (linearized) augmented plane-wave (FP/LAPW) method, confirms the experimental investigations. The calculations are important as a way of predicting the best cathode and storage materials. (C) 2009 Elsevier B.V. All rights reserved.

The hyperfine quadrupole interaction parameters and their temperature dependences for (181)Ta and (111)Cd probes in Ti(2)Pd compound were determined experimentally by means of the perturbed angular correlation (PAC) technique. In addition, ab initio calculations using all-electron augmented plane wave plus local orbitals (APW + lo) method have been performed. Through the comparison of the calculated and measured electric field gradients (EFG) it was established that both Ta and Cd occupy 4(e) Ti position in the Ti(2)Pd compound. (C) 2008 Elsevier B.V. All rights reserved.

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. (c) 2008 Elsevier B.V. All rights reserved.

The hyperfine magnetic field (H-hf) in 0.2 at.% Hf-Ni alloy is measured at the Ta-181 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 ( LT 1 at.%) mainly substitute on Ni lattice sites in the fee 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. (C) 2008 Published by Elsevier Ltd.

The stability and structure of titanium doped gold clusters (AunTi; n = 1-32) are studied by density functional theory calculations, as implemented in the first principles code SIESTA. The exchange and correlation effects were calculated within the generalized gradient approximation (GGA) parametrized by Perdew, Burke and Ernzerhof (PBE). We used norm conserving Troullier-Martins pseudopotentials for the 10-electron valence configuration of Ti and 11-electron valence configuration of Au. All calculations were spin-polarized. The global energy minimum geometries of the clusters were searched for by using the simulated annealing technique. The stability of the clusters is discussed on the basis of the binding energy per atom, second-order energy difference, vertical ionization potential, vertical electron affinities, HOMO-LUMO energy gap and vibrational frequencies. Based on the simultaneous criteria of high binding energy, high band gap, high vertical ionization potential, and low electron affinity, it is found that Au4Ti and Au14Ti clusters have a higher stability and are candidates for magic clusters, which confirms the already known results from previous works. The new result presented in this paper is that the Au20Ti and Au30Ti clusters have a higher stability too. In general, the clusters with even n are more stable than the clusters with odd n. Most of the clusters with even n are non-magnetic (total magnetic moment is zero). Our results also suggest that only the Au3Ti, Au7Ti and Au8Ti clusters have a planar structure. (C) 2016 Elsevier B.V. All rights reserved.

Hypervalent molecules of Li3F and Li4F were detected experimentally for the first time. Ionization energies determined by the thermal ionization mass spectrometry were 3.8 +/- 0.2 eV for Li2F, 4.0 +/- 0.2 eV for Li3F and 3.9 +/- 0.2 eV for Li4F. The ionization energies obtained by ab initio calculations support the experimental findings. The lowest energy structures of the LinF (n = 2-4) molecules and their cations were also predicted. (C) 2007 Elsevier B.V. All rights reserved.

The role of Ti, Fe, and Mn on H diffusion in MgH2 is studied by DFT calculations using the PW91 generalized gradient functional on a 71 atom 2 x 2 x 3 supercell of rutile-type MgH2 in which one Mg host atom is replaced by the dopant.

The electric field gradients (EFG) of the Hf2Fe intermetallic compound were calculated Using the full-potential linearized augmented plain-wave (FP-LAPW) method as embodied in the WIEN 97 code. The obtained values are compared with other ab-initio calculations and on a qualitative basis with the previously reported experimental data obtained from TDPAC. The calculated results, -23.1.10(21) V/m(2) and 2.7.10(21) V/m(2) for Hf 48f and Fe 32e position, respectively, are in excellent agreement with experimental data (23.4.10(21) V/m(2) and 2.7.10(21) V/m(2)), better than those reported in earlier calculations. The calculated EFG for Hf 16c position (4.2.10(21) V/m(2)) is stronger than the experimental one (1.1.10(21) V/m(2)). Joint Meeting of the 13th International Conference on Hyperfine Interactions/17th International Symposium on Nuclear Quadrupole Interactions (HFI/NQI 2004), Aug 22-27, 2004, Bonn, Germany

We have investigated the development of local bond lengths with composition in the Cd1−xZnxTe mixed system by measuring the fine structure in X-ray absorption (EXAFS) at all three constituent atoms. The bond strength is found to dominate over the averaging of the bulk so that the local bond length deviates only slightly from its natural value determined for the pure binary components ZnTe and CdTe, respectively. The deviations are significantly less than predicted by a simple radial force constant model for tetrahedrally co-ordinated binary systems, and the bond-length variation with concentration is significantly non-linear. For the second shell, bimodal anion–anion distances are found while the cation–cation distances can already be described by the virtual crystal approximation. In the diluted regime close to the end-point compounds, we have complemented our experimental work by ab initio calculations based on density functional theory with the WIEN97 program using the linearised augmented plane wave method. Equilibrium atomic lattice positions have been calculated for the substitutional isovalent metal atom in a 32-atom super cell, Zn in the CdTe lattice or Cd in the ZnTe lattice, respectively, yielding good agreement with the atomic distances as determined in our EXAFS experiments.

We have measured the lattice relaxation around As in Si at a homogeneous As concentration of 4×1018 cm−3 by EXAFS spectroscopy. From the absorption spectra, distances up to the 4th shell could be extracted. A sizeable misfit due to an increased distance is only observed for the 1st shell. Complementing our experimental work we have performed ab initio calculations based on the density functional theory with the WIEN97 package which uses the linearised augmented plane wave method and with the FHI96md program which uses first-principles pseudo-potentials and a plane wave basis set to investigate the size dependence of the super-cells constructed around one substitutional As atom. The calculations yielded good agreement with our EXAFS experiment so that the determined relaxations can be used as a solid basis for further interpretations of derived parameters such as hyperfine interaction parameters in defect complexes.

Thermodynamics of hydride formation is one of the key properties of metal-hydrogen system and determines its applicability. Therefore, numerous researches are focused on the use of first-principles calculations as the predictive tool when investigating the stability of the hydrides. In this paper we use density functional theory to address two-step process of hydride formation in the MNi (M = Ti, Zr, Hf). Through systematic study of experimentally verified and hypothetical hydride phases, we examine the influence of crystal structure and intermetallic composition on electronic structure, stability and bonding of the hydrides. The unique properties and advantages of gamma-phase hydrides having orthorhombic crystal structure (space group Cmcm) for near-ambient hydrogen storage applications are pointed out, as well as the importance of further investigation of the crystal structure of the beta-phase hydrides. Calculated enthalpies of hydride formation/decomposition and desorption temperatures show good agreement with the wide range of experimental data taken from literature, demonstrating predictive power of the used approach for addressing structure-property relationship and giving complete overview of the important representatives of AB class of intermetallic compounds. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

The evolution of the environment of Hf sites in a hafnium-alkoxide-modified polysilsesquioxane upon polymer-to-ceramic transformation was investigated via the perturbed angular correlation (PAC) method. The results of the PAC measurements on samples thermally treated at temperatures from 400 to 1300 degrees C indicate that Hf is surrounded only by oxygen at all studied temperatures. This finding is in agreement with the evolution pathway of polymer-derived SiHfOC ceramics, which were reported to be generated as single-phase amorphous materials upon pyrolysis of alkoxide-modified polysiloxanes and subsequently to phase separate and crystallize towards HfO2/SiOC nanocomposites. Thus, the results presented here support our previous statement that there is a thermodynamic control on the phase separation and crystallization behavior of Si-M-O-C (M = metal) ceramics (Ionescu et al. 96;2013:1899-1903), which in the case of Hf leads to the precipitation and crystallization of hafnia nanoparticles within a SiOC matrix. (C) 2014 Elsevier Ltd. All rights reserved.

Substitutional N to O and M to Ti (M = Pt, V, Sb) codoped rutile TiO2 was investigated using density functional theory (OFT) based calculations with both standard and hybrid exchange-correlation functionals. The band gaps calculated using generalized gradient approximation (GGA) exhibited narrowing compared to the pure rutile TiO2 in all the investigated cases. In contrast, the results obtained with hybrid exchange-correlation functional showed that there was no band gap narrowing, but doping induced localized states within the band gap just above the valence band, as well as below the conduction band for Pt doped TiO2. The presence of broad intermediate states (IS) in the band gap could enhance visible light absorption through a two step optical transition from the valence to the conduction band via the IS and at the same time lower recombination of the photo-generated charges. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

X-ray absorption spectroscopy has been used to obtain information on the local structure around Zn atoms in CdTe. The Zn–Te distance is 5% smaller than the Cd–Te bond-length in the bulk, but slightly larger than in pure ZnTe. A similar effect is predicted by FLAPW calculations.

The time differential perturbed angular correlation technique has been used to measure the electric field gradient at impurities in the intermetallic compound . The results of the measurements show the presence of two independent quadrupole interactions. At room temperature the interaction frequencies are and . The electric field gradient , the corresponding asymmetry parameter and the distribution parameter exhibit a pronounced temperature dependence from 78 to 1223 K.

The aim of this study is to evaluate the size and stability of radiolytically produced silver clusters. The clusters obtained by the gamma irradiation of a mixture of a silver salt and polyvinyl alcohol (PVA) were studied both theoretically and by mass spectrometry. MALDI TOF mass spectrometry of the Ag-n clusters showed the presence of clusters containing between 5 and 29 atoms. Both the theoretical and experimental results of this paper show that the most stable clusters are small clusters with an odd number of atoms. The stability of these clusters is correlated with the clusters electronic configuration. (C) 2012 Elsevier Ltd. All rights reserved.

Intermetallic compound TiFe, known for its hydrogen storage applications, is modified by substituting iron by nickel and related changes of properties and applicability of the obtained compounds are studied. Samples TiFe 1− x Ni x ( x = 0.2–0.6) are synthesized by melt-spinning and their crystal structure, desorption characteristics and electronic structure are investigated by TPD, 1 H NMR and Mossbauer spectroscopy. State-of-the-art DFT calculations give further insight into the changes in electronic structure and bonding related to the hydrogen absorption and substitution of iron by nickel. The increase of Ni/Fe ratio in the TiFe 1− x Ni x is found to result in the increase of hydride cohesive energies and in the systematic shifting of Fermi energy ( E F ) to lower values, in both pure intermetallics and appropriate hydrides. Hydride formation was found to influence the Fermi energy lowering and the increase of number of states at E F .

Hydrogen induced modifications to the structural, electronic and bonding properties of HfNi are investigated by performing first principles calculations. The full-potential linearized augmented plane waves (FP-LAPW) code based on the density functional theory (DFT) was used. The charge transfer and bonding between the constituent atoms is examined by means of the Baders atoms in molecule (AIM) theory. The calculated enthalpies of formation of HfNi, HfNiH and HfNiH(3) are -53.5 kJ/mol atom, -17.3 kJ/molH and -34.6 kJ/molH. They are found to be in a good agreement with the experimental and semi-empirical values. The calculated stability of the hydrides is in agreement with their hydrogen absorption ability. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

The time differential perturbed angular correlation technique was used to observe the magnetic dipole and electric quadrupole interactions of (181)Ta probe in the Ni-5 at.% Hf alloy from 78 to 1131 K. The presence of the Larmor precession frequency of (181)Ta in the Ni(Hf) solid solution phase confirms the Ni dominant concentration for the onset of ferromagnetism in the Ni-Hf alloys. A very weak quadrupole interaction originates from imperfections in the HfNi(5) phase formed during melting procedure. The third observed quadrupole interaction corresponds to the Ta probe in the HfO(2) contamination due to internal oxidation of the sample. (C) 2008 Elsevier B.V. All rights reserved. 16th International Conference on Solid Compounds of Transition Elements, Jul 26-31, 2008, Dresden, Germany

The hyperfine quadrupole interaction parameters and their temperature dependence for Hf-181/Ta-181 probe in TiAl3 compound were determined using the time differential perturbed angular correlation (TDPAC) method. in addition, ab initio calculations using the augmented plane wave plus local orbitals (APW+ lo) method as implemented in the WIEN2k code, within the framework of the density functional theory(DFT) have been performed. Through the comparison of the calculated and measured electric field gradients (EFG) it was established that Ta occupies Ti 2(a) position, while Cd probe is settled at the Al 4(d) site. (c) 2009 Elsevier B.V. All rights reserved.

Ab initio calculations of the structural and electronic properties of ${\text{Mo}}_{3}\text{Pt}$, ${\text{MoPt}}_{2}$, and ${\text{MoPt}}_{3}$ are presented. In particular, the enthalpy of formation of the investigated intermetallics is considered and their bonding properties, as obtained from the atoms in molecules theory, are analyzed. The results are discussed in view of the potential application of these systems as cathode materials in the process of electrocatalytic production of hydrogen. Our calculations predict ${\text{MoPt}}_{3}$ to be the best catalyst, which is in agreement with experimental trends.

The lattice relaxation around Mo, Ru, Hf, Ta, W and Re in Ni is investigated by means of X-ray absorption spectroscopy. For all of the investigated concentrations the substitutional lattice position is confirmed except for the higher Hf concentration where different phases are observed. Ail outward relaxation of the neighboring atoms is detected with clear trends of increasing nearest neighbor distances with decreasing valence for the 5d impurities, and separate trends for the 4d impurities. Ab initio supercell calculations within the linearized augmented plane wave formalism are used to complement the experimental results and allow for a better interpretation of the experimental trends in terms of the electronic structure of the impurity. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The stability and bonding properties of Hf2Fe, Hf2Co and Hf2Rh are investigated by means of ab initio quantum mechanical simulations. All three compounds belong to the family of complex intermetallic compounds of hafnium with Ti Ni structure. The relative stability of these intermetallics is investigated by calculating their enthalpies of formation and cohesive properties. In addition, the charge transfer between the constituent atoms is examined by means of the atoms in molecule (AIM) theory. The computed structural and electronic properties are discussed in view of the possible application of these compounds as cathode materials in the process of electrocatalytic hydrogen production. (C) 2007 Elsevier B.V. All rights reserved. 15th International Conference on Solid Compounds of Transition Elements, Jul 15-20, 2006, Jagiellonian Univ, Inst Phys, Cracow, Poland

A detailed theoretical study of the structure, electronic properties and electric field gradients (EFG) of the HfAl2 and ZrAl2 Laves phases is presented. Using all-electron augmented plane waves plus local orbitals (APW + lo) formalism, the equilibrium volumes, bulk moduli and EFGs for the two compounds are calculated. The obtained results are compared with the available experimental and theoretical data. Better agreement with the experimental data is found by employing supercell, calculations with Ta and Cd impurities. (c) 2007 Elsevier B.V. All rights reserved.

We have measured the lattice distortion around As (acceptor), Se (isovalent), and Br (donor) in CdTe with fluorescence detected X-ray absorption spectroscopy (XAFS). The experimental challenge lies in the compromise between a concentration high enough for X-ray absorption and low enough to avoid compensation and clustering. We could experimentally verify the lattice relaxation with a bond length reduction of 8% around the As atom as inferred indirectly from ab initio calculations of the electric field gradient in comparison with the measured value in a Perturbed Angular Correlation (PAC) experiment as recently reported. Our calculations of relaxation were performed with the WIEN97 package using the linearised augmented plane wave (LAPW) method and the FHI96md pseudopotential (PP) program. Using a super-cell approach for As on the Te site in CdTe, we find good convergence with increasing cell size so that the result can be taken as representative for the low doping limit. The extension to the neighbouring isovalent element Se again yields perfect agreement between experiment and model calculation with a sizeable lattice mismatch. In the case of Br, we find a sizeable increase of the bond length which may be taken as evidence for the high tendency of Br to form a Br A-center (Br plus metal vacancy) in CdTe. (c) 2004 Elsevier B.V. All rights reserved. EMRS Symposium on Thin Film Chalcogenide Photovoltaic Materials, May 24-28, 2004, Strasbourg, France

A detailed theoretical study of the structure, electronic properties and the electric field gradients of the Hf2Fe 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 Mossbauer 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. (c) 2005 Elsevier Ltd. All rights reserved.

We have measured the lattice distortion around As (acceptor) and Br (donor) in CdTe With fluorescence detected X-ray absorption spectroscopy. We Could experimentally verify the lattice relaxation with a bond length reduction of 8% around the As atom as inferred indirectly from ab initio calculations of the electric field gradient performed with the WIEN97 package in comparison with the measured value in a Perturbed Angular Correlation experiment as recently reported. We have complemented Our own calculations of relaxation with WIEN97 with calculations using the FH196md pseudo-potential program, which allows the use of larger super-cell sizes. Encouraged by the good agreement between experiment and model calculation for As in CdTe as well as similarly for the isovalent Se in CdTe, we extended Our investigation to Br in CdTe, where the electric field gradient has also been Measured, and Could not only verify the derived lattice expansion around Br with Our EXAFS analysis but additionally observe fractions of Br in the A-center as well as in a DX-center configuration. Joint Meeting of the 13th International Conference on Hyperfine Interactions/17th International Symposium on Nuclear Quadrupole Interactions (HFI/NQI 2004), Aug 22-27, 2004, Bonn, Germany

The Time Differential Perturbed Angular (TDPAC) measurements of nuclear quadrupole interactions (NQIs) at 181Ta ion probe in the polycrystalline intermetallic alloy γ′–Ni3Al doped with 0.2 at. % Hf were performed in the temperature range 78-1230 K, in order to determine the lattice location of Hf atoms in the ordered γ′–Ni3Al structure. The two NQIs obtained are discussed within the present L12 cubic structure and a tetragonal distortion of L12 to another two DO22 and L60 type structures. The first low frequency NQI at the site of the 181Ta ion-probe after substitution of aluminum for hafnium in DO22 at ambient temperature, is vQ1(300 K) = 39(1) MHz with η1 = 0. The corresponding high frequency value on the second crystallographic site in L60, is vQ2(300 K) = 204(14) MHz with η2 = 0.47(11). These two NQI's have different temperature behavior. The presence of both DO22 and L60 tetragonal distortions of the parent cubic L12 lattice, detected after adding 0.2 at. % Hf, are with modulations to the lattice con...

The quadrupolar hyperfine interactions of in-diffused (111)In --(111)Cd probes in polycrystalline isostructural Zr(4)Al(3) and Hf(4)Al(3) samples containing small admixtures of the phases (Zr/Hf)(3)Al(2) were investigated. A strong preference of (111)In solutes for the contaminant (Zr/Hf)(3)Al(2) minority phases was observed. Detailed calculations of the electric field gradient (EFG) at the Cd nucleus using the full-potential augmented plane wave + local orbital formalism allowed us to assign the observed EFG fractions to the various lattice sites in the (Zr/Hf)(3)Al(2) compounds and to understand the preferential site occupation of the minority phases by the (111)In atoms. The effects of the size of the supercell and relaxation around the oversized In and Cd probe atoms were investigated in detail.