PAC study of dynamic hyperfine interactions at 111In-doped Sc2O3 semiconductor and comparison with ab initio calculations

Time-Differential $\upgamma$ - $\upgamma$ Perturbed-Angular-Correlation (PAC) measurements were performed in ¹¹¹In-difussed Sc₂O₃ polycrystals in order to characterize the electric-field-gradient tensor at ¹¹¹Cd nuclei located at the two non-equivalent cation sites of the host lattice. The experimental data were compared with ab initio calculations performed using the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) method in the framework of the Density Functional Theory. The PAC experiments were carried out in air in the temperature range 10–900 K. The spectra present a strong damping below 650 K. This damping is associated with dynamic hyperfine interactions that were analyzed with the perturbation factor proposed by Bäverstam et al.. A model based in the population of impurity levels that are introduced by the Cd probes (supported by FP-APW+lo results) is proposed in order to explain the origin of the observed dynamic interactions.     

Experimental and ab initio study of the nuclear quadrupole interaction of 181 Ta-probes in an α-Fe2O3 single crystal

We report perturbed-angular-correlation (PAC) experiments on ¹⁸¹Hf (→¹⁸¹Ta)-implanted corundum α-Fe₂O₃ single crystal in order to determine the magnitude, symmetry and orientation of the electric-field-gradient (EFG) tensor at Ta donor impurity sites of this semiconductor. These results are analyzed in the framework of ab initio full-potential augmented-plane wave plus local orbital (FP-APW+lo) calculations. This combined analysis enables us to quantify the magnitude of the lattice relaxations induced by the presence of the impurity and to determine the charge state of the impurity donor level introduced by Ta in the band gap of the semiconductor.     

Electric-field gradients at Ta donor impurities in Cr2O3(Ta) semiconductor

We report perturbed-angular-correlation (PAC) experiments on ¹⁸¹Hf(→¹⁸¹Ta)-implanted corundum Cr₂O₃ powder samples in order to determine the magnitude and symmetry of the electric-field gradient (EFG) tensor at Ta donor impurity sites of this semiconductor. These results are analyzed in the framework of ab initio full-potential augmented-plane wave plus local orbitals (FP−APW+lo) calculations. The results are also compared with EFG results coming from PAC experiments in isomorphous α-Al₂O₃ and α-Fe₂O₃ doped with ¹¹¹In→¹¹¹Cd and ¹⁸¹Hf→¹⁸¹Ta tracers. This combined analysis enables us to quantify the magnitude of the lattice relaxations induced by the presence of the impurity and to determine the charge state of the impurity donor level introduced by Ta in the band gap of the semiconductor.     

Ab initio study of the EFG tensor at Cd impurities in Sc2O3 semiconductor

We present an ab initio study of diluted Cd impurities localized at both cation sites of the semiconductor Sc₂O₃. The electric-field-gradient (EFG) tensor at Cd impurities located at both cationic sites of the host structure was determined from the calculation of the electronic structure of the doped system. Calculations were performed with the full-potential augmented-plane wave plus local orbitals (APW+lo) method within the framework of the density functional theory. We studied the atomic structural relaxations and the perturbation of the electronic charge density induced by the impurities in the host system in a fully self-consistent way. We showed that the Cd impurity introduces an increase of 8% in the nearest oxygen neighbors bond-lengths, changing the EFG sign for probes located at the asymmetric cation site. The APW+lo predictions for the charged state of the Cd impurity were compared with EFG results existent in the literature, coming from time-differential γ–γ perturbed-angular-correlations experiments performed on ¹¹¹Cd-implanted Sc₂O₃ powder samples. From the excellent agreement between theory and experiment, we can strongly suggest that the Cd acceptor impurities are ionized at room temperature. Finally, we showed that simple calculations like those performed within the point-charge model with antishielding factors do not correctly describe the problem of a Cd impurity in Sc₂O₃.     

Electronic structure of α-Al2O3 slabs: A local environment study

In this work we performed an ab initio/Density Functional Theory (DFT) study of structural and electronic properties of the (0 0 1) α-Al₂O₃ surface. For this study we used two methods with different basis set: the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) and a linear combination of numerical localized atomic orbital basis sets, employing the WIEN2k code and the SIESTA code, respectively. In order to calculate the structural and electronic properties of the reconstructed surface, we calculated the final equilibrium atomic position with the SIESTA code and then the electric-field gradient (EFG) at Al sites was calculated with the FP-APW+lo code using the optimized positions. Using this procedure we found equilibrium structures with comparative lower energy than those obtained using only the FP-APW+lo method. The EFG tensor and the local structure for Al were studied as a function of the depth from the surface for the relaxed structures. We found that distances down to 6 Å from the surface are sufficient to converge the EFG and the Al–O distances to bulk values. The predicted bulk EFG at the Al site is in good agreement with available experimental values. These results can be used for local probes purposes, e.g., in the case of doping, with important sensitivity for probes located close to the top of the surface, in particular for distances smaller than 6 Å.     

The Nonanuclear [Mo(IV){(CN)Fe(III)(3-ethoxy-saldptn)}8]Cl4 Complex Compound Exhibits Multiple Spin Transitions Observed by Mössbauer Spectroscopy

The hyperfine interactions at ¹⁸¹Ta ions on Fe³⁺ sites in α-Fe₂O₃ (hematite) were studied in the temperature range 11–1100 K by means of the perturbed angular correlation (PAC) technique. The ¹⁸¹Hf(β⁻)¹⁸¹Ta probe nuclei were introduced chemically into the sample during the preparation. The hyperfine interaction measurements allow to observe the magnetic phase transition and to characterize the supertransferred hyperfine magnetic field B_hf and the electric field gradient (EFG) at the impurity sites. The angles between B_hf and the principal axes of the EFG were determined. The Morin transition was also observed. The results are compared with those of similar experiments carried out using ¹¹¹Cd probe. ^aAlso at Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, Argentina.     

TDPAC study of Cd-doped SnO

The combination of hyperfine techniques and ab initio calculations has been shown to be a powerful tool to unravel structural and electronic characterizations of impurities in solids. A recent example has been the study of Cd-doped SnO, where ab initio calculations questioned previous TDPAC assignments of the electric-field gradient (EFG) in ¹¹¹In-implanted Sn-O thin films. Here we present new TDPAC experiments at ¹¹¹In-difused polycrystalline SnO. A reversible temperature dependence of the EFG was observed in the range 295–900 K. The TDPAC results were compared with theoretical calculations performed with the full-potential linearized augmented plane wave (FP-LAPW) method, in the framework of the density functional theory. Through the comparison with the theoretical results, we infer that different electronic surroundings around Cd impurities can coexist in the SnO sample.     

Hyperfine interactions of 111Cd in TiO2 (Rutile) studied by perturbed angular correlations

The time-differential perturbed angular correlation for ¹¹¹Cd nuclei has been measured after ¹¹¹In implantation into polycrystalline TiO₂. The observed perturbation functions are characterized by a well-defined electric field gradient with the quadrupole coupling constant ν_Q = 105(1) MHz and the asymmetry parameter η = 0.18(1). We attribute these hyperfine parameters to ¹¹¹Cd on the (distorted) substitutional cation site in rutile. The PAC results are compared with those in SnO₂ rutile as well as with X-ray diffraction, RBS channeling experiments and point charge model calculations including relaxation of the probe atom surrounding.     

Ferromagnetic Semiconductors Studied by Hyperfine Interaction of Nuclear Probes

The ferromagnetic spinels CdCr₂Se₄ and CuCr₂Se₄ were investigated by PAC (perturbed angular correlations) after implantation of the probes ¹¹¹In(¹¹¹Cd), ^111mCd, ¹¹¹Ag(¹¹¹Cd) and ⁷⁷Br(⁷⁷Se). The site occupation of different probes was determined by magnetic and electric hyperfine interactions. Theoretical calculations of hyperfine parameters by the WIEN97 code gave satisfactory efg (electric field gradients) but the magnetic hyperfine fields are inconsistent with the experiments. This revised version was published online in September 2006 with corrections to the Cover Date.     

Electric field gradients in dilute alloys of Cu,Ag and Au

In this paper we present perturbed angular correlation (PAC) measurements on¹⁰⁰Rh and¹¹¹Cd in a number of dilute alloys of Cu, Ag and Au. We discuss the electric field gradient (EFG) values derived from these and other experiments in the framework of a theory of the EFG at nearest neighbours of point defects in metals, essentially based on the screening of a point charge in a free electron gas. We include in our discussion the EFG due to a neighbour monovacancy for a number of fcc metals.     

Angular dependence of the PAC spectrum in a quenched single crystal of cadmium

To examine the orientation and magnitude of the electric field gradient (EFG) created by a vacancy at the¹¹¹In probe site, PAC spectra were measured in a Cd single crystal as a function of the crystal orientation after a quenching and aging treatment at 120 K. The results are compared with calculated spectra based on a point charge model of the EFG created by a vacancy at the site of the probe atom.     

Measurement of Quadrupole Interactions in LaMO3 (M = Cr,Fe, Co) Perovskites by TDPAC

The perturbed angular correlation (PAC) technique has been used to study the electric field gradient (EFG) in LaCoO₃ perovskite. The results are compared with those for LaCrO₃, LaFeO₃ measured earlier. The PAC probe, ¹¹¹In → ¹¹¹Cd, was introduced in the oxide lattice by means of chemical reaction during sample preparation. In the present work, the temperature dependence of the electric quadrupole interaction parameters, for LaCoO₃ was investigated. The resulting systematics of EFG at ¹¹¹Cd, in La(Cr,Fe,Co)O₃ perovskites, reveals a linear dependence with temperature. This revised version was published online in September 2006 with corrections to the Cover Date.     

Hyperfine interactions in 111Cd-doped lutetium sesquioxide

We report here first Perturbed Angular Correlation (PAC) results of the electric field gradient (EFG) characterisation at ¹¹¹Cd impurities located at both non-equivalent cation sites of the bixbyite structure of Lutetium sesquioxide, between room temperature (RT) and 1273 K. The comparison with results coming from a systematic ¹¹¹Cd PAC study in bixbyites and with point-charge model (PCM) predictions shows the presence of a trapped defect at RT in the neighbourhood of the asymmetric cation site, which is completely removed at T > 623 K. The anomalous EFG temperature dependence in Lu₂O₃ can be described in the frame of a “two-state” model with fluctuating interactions, which enables the experimental determination of the acceptor energy level introduced by the Cd impurity in the band-gap of the semiconductor and the estimation of the oxygen vacancy density in the sample. This revised version was published online in August 2006 with corrections to the Cover Date.     

Search for magnetic interaction in In doped AlN using perturbed angular correlation spectroscopy

The possible presence of a large magnetic field due to spin polarization of a Cd nucleus (decay product of ¹¹¹In) at an Al substitutional site in AlN is investigated with perturbed angular correlation (PAC) spectroscopy. The PAC spectra of ¹¹¹In/¹¹¹Cd in AlN show two probe environments: a weak quadrupole interaction (quadrupole interaction constant, $\nu _{\rm Q}^{\,\,\,\rm lattice} = 30$  MHz) due to ¹¹¹In probes at a defect free Al substitutional site and an unknown large interaction ( $\nu _{\rm Q}^{\,\,\,\rm complex} = 300$  MHz) tentatively attributed to a nearest neighbour pair between ¹¹¹In and a nitrogen vacancy (V_N) aligned along the c-axis. Surprisingly, in density functional theory (DFT) calculations, such a large electric field gradient (EFG) could not be reproduced. However, an inclusion of spin polarization in the calculations indicates a strong magnetic field at ~50 % of the ¹¹¹In/¹¹¹Cd site. An attempt to verify the presence of the strong magnetic field and to explain the origin of the strong interaction is made. Orientation measurements show, the large interaction is not characterised by a magnetic interaction and is predominantly due to the EFG. However, in the presence of an external magnetic field, the strong interaction probe environment becomes more uniform and the EFG increases by ~10 %. This definitely hints towards some sort of magnetic interaction at the strong interaction probe site.     

PAC Investigation of Amorphous Ferromagnets

The amorphous ferromagnetic alloy Fe79/B16/Si5 was investigated by PAC. For that the probe atoms ¹¹¹In(¹¹¹Cd) and ^111mCd were implanted at the ISOLDE on-line separator. The distributions of magnetic hyperfine fields were determined. Below the crystallization temperature α-Fe clusters could be observed. For comparison Mössbauer transmission experiments were performed.     

Effect of Mg2+ on the Magnetic Compensation of Lithium–Chromium Ferrite

PAC measurements on ¹¹¹In(¹¹¹Cd) implanted and thermally treated α-Fe have shown an indication for a cubic defect with the ¹¹¹Cd probe in the centre of it. The measured room temperature (R. T.) magnetic hyperfine fields are B _hf1 = −38.4(8) T for substitution and B _hf2 = +11.5(3) T for the cubic defect. Additionally, probes with pure quadrupole frequency distributions were observed, which are incorporated in surface contaminations.     

Calculated Electric Field Gradients and Electronic Properties of Acceptors in CdTe

Linearised augmented plane wave (LAPW) calculations are performed in order to determine the electric field gradients (EFG) induced by group V and group Ib acceptors in CdTe at the nearest neighbour (NN) atomic site. Experimentally, the EFG are measured by the perturbed γγ-angular correlation spectroscopy using the radioactive probe isotopes ¹¹¹In and ⁷⁷Br. Besides the identification of the experimentally observed defects, the LAPW calculations provide information about fundamental properties of the EFG. For the group Ib acceptors, the dependence of the EFG on the NN distance is opposite to the expectation from the simple point charge model. This revised version was published online in September 2006 with corrections to the Cover Date.     

Precision PAC measurements in Er2O3 and Ho2O3 single crystals and structure refinement

The structure of C-form Ho₂O₃ and Er₂O₃ single crystals and powder samples was investigated by the electric quadrupole hyperfine interaction of¹¹¹In(EC)¹¹¹Cd probe ions using the perturbed - angular correlation method (PAC). The resulting set of refined atomic coordinates is compared to X-ray data and used to calculate the orientations of the electric field gradients (EFG) which are reproduced by the PAC measurements in single crystals. The temperature dependence of the coordinates was measured for both substances.     

Site occupation of indium and jump frequencies of cadmium in FeGa<Subscript><Emphasis Type="Bold">3</Emphasis></Subscript>

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 ω ₁= 513.8(1) Mrad/s and η= 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 ±0.1 eV.