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The magnetic and hyperfine properties of iron impurities in 3C- and 6H- silicon- carbide are calculated using the abinitio method of full-potential linear-augmented-plane- waves. The iron atoms are introduced at substitutional carbon, FeC, and silicon, FeSi ,s ites as well as at the tetrahedral interstitial sites with four nearest neighbours carbon atoms, FeI (C), and four nearest neighbours silicon atoms, FeI (Si). The effect of introducing vacancies at the neighbours of these sites is also studied. Fe atoms with complete neighbors substituted at Si or C sites are found to be nonmagnetic, while Fe atoms at interstitial sites are magnetic. Introduction of a vacancy at a neighboring site reverse the picture.

119Sn Mossbauer measurements have been made on a 3C-SiC single crystal implanted with 60 keV precursor radioactive 119In ∗ ions at ISOLDE/CERN. Spectra collected at sample temperatures of 300–670 K have been analysed in terms of two single lines and a quadrupole split doublet, which based on their isomer shifts are assigned respectively to Sn ions located on substitutional Si sites (SnSi) and interstitial sites (SnI) and in defect complexes near substitutional sites. The substitutional SnSi fraction increases from 25% at room temperature to 60% at 680 K.

Films of ytterbium doped with iron (0.3–1.6 at.% Fe) have been prepared by co-deposition of atomic beams of the elements onto Kapton substrates under high vacuum. Iron is immiscible in ytterbium and various iron species are expected to occur. XRD reveals a mixture of fcc and hcp ytterbium. The Mossbauer spectra are interpreted with contributions from monomeric iron on interstitial sites, two types of neighboring substitutional iron aggregates and two types of clusters formed upon diffusion. The present data supplement earlier data obtained for 0.5 and 5 at.% Fe. There is no systematic correlation of cluster formation with increasing iron concentration. There are indications that clusters are formed more easily for thin films. We conclude that cluster formation occurs mainly on the film surface and is limited by the slow diffusion from inside films.

Ion implantation provides a precise method of incorporating dopant atoms in semiconductors, provided lattice damage due to the implantation process can be annealed and the dopant atoms located on regular lattice sites. We have undertaken 57Fe emission Mössbauer spectroscopy measurements on GaAs and GaP single crystals following implantation of radioactive 57Mn + ions, to study the lattice sites of the implanted ions, the annealing of implantation induced damage and impurity-vacancy complexes formed. The Mössbauer spectra were analyzed with four spectral components: an asymmetric doublet (D1) attributed to Fe atoms in distorted environments due to implantation damage, two single lines, S1 assigned to Fe on substitutional Ga sites, and S2 to Fe on interstitial sites, and a low intensity symmetric doublet (D2) assigned to impurity-vacancy complexes. The variations in the extracted hyperfine parameters of D1 for both materials at high temperatures (T > 400 K) suggests changes in the immediate environment of the Fe impurity atoms and different bonding mechanism to the Mössbauer probe atom. The results show that the annealing of the radiation induced damage is more prominent in GaAs compared to GaP.

External bending stress was applied to a 57Fe-doped n-type silicon wafer, and transmission Mossbauer spectra were measured at room temperature in order to observe directly stress induced diffusion of 57Fe atoms in the silicon matrix. As the stress is increased up to 23 MPa, the resonance area of the substitutional Fes component is found to decrease by more than 30%. In addition, a line broadening can be clearly seen in the interstitial Fe $_{\rm i}^{0}$ component, corresponding to a jump frequency of 106 s − 1. This diffusion process is interpreted to be due to the fast jumps between interstitial sites, following a formation jump of interstitial Fe $_{\rm i}^{0}$ from substitutional Fes.

Radioactive 57Mn+(T 1/2 = 1.5 min) ions have been implanted at the ISOLDE facility at CERN with 60 keV energy to fluences

The effect of different milling conditions on the formation of Fe-doped TiO2 powders by mechanical alloying was investigated by Mossbauer spectrometry. The milling conditions investigated were ball to powder weight ratio, milling time, rotation velocity of supporting disc, and the type of starting reactive iron and its concentration. X-ray diffraction shows that high energy mechanical milling of undoped anatase TiO2 induce the anatase to rutile phase transformation via high pressure srilankite. Mossbauer spectra for the majority of the doped samples were decomposed into one sextet and one or two doublets. The sextets was attributed to the presence of α-Fe or hematite impurities. The doublets were assigned to Fe3 + incorporated in the TiO2 structure, and to the Fe2 + located either at the surface or the interstitial sites of TiO2. A greater incorporation of Fe in the TiO2 structure was observed when samples were prepared from hematite instead of α-Fe.

The β-NMR of short-lived β-emitter 12N(I π = 1+, T 1/2 = 11ms) in a rutile TiO2 single crystal has been measured as functions of temperature and external magnetic field. Atomic motion induced spin lattice relaxation was observed for two known sites, O substitutional and interstitial sites. The data were analyzed in terms of the thermal atomic jump, which suggests that the motion of defects around the substitutional 12N atom for O, and of the interstitial 12N atom are attributed to the spin lattice relaxation. The electric field gradients have shown temperature dependence for both sites, which is probably due to the thermal expansion of rutile.

Radioactive 57Mn isotopes have been implanted into Si1−xGex crystals (x ≤ 0.1) at elevated temperatures for Mossbauer studies of the diffusion of interstitial 57Fe daughter atoms. The atomic jump frequency is found to increase upon Ge alloying. This is attributed to a lowering of the activation energy, i.e. the saddle point energy at hexagonal interstitial sites with Ge neighbour atoms.

By studying the quadrupolar interaction of 12B in silicon-germanium mixed crystals with the β-NMR method, the boron-germanium pair was identified and the saturation amplitudes for boron in differently composed crystals were measured. The relative saturation amplitudes agree with statistical predictions. At low temperatures boron is preferentially implanted into stable interstitial sites. These sites are converted into substitutional sites by diffusion processes which take advantage of reorientation jumps.

A thermodynamic model for site preferences of solute atoms in ordered binary phases is applied in this paper for substitutional and interstitial sites of the Cu3Au, or L12, structure. Site preferences are found to depend on formation energies of combinations of elementary point defects and on energies for transfer of solutes among different sites. The composition dependence in compounds having a wide phase field is examined in detail. A phenomenology of site preference behavior is outlined for regular lattice sites as well as for non-lattice sites such as grain boundaries.

The study of trapping of He+ ion in solid hydrogen is important both as a problem in solid state physics and also as an applied physics problem in the field of muon catalyzed fusion (μCF). In μCF, He+ ion acts as a trap for μ−, interrupting the chain reaction aspect of the catalytic role of μ− in producing fusion of deuteron and triton and of triton and triton in solid hydrogen composed of 2H–3H and 3H–3H molecules, respectively. Using the Hartree–Fock procedure, combined with procedures for including many-body effects, as well as relaxation effects associated with the He+–H2 distances and the adjustment of the H–H separation, we have investigated the trapping of He+ in gaseous and solid state environments. For the former, the environment of He+ is simulated by a single hydrogen molecule and for the solid by clusters appropriately chosen to represent the hexagonal close-packed structure. Our results for the gaseous state indicate that the trapping is rather strong with a binding energy of 8.5 eV, with almost equal binding energy in the linear and triangular configurations with respect to the H–H direction. For the solid, both the likely sites for He+ trapping, namely the tetrahedral and octahedral interstitial sites, are also found to provide deep traps (8.6 eV) of almost equal strength, independent of the orientations of the neighboring molecules, showing that the trapping is not influenced by the orientational disorder in the surrounding hydrogen molecules. Further, the influence of next nearest neighbor hydrogen molecules is found to enhance the trapping energy for He+ substantially, by 0.6 eV, with the incorporation of the third nearest neighbors having a much smaller added effect, demonstrating the convergence of our results with respect to the size of the cluster chosen to simulate the solid. The substantial influence on the He+ trapping energy found for the neighbors beyond the nearest ones provides an explanation of the greater accumulation of helium in the solid state of hydrogen in μCF experiments as compared to the liquid. Suggestions are made regarding the possible reasons for the almost negligible accumulation of helium in the liquid state.

TDPAC study of InBi1−x Te x with x=0.0, 0.01, 0.05, 0.1 and 0.2 as a function of temperature is done. Both pure and Te-doped samples show a semi-metallic character up to around room temperature. Above room temperature a steep transition to semiconducting state was observed, which is independent of the doping concentrations. Hence Te may be in the interstitial sites. Conductivity measurement of the above samples showed the same trend as observed in the EFG studies. Thus the importance of the conduction electron density is prominently seen in these systems.

The properties of the ZrCo-hydrogen system as a function of composition ratio H/ZrCo were investigated by combining TDPAC and X-ray diffraction techniques with the full potential linearized augmented plane wave (LAPW) band structure calculations. It was found that during hydrogenation ZrCo transferred directly into ZrCoH3 and no transition phase existed. The z component Vzz of the efg and the asymmetry parameter η at the Zr and Co sites were calculated for ZrCoH3 by using the structure parameters of Zr and Co, obtained in the present work, combined with the hydrogen structure parameters obtained from neutron diffraction on ZrNiH3. The calculations are in good agreement with the measured values for Vzz and η. This agreement is consistent with the proposed 4c and 8f interstitial sites occupancy of the hydrogen atoms in ZrCoH3.

We report on the measurements of the muon relaxation rate in gadolinium (Gd) in the critical regime down to T-TC=0.1 K. The results are compared to the predictions of mode‐coupling calculations for a uniaxial and dipolar ferromagnet. One observes that even if the uniaxiality in Gd is quite small, it should not be neglected in order to get a consistent description of our data. Most striking is the fact that these investigations support the assumption that above TC the muon resides on the tetrahedral interstitial sites and not on the octahedral sites which are chosen in the low temperature phase.

Muon spin rotation (μSR) data on a single crystalline sample of Dy have been obtained as function of temperature and applied hydrostatic pressure. The discontinuity observed in our new data at the phase transition at 85 K strongly suggests that the muons occupy tetrahedral interstitial sites, with a contact field BC=-2.5 T at the lowest temperatures.

Transverse and zero‐field muon spin relaxation reveal several diamagnetic muonium states in InP characterized by their static linewidths and diffusion properties. We tentatively associate low‐temperature diamagnetic states with Mu+ in the BC and TP interstitial sites and a missing fraction with Mu0 rapidly diffusing through TIn interstices. Trapping peaks above 250 K imply static centers which depend on doping type, consistent with Mu- at TIn for n‐type samples and Mu coupled with a dopant or other defect for p‐type.

We have utilized the electron-capture decay of119 Te to119Sb to produce isolated single Frenkel pairs in InSb. This effect is caused by the neutrino emission in the decay process which imparts a monoenergetic recoil of 12 eV to the119Sb atoms, thereby displacing about 20% of them to interstitial sites. Two distinct interstitial components can be observed. The process is traced by Mossbauer emission spectroscopy following the decay of119Sb to119Sn. The displacement thresholdE d is confined to 6 eV

57Fe- and119Sn-Mossbauer effects become a very effective tool for the local structural study of inorganic glasses. Fe3+ and Sn4+ occupy network former (NWF) sites in several oxide glasses, while they are present at interstitial sites as network modifier (NWM) in phosphate, germanate and sulfate glasses. Characteristicγ-ray or thermal neutron irradiation effect indicates the structural role of Mossbauer ions; an increase in the 4s electron density or a reduction of Fe3+ to Fe2+ is observed due to the charge transfer from oxygen to Fe3+ when iron occupies NWF sites. By contrast, oxidation of Fe2+ to Fe3+ takes place when iron occupies NWM sites. Debye temperatures (Θ D) obtained from low-temperature Mossbauer measurements are higher than 280 K when Fe3+ and Sn4+ are covalently bonded to oxygen at NWF sites, while theΘ D is lower than 270 K when these ions are ionically bonded to oxygen or halogen at NWM sites. A linear relationship between the glass transition temperatureT g and the quadrupole splitting (Δ) of Fe3+, named “Tg-Δ rule”, also indicates the structural role of Fe3+, i.e.,T g versusΔ plot yields a large “slope” of 680°C/mm s−1 when Fe3+ occupies NWF sites, whereas it is only 35°C/mm s−1 when the iron is present at NWM sites.

The119Sn-Mossbauer spectra ofxSnO·(70−x)SnF2·30P2O5 glasses (0 ≤x≤70) measured at 78 K comprised a doublet due to Sn2+ (δ=3.30–3.36 mm s−1, Δ = 1.70–1.72 mm s−1) and a weak singlet due to Sn4+ located at −0.23 mm s−1 with respect to BaSnO3. The δ and Δ of Sn2+ were comparable to those of Sn2P2O7. Small Debye temperatures (146 and 155 K) were obtained from the low-temperature measurements. These results indicate that Sn2+ and Sn4+ occupied interstitial sites, being loosely and ionically bonded to distorted PO4−xF x tetrahedra.

We have calculated distributions and lifetimes of positrons in the infinite-layer compound SrCuO2 and those trapped at possible point defects therein. In the delocalized state, positrons show their density maxima at interstitial sites in the Sr planes and have a significant overlap also with Cu and O atoms. The corresponding positron lifetime is 149 ps. It has been revealed that the Sr vacancy strongly localizes positrons with the binding energy of 2.8 eV and the lifetime of 238 ps, while the O vacancy does not trap positrons. Calculations are also performed on related materials Sr2Cu4O6 and Sr4Cu6O10, which are characterized by one-dimensional networks of edge-sharing CuO4 squares. Positrons are predominantly distributed between these networks in these materials and their corresponding lifetimes are 170–171 ps.

In-beam Mossbauer spectroscopy (IBMS) is used to study single isolated57Fe impurities after implantation in metals and semiconductors with very restricted or even vanishing solubility for Fe. From the Mossbauer parameters it can be inferred that the Fe implants take up substitutional as well as interstitial sites. The strongly increased electron density at the interstitial position in metals is qualitatively explained by the pressure resulting from the small interstitial volume. In Si, Sc and Pb exponential line broadening due to interstitial diffusion has been observed. Additional information on the dynamic behavior and local magnetic structure in some of the systems presented comes from perturbed angular distribution experiments (PAD) performed on an isomeric state of54Fe.

Super-cell band structure calculations with the Korringa-Kohn-Rostoker method in the framework of the local-spin-density approximation are performed on ferromagnetic iron with typical elements (B, C andN) at the octahedral interstitial sites for the purpose of studying hyperfine fields of light interstitials. Lattice relaxations around the interstitial atoms are allowed in these calculations. Calculated hyperfine fields of these interstitial impurities are in better agreement with experimental values than the results obtained previously for unrelaxed lattices, showing that the inclusion of the lattice relaxation is crucial in these systems.

In-beam Mossbauer spectroscopy is applied to study implanted Fe atoms in the alkali metals Li, Na and K. From the Mossbauer parameters we infer that the Fe implants take up substitutional as well as interstitial sites. The strongly increased electron density at the interstitial position is qualitatively explained by the pressure resulting from the small interstitial volume. It is concluded that recently reported local moment formation in these systems cannot be due to substitutional Fe only.

Fe has been studied in the semiconductors Si and Ge with the Coulomb excitation recoil implantation technique in a wide temperature range. In the case of Fe inSilicon it was found that one third of the implanted atoms land on interstitial sites. The long range diffusion of these atoms could be observed microscopically at temperatures around 600 K. The isomer shift of interstitial Fe in Si was determined. The remaining atoms exhibit a strong quadrupole splitting on disturbed sites. This component seems to relax into a state with higher symmetry above 700 K. InGermanium a similar situation is found. Whereas iron on disturbed sites dominates the spectra, the direct implantation into interstitial sites is also observed below 200 K. At higher temperatures the substitutional position is preferred. The isomer shifts for interstitial and substitutional Fe in Si and in Ge are in good agreement with calculated electron densities.

The lattice sites of positive pions in gold have been determined as a function of temperature by means of π+/μ+ channelling experiments. A blocking pattern in the direction at low temperatures indicates that only octahedral interstitial sites are occupied. The vanishing of this pattern and the reduction of muon flux enhancement observed in ifT is raised above the Debye temperature (165 K) may be interpreted in terms of muon dechannelling owing to the thermal vibrations of the lattice atoms.

First principles calculations of the properties of hydrogen and muonium in silicon are presented. H+ and H− are shown to have definite preferences for bond-centred and tetrahedral interstitial sites respectively whereas H0 (or a muon) is shown to be stable at two sites with almost equal energies, the bond-centred and antibonding sites. The structures of normal and isotropic muonium are discussed. In contrast to common belief the tetrahedral site is shown to be unstable with the muon moving spontaneously towards one of the neighbouring silicon atoms. The barrier to motion between equivalent antibonding sites is low suggesting that the normal muonium signal is isotropic because of motional averaging, not due to the symmetry of a well defined equilibrium site.

The spin-precession frequencies and the transverse spin relaxation rates of positive mouns (μ+) have been measured on two elastically strained α-Fe single crystal platelets as well as on an unstrained reference α-Fe crystal at temperatures down to 2.7 K in applied magnetic field 0≤Bappl≤3 T. The drastic effects of the strains may be qualitatively understood in terms of their influence on both the magnetic domain structure and theμ+ energies at the various interstitial sites. This leads to the conclusion that at low temperaturesμ+ in α-Fe occupy configurations related to octahedral interstitials with dipolar fieldBdip=0.70 T.

Multi-frequencyμ+ SR spectra were observed in PrNi5 at 23 K in an external transverse field of 3010 G. Three types of interstitial sites are identified as the most likely muon stopping positions. A substantial amount of the magnetic moment sampled by the μ is found to be localized on the Ni sites.

The electronic structure of a hydrogen-like atom located at interstitial sites of the silicon and diamond crystals is calculated by the intermediate neglect of differential overlap (INDO) method. Calculations of the electronic g- and hyperfine interaction tensors of the impurity atom are performed. The results obtained are compared with the experimental properties of both “anomalous” muonium and hydrogen centers. It is shown that the most likely model for these centers in silicon and diamond is that in which interstitial neutral hydrogen-like atom locates at the bond-center site.

Cross-relaxation resonances of the polarization of12B nuclei in single-crystal Vanadium were observed at various values of the external B field. From the comparison with calculated relaxation spectra the electric field gradients at the neighbouring host nuclei were determined and the locations of12B were found to be the substitutional and the octahedral interstitial sites.

Positive muons are expected to be easily trapped by imperfections including impurities in metals (Seeger 1975). Therefore it is essential to study muon diffusion and localization on well-characterized high-purity materials. The sensitivity to impurities has been confirmed by recent ~SR studies on Nb (Birnbaum et al. 1978, Borghini et al. 1978). In this note we report results of DSR experiments on high-purity monocrystals of beryllium, tantalum, and niobium. Starting from high-purity material, the Be target was vacuum melted and zone-refined i0 times resulting in a residual resistivity ratio F ~ R2g3K/R4.2K = 400, corresponding to a purity of 99.993 wt% (Aldinger 197~).0f the remaining 70 wt.ppm impurities, the principal constituent was iron. The preparation of the Ta (F = 5000, interstitial C+O+N

μ+ SR-measurements in transversally applied magnetic fields of 2000 G and 4000 G on heavy-electron single crystal U2Zn17 are presented. They reveal that at least two types of interstitial sites are occupied by the positive muons. One of these sites (1/3, 2/3, 5/6) could be identified via induced local dipolar fields which aboveTN=9.7 K can exactly be derived from the magnetic susceptibility. The corresponding component of the μ+-signal exhibits a steplike decrease by about 40% atTN which is caused by the onset of a very broad distribution of static internal magnetic fields (ΔB≈1000 G) with zero average. Such a field distribution is in distinct contrast to dipolar-field calculations performed for the simple antiferromagnetic structure deduced from neutron diffraction. The remaining 60% of the muons contributing to this component belowTN are subject to a narrow static field distribution (ΔB≈1 G). The induced dipolar fields at the site (1/3, 2/3, 5/6) are temperature-independent belowTN. A weak dipolar coupling to the U-moments renders similar observations for muons occupying the second type of interstitial impossible.

The motion of a particle in a metallic crystal is studied for low temperatures where transitions between adjacent interstitial sites are caused by quantum tunneling. The influence of electrons and phonons on the hopping rate is taken into account by means of a functional integral method. The electronic influence may effectively be described by Ohmic damping which dominates the low temperature behavior of the defect motion. When subsequent tunneling transitions are statistically independent, the diffusion constant is found to obey a power law, D∼T2K−1, where K depends on the defect-electron interaction. This power law is limited at low temperatures by the effects of phonon excitations. Near the transition between electron and phonon dominated behavior the diffusion constant has a minimum where the precise temperature dependence of the rate depends not only on phonon spectra but also on the processes limiting phonon lifetimes.

Light elements like hydrogen, nitrogen, and carbon, residing on interstitial sites of the host lattice, are capable of inducing noticeable changes in the physical properties of the material affected by their presence. Hydrogen uptake by different intermetallic compounds can lead to completely different results, depending on the chemical affinity between hydrogen and the constituent elements of the compounds. In the cases of considerable hydrogen concentrations, several distinct hydride phases can usually be found and identified by Mossbauer spectroscopy. Alterations of the magnetic behaviour of the intermetallic hydride are frequently observed. Nitrogen and carbon serve as agents for solid solution and precipitation hardening in commercial steels. Among the various methods for alloying with these elements and for controlling their distribution, implantation and surface treatment by use of a laser beam are of particular interest.

The orientation dependence of the inhomogeneously broadened μSR line width has been measured in a single crystal α′-PdH0.74 sample between 20 and 150 K. Protons and muon are thought to occupy octahedral interstitial sites in the Pd lattice. A good fit to the data is only obtained assuming a decrease in the proton occupancy of the octahedral sites nearest to the μ+ with increasing temperature. The proton occupancy of the remaining octahedral sites at all temperatures as well as the occupancy of the proton sites nearest to the μ+ at 20 K is compatible with the bulk nominal hydrogen concentration x=0.74.

Owing to the staggered sequence of hexagonal layers in the rhombohedral crystal structure of Bi two different interstitial sites are in principle available to the μ+. It is found that below 10 K the μ+ is localized at one of these sites while above 80 K up to 110 K the μ+ is localized at the other site. In both instances a latticecontraction around the μ+ is observed, amounting to a reduction of 12% at 8 K and 7% at 100 K in the nearest Bi μ+ distances. At both sites the second moments reveal strong quadrupole interaction effects, which dominate at 8 K in fields up to 0.746 T but which are slightly quenched at this field strength at 100 K.

The effects of symmetric lattice relaxation around the tetrahedral (T) and the hexagonal (H) interstitial sites are calculated for normal muonium (Mu) in diamond using the non-semiempirical method of Partial Retention of Diatomic Differential Overlap (PRDDO). We use clusters containing 3 and 4 host atom shells around the T and the H sites (C20H32 and C30H40). The only significant relaxations are the outward displacement of the 6 nearest neighbors (NN) to the H site and, when the muon is at the T site, the simultaneous outward displacement of the first and second NN to the T site. In the case of diamond, the effects of these relaxations on the energy profiles and spin densities are small.

It is discussed that applicability of the diffusion approximation depends critically on the magnitude of the phonon-induced incoherency between muon levels in adjacent interstitial sites. By means of two examples it is demonstrated that the temperature below which the diffusion approach breaks down may be different for different physical situations.

An in-beam Mossbauer spectrometer was set up at the VICKSI heavyion accelerator in Berlin. Coulomb excited Mossbauer nuclei can be implanted into arbitrary sample materials. Measuring temperatures may be varied from 5 K to 650 K. A typical scpectrum shown in this work can be measured within a few hours.57Fe was implanted into metals having large atomic volumes: Al, α-Zr, β-Sn. In these matrices the strongly undersized iron atom tends to occupy interstitial sites and may undergo dynamic processes.

We have studied by transverse field positive muon spectroscopy μ+SR, the muon diffusion in V3Si. We found that the muon is static and localized at tetrahedral interstitial sites below 200 K. Above 200 K the muons diffuse with an activiation energy 2550 (220) K. The nature of this diffusion process is discussed.

Hyperfine fields of impurities of the atomic number Z=1–56 at the substitutional site and those of light impurities of Z=1–9 at the interstitial sites in ferromagnetic iron are calculated by the KKR method adapted to the system containing a single impurity atom. The potential of the impurity atom is determined self-consistently by use of the local spin density functional formalism. The results for nonmagnetic sp valence impurities agree with those of the previous nonself-consistent calculation by Katayama-Yoshida, Terakura and Kanamori except for a few cases, confirming their theory of the systematic variation of hyperfine fields. The calculation for magnetic impurities of transition elements is presented for the first time in this paper. The calculations mentioned so far assume that impurities are situated at the center of each site. For the purpose of discussing the stability of the impurity positions, the change of the adiabatic potential due to displacements from the center is calculated by carrying out similar self-consistent calculations for off-center impurity positions. It is concluded that positive muon and some light impurities including boron will be displaced from the center when trapped in a vacancy.

Using a nonlinear-screening approach the potential energy of positively charged light interstitials in Cu and lattice distortions around these particles have been determined numerically. The calculations predict a marked ‘isotope’ effect for the lattice distortions around positive muons and protons at octahedral interstitial sites. They indicate that in Cu the diffusion mechanisms for positive muons and for hydrogen isotopes may be different.

The positive muon is widely used as a microscopic probe of internal fields at interstitial sites in magnetically ordered materials. Recently, we have demonstrated that the hyperfine fields on the neighboring host nuclear spins can be measured using a novel muon level-crossing resonance technique, thus providing a more detailed picture of the electronic and magnetic environment around the muon. In this paper I will describe the fundamentals of muon level-crossing resonance as applied to magnetically ordered materials, and report an example in MnF2.

With the aim of obtaining direct information on the sites occupied by positive pions (π+), channelling experiments have been performed on the decay muons (μ+) from π+ implanted into high-purity copper crystals. At 100 K virtually exclusive occupancy of octahedral interstitial sites (0) is found in good accordance with earlier μSR results. At room temperature a significant fraction of the implanted pions decay on metastable tetrahedral interstitial sites (T). The data suggest strongly that above about 100 K both μ+ and π+ diffuse by phonon-assisted tunnelling between stable 0 sites and metastable T sites. The enthalpy difference between these two sites is estimated to be about 50 meV for μ+ and of the same order of magnitude for π+. These values are much smaller than the available theoretical predictions. The implications of low-lying metastable interstitial sites for the μ+ stopping sites of low temperatures are discussed.

With this in mind, we present and discuss the results on hyperfine fields at positive moons in the ferromagnetic metals Ni, Fe, Co and Gd. In Co and Gd one has to consider - besides the hyperfine field due to the spin polarized interstitial electrons - dipolar contributions from the local magnetic moments of the host. Recent Knight shift measurements in ferromagnetic and paramagnetie Ni allow some conjectures on the origin of the interstitial magnetization. In this contribution we will discuss the application and the behavior of positive muons in metals. We are interested a) in the electronic structure of the implanted point-impurity, and b) in the internal magnetic fields that are present at the muon sites. The first item is a problem of general interest which also applies to the case of hydrogen in metals, the second item is concerned with the magnetic and electronic properties of the host lattice about which one hopes to obtain information by the use of the p SR [muon spin rotation] method I) The attractive features of the applicability of polarized positive muens rest in the facts that muons can be implanted easily in any metal, that it is present in infinite dilution and that it does not possess a complicated electron core. Positive muons, implanted in metals, seem to reside in the interstitial sites in the metbl lattice. They will interact electrostatically with the conduction electrons by which the positive charge is screened over some distance comparable with the radius of a hydrogen atom. There will be a magnetic hyperfine interaction of the Fermi contact type with the conduction electrons. In a non-magnetic metal this will cause the Knight shift. In a ferromagnetic metal, where the conduction electrons are spin polarized, a large hyperfine field at the muon is to be expected in addition to possible dipolar fields from the magnetic ion cores of the host lattice. In e magnetic alloy the internal fields may vary from site to site causing a dephasing of the muon precession. The hyperfine field at a positive muan may most generally be

The electronic structure of muonium (Mu) located at different interstitial sites of the silicon crystal is calculated by the complete neglect of differential overlap (CNDO) and intermediate neglect of differential overlap (INDO) methods. Calculations of the electronicg- and hyperfine interaction tensors of the impurity atom are performed. The results obtained are compared with the experimental properties of both “normal” (Mu′) and “anomalous” (Mu*) muonium centers. It is shown that the most likely dynamic model for Mu′ is that in which neutral Mu diffuses rapidly in the silicon lattice, whereas for Mu* it is the model wherein interstitial Mu is located at the bond-center site. A correlation is made between the characteristics of the hydrogen-bearing Si-AA9 center, recently observed by EPR in a silicon crystal, and those of Mu*. The Si-AA9 center is shown to be a hydrogen-bearing paramagnetic analogue of the Mu* center.

On polycrystalline and monocrystalline iron muon-spin precession frequencies and transverse relaxation rates have been measured down to 0.5 K. In the polycrystalline sample two distinct precession frequencies were observed at and below 1.4 K. They are attributed to the different dipolar fields at magnetically inequivalent tetrahedral interstices seen by muons moving locally around impurities. By contrast, in monocrystalline iron we observed only one precession frequency in monocrystalline iron with a damping rate which increased with decreasing temperature down to 0.5 K. We attribute the difference between the monocrystalline and the polycrystalline sample to different impurity contents. The single-crystal data are discussed in terms of μ+ diffusion by hopping between interstitial sites of tetragonal symmetry. The answer to several open questions is expected from an extension of the measurements to lower temperatures.

The ternary hydride phases ErFe3Hx (with x=1.5 and 2.7) has been studied by57Fe Mossbauer effect. The spectra show that the spin reorientation temperature is increased by the presence of hydrogen in the ErFe3 lattice and is independent of hydrogen concentration for the two hydrides investigated. A preferential filling of the interstitial sites in the RFe3 structure is indicated by the data.