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We report first-principles calculations of the structure and electronic structure of nitrogen-doped TiO2 anatase as a function of the dopant depth below the (101) surface. Specifically we evaluate the depth dependence of the formation energy for a few positions of the N impurity, considering for both substitutional and interstitial sites. We find a significant advantage of interstitial over substitutional positions, and a mild dependence of this formation energy on depth. The lengths of the bonds surrounding the impurity also evolve smoothly with depth. Regarding the electronic structure, we report the main features of the intragap impurity states and the hole-related spin magnetization density surrounding the N impurity.

Recent investigations on spinel CoMn2O4 have shown its potential for applications in water splitting and fuel cell technologies as it exhibits strong catalytic behavior through oxygen reduction reactivity. To further understand this material, we report for the first time the synthesis of single-crystalline Co1+x Mn2−x O4 thin films using molecular beam epitaxy. By varying sample composition, we establish links between cation stoichiometry and material properties using in-situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, x-ray absorption spectroscopy, and spectroscopic ellipsometry. Our results indicate that excess Co ions occupy tetrahedral interstitial sites at lower excess Co stoichiometries, and become substitutional for octahedrally-coordinated Mn at higher Co levels. We compare these results with density functional theory models of stoichiometric CoMn2O4 to understand how the Jahn–Teller distortion and hybridization in Mn–O bonds impact the ability to hole dope the material with excess Co. The findings provide important insights into CoMn2O4 and related spinel oxides that are promising candidates for inexpensive oxygen reduction reaction catalysts.

Using a recently developed approach to represent ab initio based force fields by a neural network potential, we perform molecular dynamics simulations of lead telluride and cadmium telluride crystals. In particular, we study the diffusion of a single cation interstitial in these two systems. Our simulations indicate that the interstitials migrate via two distinct mechanisms: through hops between interstitial sites and through exchanges with lattice atoms. We extract activation energies for both of these mechanisms and show how the temperature dependence of the total diffusion coefficient deviates from Arrhenius behaviour. The accuracy of the neural network approach is estimated by comparing the results for three different independently trained potentials.

International audience; Overlap of collision cascades with previously formed defect clusters become increasingly likely at radiation doses typical for materials in nuclear reactors. Using molecular dynamics, we systematically investigate the effects of different pre-existing self-interstitial clusters on the damage produced by an overlapping cascade in bcc iron and tungsten. We find that the number of new Frenkel pairs created in direct overlap with an interstitial cluster is reduced to essentially zero, when the size of the defect cluster is comparable to that of the disordered cascade volume. We develop an analytical model for this reduced defect production as a function of the spatial overlap between a cascade and a defect cluster of a given size. Furthermore, we discuss cascade-induced changes in the morphology of self-interstitial clusters, including transformations between 1/2 and dislocation loops in iron and tungsten, and between C15 clusters and dislocation loops in iron. Our results provide crucial new cascade-overlap effects to be taken into account in multi-scale modelling of radiation damage in bcc metals.

The distribution of carbon and nitrogen atoms on the octahedral interstitial sites of the face-centred-cubic austenite phase in Fe–C and Fe–N alloys, especially in austenitic stainless steel, is still causing controversy. In this work, results of Mossbauer experiments are presented in order to advance the understanding of this interstitial occupation. Therefore, laser carburized and laser nitrided austenitic stainless steel was investigated by means of x-ray diffraction and Mossbauer spectroscopy. Three subspectra in terms of different iron sites were resolved in the Mossbauer spectra for these iron–carbon and iron–nitrogen austenites. The isomer shifts, the quadrupole splittings and in particular the subspectra fractions depend on the type of the introduced atom and undergo changes when increasing the carbon or nitrogen content. This is discussed in connection with the existing ordering models for interstitial atoms. No clear evidence could be found for a perfect random occupation, nor for a perfect ordered occupation of the interstitials. Nevertheless, there seems to a tendency for a weak attractive interaction for nitrogen interstitials, and for a stronger repulsive force for the carbon interstitials in laser nitrided/carburized austenitic stainless steel.

By using first-principles DMol and the discrete-variational method (DVM) based on density functional theory, we investigated the effect of some light impurities, H, B, C, N and O, on the electronic structure of their corresponding different impurity-doped systems in gamma-TiAl. The impurity formation energy, Mulliken occupation, bond order and charge density difference have been calculated to study the impurity-induced changes in the energetics and electronic structure. According to the impurity formation energy, it is found that the impurities energetically prefer to occupy the Ti-rich octahedron interstitial sites in the order H < B < O < N < C. Charge transfer suggests that the effect of the impurities is localized and the bond order as well as charge density difference results show that B, C and N are beneficial for the ductility of TiAl, while H and O are not.

KY3F10 (Fmm, Z = 8) is an anion-excess fluorite-related superstructure, which is employed as a room-temperature laser, when doped with rare-earths. Earlier conductivity measurements have revealed two thermally activated processes below and above 600 K. In this work we studied the high-temperature behaviour of KY3F10 in the temperature range from 293 K up to 800 K at ambient pressure using x-ray powde ra nd single crystal diffraction. No structural phase transition is observed in this temperature range and the structure determinations reveal that the change of structural parameters, interatomic distances, and angles with temperature is very small. A prominent maximum observed in the difference Fourier maps, which corresponds to the centre of an empty F8 cube, might be one of the interstitial sites involved in the conduction mechanism.

We have performed an ab initio study of the energetics, structural, electronic, and optical properties of the CH4-doped ultrathin 4 A and large diameter carbon nanotubes (CNTs). No adsorption of the CH4 molecule has been seen either on the groove or on the interstitial sites of the achiral 4 A nanoropes. In the case of weakly bonded systems such as the adsorption of the CH4 molecules on the nanotubes, a prominent role of the dispersion forces in the binding is observed. The negative contribution of the zero point vibrational energy to the binding energy is seen to be appreciable. The effects of the tube diameter and the different chiralities of the carbon nanotubes on the adsorbate-induced physical properties have been investigated. In the large diameter tubes, the binding of the CH4 molecule in the endohedral adsorption is much stronger than that in the exohedral adsorption. The binding of the CH4 molecules depends upon the chirality of the nanotube and we find no adsorption on the chiral (4, 2) tube. We find that the local density approximation (LDA) over-binds the CH4 molecule and the generalized gradient approximation (GGA) under-binds it, and for a reliable theoretical estimate one should take some weighted average of the binding energies (BEs) determined in the LDA and the GGA. The currently calculated BE and the adsorbate concentration are in reasonable agreement with the measured data available for the (10, 10) nanotubes. The electronic structure of the pristine tube is quite altered by the adsorption of the CH4 molecule on the surface of the tube because of the breaking of the symmetry of the host lattice except the chiral (4, 2) tube, which has practically no symmetry. The adsorption incurs splitting in the states in the whole energy range, especially in the large curvature 4 A tubes. The bandgap of the semiconducting achiral zigzag nanotube is reduced, whereas that of a chiral semiconducting tube is enhanced, by the adsorption of the CH4 molecules. The adsorption of CH4 molecules does not alter significantly the peak structure in the optical absorption of the pristine tube, except for some changes in the energy locations and the relative intensities in the achiral tubes. Most of the calculated peaks in the optical absorption of the pristine large diameter (10, 0) and (10, 10) nanotubes have been observed in the experimental measurements.

An exact algebraic expression is derived for the diffusivity for diffusion between interstitial e and g sites in the cubic C15 Laves phase AB2 in terms of four rates of jumping between the interstitial sites. The result is for the low concentration limit of diffusing atoms and is valid for arbitrary positions of the e and g sites. The expression reduces to simple forms in some special cases. Some examples of diffusion of H in C15 intermetallic compounds are discussed.

Site-specific spectroscopy is critical for a molecular-level understanding of the mechanisms and dynamics of the inhomogeneous chemical processes crucial for catalysis, surface and interfacial chemistry, and membrane protein dynamics in living cells. Surface-enhanced Raman scattering (SERS) spectroscopy and microscopy combined with atomic force microscopy (AFM) using metal-coated AFM tips have proven to be powerful in spectroscopic analysis of inhomogeneous processes, providing correlated topographic and spectroscopic information at the nanoscale from sites in highly heterogeneous environments. It has recently been observed that SERS spectral fluctuations are pertinent to site-specific spectroscopy and microscopy. Such fluctuations are important in that they hold promise for the study of molecular structure and dynamics at a single-molecule level. This article reviews our recent work on characterization and analysis of SERS spectral fluctuation dynamics at the nanoscale in metallic interstitial sites. Fluctuations were found to accompany nanoscale confined electromagnetic near-field enhancement. The result of such confinement is that only a few molecules dominate the far-field SERS spectral signal detected in microscopic measurements that probe one nanoscale 'hot' site at a time. The fluctuation amplitude significantly decreased with the number of molecules confined at the nanoscale-local field. A new AFM-coupled two-channel photon time-stamping system, enabling in situ correlation of the topographic and spectroscopic information for single nanoparticle clusters, was used to record the Raman intensity fluctuation trajectories at a sub-microsecond resolution. Experimentally, we found that SERS fluctuation dynamics are highly inhomogeneous amongst nanocluster interstitial sites although molecular translational and rotational motions at the interstitial sites can account for the SERS spectral fluctuations. To further understand these fluctuations at the nanoscale interstitial sites and nanostructures, field enhancement and field distribution at different interstitial site topographies and rough fractal surfaces were studied using finite element method computational simulation in a classic electrodynamics approach.

Experimental data for proton nuclear spin relaxation and diffusion of H in HfTi2Hx are analysed by simultaneously fitting the temperature-dependent relaxation and diffusion data with a common set of parameters. HfTi2Hx has the C15 structure with the H occupying the inequivalent interstitial e and g sites. The fitting of the relaxation data uses a rigorous theory of nuclear spin relaxation between inequivalent sites and makes no assumptions about which types of H jumps are significant for the relaxation. The diffusion data is fitted by developing the theory of diffusion between the inequivalent e and g interstitial sites, which enables the diffusivity to be calculated rigorously as a function of temperature from the H jump rates in the low concentration limit. Monte Carlo simulations are used to estimate the effect of diffusion correlation effects at higher H concentrations. Models for diffusion between inequivalent sites involve a large number of parameters and density functional theory (DFT) calculations are used to provide constraints on them. Good fits to both the relaxation and diffusion data are obtained for energy parameters that are close to those from the DFT calculations. A complete set of jump parameters for H between the interstitial sites is deduced which provides a detailed microscopic description of the diffusion as a function of temperature.

Ultrasonic measurements were made on the C15 Laves-phase material ZrCr2H(D)x over the temperature range 4–295 K for x(H) = 0.09, 0.15 and 0.31 and x(D) = 0.12. Attenuation peaks associated with H (D) motion between g-site hexagons were observed in all of these materials for measurement frequencies of approximately 1.5 MHz. A strong isotope effect was observed for similar concentrations of H and D and interpreted in terms of quantum mechanisms of diffusion. For temperatures below 200 K, the dominant diffusion mechanism appears to be tunnelling transitions between ground states of hydrogen in adjacent interstitial sites. Comparison with earlier ultrasonic and NMR results suggests that such phonon-assisted tunnelling between ground states is the dominant mechanism of long-range diffusion in many C15 Laves-phase compounds for temperatures below about 200 K. The elastic shear moduli of ZrCr2H0.09, ZrCr2H0.15 and ZrCr2D0.12 were also measured. A small shift was observed in the modulus for each material at a temperature corresponding to the relevant peak in the ultrasonic loss, which is consistent with the interpretation that these peaks are due to H (D) relaxation.

A neutron powder diffraction study of ErFe2D5, synthesized under 1 GPa hydrogen pressure, shows that it crystallizes at room temperature in an orthorhombic structure described by the Pmn21 space group with a = 5.42 A, b = 5.79 A, c = 8.00 A. The deuterium atoms order preferentially in some A2 B2 and AB3 interstitial sites. Below 5 K the erbium moments order in a canted magnetic structure, with an erbium moment of 6.6 μB at 1.4 K. The 57Fe Mossbauer spectra of ErFe2D5 from 4.2 to 300 K indicate that there are no ordered Fe moments at zero field. These results are discussed in relation to the influence of hydrogen absorption on the magnetic interactions.

The Si-B3 electron spin resonance (ESR) signal from agglomerates of self-interstitials was detected for the first time in hydrogen-doped float-zone-grown silicon samples subjected to annealing after electron irradiation. Previously this signal had been detected only in neutron- or proton-irradiated silicon samples. The absence of obscuring ESR peaks for the investigated samples at applied measurement conditions allowed an investigation of the hyperfine structure of the Si-B3 spectra. The analysis supports assignment of a tetra-interstitial defect as the origin of the signal.

The optical vibrations of hydrogen in TbNiAlH1.4 and UNiAlH2.0 were investigated by means of inelastic neutron scattering. The experimental data were analysed, including multiphonon neutron scattering contributions, calculated in an isotropic harmonic approximation. At least two fundamental H optical peaks were observed in TbNiAlH1.4, and were assigned to the vibrational modes of hydrogen atoms occupying different interstitial sites in the metal sublattice. The high-energy part of the UNiAlH2.0 spectra is characterized by strong anharmonicity, and a broad fundamental band. The latter can be accounted for by a large dispersion of phonon modes due to the strong H–H interactions, and/or different metal–hydrogen force constants, which may originate from different metal atoms surrounding the H atoms in the unit cell.

The structural and magnetic properties of the Gd2Fe17Dx compounds (x = 0, 3, and 5) have been investigated by means of x-ray powder diffraction, high energy inelastic neutron scattering and magnetic measurements. The Gd2Fe17Dx compounds crystallize in the R-3m space group with the Th2Zn17-like structure. The increase of the lattice parameters with D content reveals a two-step filling of the interstitial sites with deuterium first filling the octahedral 9e sites for x = 3 and then partially filling the tetrahedral 18g sites for x = 5. The evolution of the Jex exchange coupling versus the deuterium (D) content is discussed. Although it is well known that the permanent magnet properties of D-doped samples are considerably better than those of the pure compound Gd2Fe17 there is a reduction in the Gd-Fe exchange field. Finally, the influence of deuterium insertion on the exchange interaction between the Gd and Fe sublattice is compared to that of nitrogen (N) or carbon (C). The C and N atoms are found to be more efficient in reducing the intersublattice coupling.

The potential wells seen by hydrogen in several interstitial sites in the C-15 Laves phase compounds, ZrTi2 (ZT), ZrCr2 (ZC) and TiCr1.85 (TC), have been investigated using incoherent inelastic neutron scattering (IINS) data. Parameters describing the harmonic terms in the H-potential well in the three compounds are obtained from the first localized hydrogen vibrations as measured by the IINS technique. The H-potential well is created by the sum of the pairwise potentials between the hydrogen and the four metallic atoms forming the different types of tetrahedral site which are labelled g (A2B2), e (AB3) or b (B4) in the C-15 structure. A Born-Mayer potential has been used to model these pairwise interactions, with parameters determined from the IINS. The resulting overall potential energy surface describes well the observed site occupancies and the diffusional behaviour of H in the ZC and TC compounds. On the other hand, due to the higher H content of the ZT sample, H-H interactions have to be included in the model for a proper description of the energy potential well in this compound. The potential energy surface enables us to identify the different diffusion paths of H in the C-15 structure and hence to infer the different time scales of H diffusion (localized and long-range diffusion) that have recently been shown to exist in these compounds (Skripov A V et al 1996 J. Phys.: Condens. Matter 8 L319-24; 1997 J. Alloys Compounds 253/254 432-4).

A first-principles pseudopotential study of neutral self-interstitial defects in silicon is reported, together with calculations for Pandey's concerted exchange mechanism for self-diffusion. The energies and structures of the fully relaxed hexagonal, tetrahedral, split-110, `caged' (Clark S J and Ackland G J 1997 Phys. Rev. B 56 47), split-100, and bond-centred interstitials are calculated using supercells with up to 128 + 1 atoms. We present results obtained using the local density approximation (LDA) and the PW91 generalized gradient approximation (GGA) for the exchange-correlation energy. Both the LDA and PW91-GGA functionals give the hexagonal and split-110 defects as the lowest-energy self-interstitials. The hexagonal and split-110 defects are essentially degenerate in energy with formation energies of about 3.3 eV (LDA) and 3.80 eV (PW91-GGA). Energy barriers are studied by calculating saddle-point structures using a simple `ridge-walking' method. The energy barrier for a diffusive jump between the hexagonal and split-110 interstitial sites is calculated to be 0.15 eV (LDA) and 0.20 eV (PW91-GGA) and the barrier between neighbouring hexagonal sites is 0.03 eV (LDA) and 0.18 eV (PW91-GGA), but we have not found a low-energy path between split-110 interstitial sites. The results suggest that self-interstitial diffusion in silicon occurs via diffusive jumps between the hexagonal sites and between hexagonal and split-110 defects.

We have synthesized CeSbNi0.15 single crystal and measured its magnetic and transport properties. X-ray powder diffraction studies reveal that CeSbNi0.15 crystallizes in the cubic structure, like the host CeSb, with Ni atoms occupying interstitial sites. The magnetic susceptibility reveals an antiferromagnetic ordering at TN = 9.5 K. The magnetization isotherm at 1.5 K shows a metamagnetic transition at 6.4 T. The field dependence of the magnetization at 5 K exhibits almost the same behaviour up to a field of 5.5 T for B || [100], B || [110] and B || [111]. The resistivity exhibits a sharp rise below TN which we attribute to the formation of a superzone gap. The obvious effect of Ni incorporation in CeSb is the absence of complex and strong anisotropic magnetic properties. This indicates that the p-f mixing in CeSb is strongly reduced by the incorporation of Ni in the interstitial site. We have also determined the solubility limit of Ni in polycrystalline compounds CeSbNix to be x0.4, and studied their magnetic and transport properties. These studies show that TN for CeSbNix alloys varies non-linearly with the Ni content, x.

ZnS:Mn2+ nanocrystals embedded in Pyrex glass matrices have been studied systematically. It has been found by photoluminescence (PL) and excitation (PLE) spectra that doped Mn impurities can be classified as two types, occupying substitutional sites (Mn2+)sub and interstitial sites (Mn2+)int. EPR experiments support this conclusion. In addition, Mn clusters were also identified from the EPR spectra. We observed the increases in the g1 factors and hyperfine structure (HFS) constants with decreasing diameters of the nanocrystals. These increases are due to the hybridization between s-p states of ZnS and d states of the Mn ions by quantum confinement effects and the surface states.

Muon spin relaxation measurements have been performed on the alkali metal oxides and in order to model the behaviour of hydrogen impurities in these materials. Lithium oxide has been suggested as a suitable blanket material for fusion reactions, hence the need to understand the location and dynamics of ions in the crystalline lattice. This oxide becomes a fast ion conductor at high temperatures and our experiments also provide information on the ion dynamics. Muon implantation into these oxides at low temperatures creates mainly neutral muonium (the analogue of the hydrogen atom) with a smaller fraction (%) of positive muons at interstitial sites closely associated with ions. At higher temperatures (T > 100 K) the proportion of formed increases, and there is an increased tendency for trapping at substitutional lattice sites. A sharp decrease in relaxation rate was observed in at temperatures above 250 K and from this temperature dependence we calculate the activation energy for muon hopping relative to the ions, which are also mobile, to be approximately 90 meV.

We study the stability and migration mechanism of self-interstitials in Si through first-principles self-consistent pseudopotential calculations. The neutral Si interstitial is lowest in energy at a [110]-split site, with energy barriers of 0.15-0.18 eV for migrating into hexagonal and tetrahedral interstitial sites, while the migration barrier from a hexagonal site to a tetrahedral site is lower, 0.12 eV. These migration barriers are further reduced through successive changes in the charge state at different sites, which allow for the athermal diffusion of interstitials at very low temperatures. The [110]-split geometry is also the most stable structure for negatively charged states, while positively charged self-interstitials have the lowest energy at tetrahedral sites. Apart from the migration barrier, the formation energy of the [110]-split interstitial is estimated to be about 4.19 eV; thus, the resulting activation enthalpy of about 4.25 eV is in good agreement with high-temperature experimental data.

A method of analysing by Mossbauer spectroscopy a steel ferromagnetic matrix has been developed. It provides the fractions of atomic elements in substitutional or interstitial sites in the Fe lattice, from the comparison between the experimental hyperfine-field distribution and the calculated one . This method, applied to an M50 steel, in the simple case of ferrite (the annealed state), and extended to the most complex situation of martensites quenched from various temperatures, describes the initial state of the steel before any further tempering treatment. The atomic fractions of Cr, Mo, V, and C in the Fe lattice have been specified.

The magneto-optical properties of MnBi show some striking features which we address here by calculating the Kerr spectra of pure MnBi, as well as of (X = Mn, Si, Al, O and Pt) and (X, Y) = (Al, Al) and (Al, O), (Pt, Pt). This system of compounds constitutes an example for which we demonstrate how much first-principles calculations can achieve and how our findings can be used to advance our knowledge of the origin of magneto-optical properties. Our calculations are based on the local density functional approximation (LDA) and the augmented spherical wave (ASW) band structure method including the scalar - relativistic and spin - orbit coupling terms to determine the effective one-electron energies and wavefunctions. From this we derive the inter-band and intra-band contributions to the optical conductivity tensor. Our results allow some statements concerning the connection of material properties with the magneto-optical properties. Thus, for instance, we are led to believe that oxygen plays an important role in explaining the experimental Kerr spectrum of pure MnBi. Furthermore, the measured enhancement of the Kerr angle in Al-alloyed MnBi is most probably not due to the occupation of interstitial sites by Al atoms. In contrast to this is Pt-alloyed MnBi, for which our calculations indicate that Pt partly occupies interstitial sites.

Ho centres in mixed-polytype ZnS crystals codoped with Ag are studied by measuring polarized site-selective photoluminescence and photoluminescence excitation spectra as well as by EPR spectroscopy. The optical data give evidence of Ho in the charge state 3+ (4f10), while in the EPR measurements, which are performed under illumination in the interband range, centres in the 2+ state (4d11) are detected. From a crystal-field analysis we conclude that the dominant centres seen in the two kinds of experiment are identical with respect to the atomic configuration, apart from small nearest-neighbour relaxation effects which accompany the change of the charge state. The crystal field is found to have approximately cubic symmetry, with a small trigonal field component, whose axis is parallel to the (111)w direction. Our data indicate that the Ho occupies zincblende-type interstitial sites with four metallic ions as nearest neighbours, most probably Ag substituting for Zn ions. The Ho2+ ground state involved in the charge transfer process is thought to lie close to the bottom of the conduction band; this explains the optical behaviour observed in our experiments.

EPR and optical (4f-4f photoluminescence and photoluminescence excitation) spectra due to Er centres are studied on bulk ZnSe crystals, which were grown by the high-pressure Bridgman technique and doped with ErF3 (and partly, in addition, with Li2CO3) during crystal growth. Besides the well known, almost isotropic signal with g=5.94, which has been assigned to isolated Er3+ on Zn lattice sites, we observe three strongly anisotropic EPR spectra (A, B and B') which are due to transitions in non-cubic Er3+ (spectrum A) and Er3+ (spectra B and B') centres. The symmetry axes of these centres have directions close to (111) (A) and parallel to (110) (B and B'). The angular dependences of the signals are influenced by twinning effects. In the crystals doped with ErF3 alone, only a single type of centre, which is obviously identical with the type A EPR centre, manifests itself in photoluminescence. A crystal-field analysis of the corresponding EPR and optical spectra shows that this centre has a Gamma 6-type ground level and is characterized by a crystal-field parameter ratio As /A4 =-0.22. We think that this centre is a complex consisting of Er3+ on a Zn site and F on a nearest-neighbour interstitial site. The g-factors found for the EPR signals B and B' can be explained on the basis of non-Kramers doublet ground levels of Er2+ 4f12 which result from the splitting of cubic Gamma 5 triplets due to non-cubic crystal-field components. These two signals are ascribed to Er3+ on the two zincblende-type interstitial sites, respectively, each forming a complex with some other kind of atom on its next-nearest-neighbour interstitial site. A discussion is given of the result that the EPR spectra (unlike the optical spectra) were detected only in the case of Li2CO3 codoping.

The only intrinsic point defects of simple crystalline metals known from solid state physics are vacancies and interstitials. It is widely believed that while vacancies play a major role in crystal properties and their concentration reaches relatively big values near the melting temperature T m, interstitials essentially do not occur in thermodynamic equilibrium and their influence on properties is minor. Here, taking aluminum single crystals as an example, we present compelling experimental evidence for rapid thermoactivated growth of interstitial concentration upon approaching T m. Using high precision measurements of the shear modulus we found a diaelastic effect of up to [Formula: see text] near T m. It is argued that this effect is mostly due to the generation of dumbbell (split) interstitials. The interstitial concentration c i rapidly increases upon approaching T m and becomes only 2-3 times smaller than that of vacancies just below T m. The reason for this c i -increase is conditioned by a decrease of the Gibbs free energy with temperature, which in turn originates from the high formation entropy of dumbbell interstitials and a decrease of their formation enthalpy at high c i . Special molecular dynamic simulation confirmed all basic aspects of the proposed interpretation. The results obtained (i) demonstrate the significance of interstitial concentration near T m that could lead to the revaluation of vacancy concentration at high temperatures, (ii) suggest that dumbbell interstitials play a major role in the melting mechanism of monatomic metallic crystals and (iii) support a new avenue for in-depth understanding of glassy metals.

Neutron powder diffraction studies on PdD0.65 demonstrate that the 50 K anomaly is due to an order-disorder transition of deuterium within octahedral interstitial sites. The slow transition to the ordered phase involves diffusion of deuterium from the nearest-neighbour deuterium positions to the second-nearest-neighbour positions. The ordered crystal structure remains cubic, and is accurately described in space group Pm3n by doubling the cell constant relative to the disordered structure (space group Fm3m).

We investigated the short-range order of D interstitials in cubic ( alpha '-phase) VD0.75 by neutron diffraction (room temperature measurements, powder sample). The VDx system is particularly favourable for neutron scattering experiments studying the short-range order of hydrogen interstitials in metals since the coherent scattering cross section of D exceeds the coherent cross section of V by a factor of approximately=300. This allows a data analysis that assumes that only the D atoms contribute to the coherent scattering intensity. The measured diffuse scattering intensity shows a nearly complete blocking of interstitial sites in the two nearest shells around a given D atom, and a reduced occupation probability for the sites in the third shell. The results indicate the existence of a strong short-range and repulsive interaction between the D atoms.

The self-diffusion of H in the substoichiometric rare-earth hydride halide YBrH0.78 was investigated by incoherent quasielastic neutron scattering. YBrH0.78 exhibits a layered structure in which the H atoms occupy tetrahedral interstitial sites located in Y bilayers, which are sandwiched by Br bilayers. As this arrangement suppresses H transfer between different Y bilayers, the present study investigates an essentially two-dimensional H diffusion within a given Y bilayer. The self-diffusion coefficient of the H was determined to be between 750 and 900 K. At 900 K, its value is D=(1.2+or-0.4)*10-6 cm2 s-1. The temperature dependence can be described by an Arrhenius relation with an activation energy E=(0.48+or-0.12) eV. The momentum-transfer dependence of the measured spectra suggests the presence of noticeable correlation effects in the H diffusion.

The lattice sites of a number of trivalent co-dopants (In, Nd, Gd, Er, Tm, and Lu) have been determined in LiNbO3 crystals also with Mg, using the Rutherford back-scattering and proton-induced X-ray emission channelling techniques. In, Er and Lu have been found mainly on Li sites, but a small fraction of In and Lu may substitute for Nb ions. Gd and Tm ions also replace mainly Li, but some of these ions seem to be present in other positions, possibly related to some kind of aggregate, e.g. self-compensating pairs. A fraction of Nd ions probably occupies interstitial sites in structurally empty oxygen octahedra, while other Nd ions may substitute for host cations. In addition to the already known effect of Cr, Fe and Nd ions, In, Lu and also Sc give rise to an OH- absorption band between 3499 and 3523 cm-1, which is attributed to Mg2+ (Li site)-OH-(O site)-M3+ (Nb site) complex defects. No such band is observed for Y, Gd, Er and Tm co-doped crystals.

The authors report measurements of the real 4He superfluid density, sigma s, as a function of temperature and coverage on hydrogen and C60 substrates using a high-Q Al oscillator. Sub-kelvin measurements of the helium mass loading of a 780 nm thick C60 film deposited on the active surface of the oscillator show that helium does not significantly permeate the interstitial sites of the C60 matrix, in either its superfluid or normal phase. In fact, the Kosterlitz-Thouless superfluid transition is observed on the C60 at very modest mass loadings. The superfluid behaviour on C60 is found to be very similar to that observed for helium on solid hydrogen, namely a suppression of sigma s(Tc) in transitions near 1 K and excess vapour dissipation in the normal phase.

Nuclear magnetic resonance measurements of the proton spin-lattice relaxation times in A15-type Ti3SbHx (0.6

The dipolar contribution to the spin-lattice relaxation rate Gamma 1 of spin-carrying light interstitials is calculated assuming nearest-neighbour hopping among tetrahedral sites of a BCC lattice. As a new element, which was missing in previous treatments, the authors took into account explicitly the existence of localized excited states of the interstitials. The model considers two states per site, i.e. the ground state and one excited state, and takes into account transitions between the ground states, the excited states, and between an excited level and a ground state of two neighbouring interstitial sites as well as intrasite transitions between ground and excited states of the same site. If the intrasite transitions from the ground to the excited state are not fast compared with the tunnelling rate among the ground states of neighbouring tetrahedral sites, which is likely to be the case for hydrogen in BCC metals like Nb and Ta, the extended model predicts deviations from the results obtained for the usually considered model which describes the interstitial motion by a single effective hopping frequency among tetrahedral sites. The product alpha = Gamma 1D, in which D is the diffusivity of the interstitial, may be considerably larger for the extended model than for the single-hopping-frequency model. Moreover the various hopping frequencies do not drop out from alpha as does the effective hopping frequency in the latter case. This may lead to a strong increase of alpha with temperature in contrast to the temperature independence of alpha for the single-frequency model. The theoretical results are compared qualitatively with NMR experiments performed on the alpha -phases of NbHx and TaHx.

The energetics and electronic structure of an isolated hydrogen impurity in bulk aluminium and in the aluminium-vacancy system are studied by ab initio methods. The calculations are based on the local density approximation and the supercell approach, with a norm-conserving pseudopotential for aluminium and the bare Coulomb potential for hydrogen. The relaxed electronic ground state is determined by the conjugate-gradients technique. Results are presented for the heat of solution, the relative energies of different interstitial sites, the energy profile for migration of hydrogen between sites, and the trapping energy and equilibrium location of hydrogen bound to a vacancy. The heat of solution is close to the measured value, and the tetrahedral site is energetically favoured over the octahedral site, as indicated by experiment, though the energy difference between the sites is comparable with the uncertainties in the calculations. Hydrogen is found to be bound to the vacancy in a strongly off-centre position, with a binding energy that is somewhat smaller than the experimental value.

Ultrasonic attenuation has been measured as a function of temperature approximately over the range 10-300 K at 5, 10, 15 and 45 MHz in a single crystal of a Pd85Pt15 alloy. The measurements have been carried out for five different propagation modes, both in the H-free and the H-loaded (H/(Pd+Pt)=0.29 at.%) state of the material. An anelastic relaxation has been observed at around 225 K (f=10 MHz), whose activation energy W and limit relaxation time tau 0 are: W=0.24+or-0.02 eV; tau 0=(1.4+or-0.5)*10-14 s. The mode dependence of the relaxation strength reveals the existence of a hydrostatic relaxation due to reactions between dipoles associated with H-H and H-Pt complexes. The results can be accounted for in terms of stress-induced changes in the short-range order of both the H-H and the H-Pt bonds (or antibonds) as well as of the occupancy probabilities of octahedral interstitial sites by H.

Using the intermediate neglect of differential overlap MO LCAO calculation procedure and the embedded molecular cluster model the authors have investigated atomic and electronic structure and key stages of diffusive transfer of T+ and T0 in Li2O, containing cation vacancies and F+ centres. The activation energy for the cation vacancy diffusion in Li2O is calculated to be 0.33 eV. Comparing different models of T+ and T0 atomic structures and the mechanisms of their diffusion, the authors have shown that the lowest activation energy (

A nonequilibrium Mg hydride was prepared by low temperature hydrogen implantation in Mg thin films. In contrast to the usual compound in which H is charged under high pressure and elevated temperature, no structural phase transformation from hexagonal to tetragonal is observed here. Implanted H atoms progressively fill the tetrahedral interstitial sites of the Mg hexagonal cell up to a concentration H/Mg around one. The resulting hydride is metallic whereas the classical MgH2 is an insulator.

Quantities relevant to the theoretical determination of D/T1 are calculated, where D is the tracer diffusion coefficient and T1 is the spin-lattice relaxation time for relaxation due to magnetic dipolar coupling between diffusing hydrogen spins. Calculations are performed for hydrogen spins diffusing on the tetrahedral interstitial sites of a BCC host metal and both the like-spin (hydrogen-hydrogen) and unlike-spin (metal-hydrogen) contributions are determined. The Monte Carlo/analytic method devised by Faux, Ross and Sholl (1986) is used. Results are obtained for the hydrogen-to-metal ratio of 0.6 for a multiple-site-blocking model in which the hydrogen spins block all sites to either the second or the third neighbour. A fraction F2 of hops are assumed to occur to second-nearest-neighbour sites. Agreement is obtained for the first time with the experimental result on NbH0.6 with F2>0.6.

1H CW and pulsed NMR studies on the hydrides ZrNiAlHx, with hydrogen concentration x=0.33 and 0.53, and YNiAlHx, with hydrogen concentration 0.12 or=0.75 in the temperature range 220-320 K, whereas only a single line is observed for hydrides of YNiAl with x 0.75, indicate that YNiAlHx is a single-phase hydride. Hydrogen atoms occupy two types of interstitial sites in YNiAl, in which the mobility of hydrogen in one of the sites is higher than that in the other. The activation energies for these motions have been estimated to be 0.24 eV/atom and 0.33 eV/atom.

Si(1 1 1)-Au surfaces with around one monolayer of Au exhibit many ordered structures and structures containing disordered domain walls. Hybrid density functional theory (DFT) calculations presented here reveal the origin of these complex structures and tendency to form domain walls. The conjugate honeycomb chain trimer (CHCT) structure of the [Formula: see text]-Au phase contains Si atoms with non-bonding surface states which can bind Au atoms in pairs in interstices of the CHCT structure and make this surface metallic. Si adatoms adsorbed on the [Formula: see text]-Au surface induce a gapped surface through interaction with the non-bonding states. Adsorption of extra Au atoms in interstitial sites of the [Formula: see text]-Au surface is stabilized by interaction with the non-bonding orbitals and leads to higher coverage ordered structures including the [Formula: see text]-Au phase. Extra Au atoms bound in interstitial sites of the [Formula: see text]-Au surface result in top layer Si atoms with an SiAu4 butterfly wing configuration. The structure of a [Formula: see text]-Au phase, whose in-plane top atomic layer positions were previously determined by an electron holography technique (Grozea et al 1998 Surf. Sci. 418 32), is calculated using total energy minimization. The Patterson function for this structure is calculated and is in good agreement with data from an in-plane x-ray diffraction study (Dornisch et al 1991 Phys. Rev. B 44 11221). Filled and empty state scanning tunneling microscopy (STM) images are calculated for domain walls and the [Formula: see text]-Au structure. The [Formula: see text]-Au phase is 2D chiral and this is evident in computed and actual STM images. [Formula: see text]-Au and domain wall structures contain the SiAu4 motif with a butterfly wing shape. Chemical bonding within the Si-Au top layers of the [Formula: see text]-Au and [Formula: see text]-Au surfaces is analyzed and an explanation for the SiAu4 motif structure is given.

Li co-doped ZnO:Co (Zn0.96-yCo0.04LiyO , y ≤ 0.1) nanoparticles were synthesized by the sol-gel technique and the correlation between the structural, electronic and magnetic properties was investigated. All the samples show a single phase hexagonal (wurtzite) ZnO structure and no secondary phases were detected. Variational trends in lattice parameters suggest the incorporation of Li in the ZnO:Co system in both substitutional and interstitial sites. Detailed electronic studies have been performed by high-resolution x-ray photoelectron spectroscopy (XPS) to determine the states of Zn, O, Co and Li. It was determined that Co substitutes at Zn sites (CoZn) while the O vacancy and Zn defects did not show much variation with increasing Li concentration. Deconvolution of the Li XPS peak showed a clear non-monotonic trend in the variation of the substitutional Li (LiZn) and interstitial Li (Lii) defects with increasing Li concentration in the particles. The magnetization study of the samples showed that the variation of the moment closely followed the trend of variation of the LiZn defects. The data are interpreted in terms of substitutional Li acting as a hole dopant and optimizing the conditions for ferromagnetism in Co-doped ZnO. Interstitial Li is not seen to be playing this role.

Here we report the dynamical behaviour of water in Prussian blue analogue (PBA) Cu(0.75)Mn(0.75)[Fe(CN)(6)]·7H(2)O molecular magnet in the temperature range 260-360 K as studied using the quasielastic neutron scattering technique. While significant quasielastic broadening is observed in the hydrated sample, no broadening was observed in the dehydrated one. Data analysis showed that the observed quasielastic broadening in Cu(0.75)Mn(0.75)[Fe(CN)(6)]·7H(2)O corresponds to the dynamics of the non-coordinated water molecules at the 32f site and the coordinated water molecules at the 24e site, existing in the cavities created by the absence of Fe(CN)(6) units. The non-coordinated water molecules at 8c interstitial sites do not contribute to the broadening, suggesting that they are immobile at least within the time window of the spectrometer used. Behaviour of the elastic incoherent structure factor is consistent with the model where the water molecules undergo translational diffusion localized within the cavity of 5.1 Å. While all the non-coordinated water molecules at the 32f site are dynamic over the entire range of temperatures, the coordinated ones at the 24e site become progressively dynamic with temperature. The water molecules were found to undergo hindered (~1.16 × 10(-5) cm(2) s(-1) at 300 K) diffusion compared to bulk water and the diffusivity followed Arrhenius behaviour within the measured temperature range with an activation energy of 1.26 kcal mol(-1).

We investigate the ferromagnetism in ZnO clusters due to vacancy defects and C impurities doped at substitutional O or Zn sites, and interstitial sites. The total energy calculations suggest C at the O site is more stable than that at the Zn site in ZnO clusters. The total magnetic moments of Zn(n)O(n-m)C(m) clusters are 2.0 μ(B)/C. However, when two C atoms are bonded to the same Zn atom they interact antiferromagnetically and the total magnetic moment becomes less than 2.0 μ(B)/C. The interstitial C defects in ZnO clusters induce small magnetic moments. The combination of substitutional and interstitial C defects in ZnO clusters leads to magnetic moments of 0.0-2.0 μ(B)/C. The presence of vacancy defects in addition to substitutional C defects gives magnetic moments of greater than 2.0 μ(B)/C. These results suggest that the experimentally observed sample dependence of magnetic moments in ZnO systems is largely due to the different concentrations of substitutional and interstitial C impurities and the presence of vacancy defects in ZnO samples prepared under different growth conditions.

Ab initio calculations based on density functional theory have been performed to determine the relative stabilities and migration of H and He atoms in hcp-Sc. The results show that the formation energy of an interstitial H or He atom is smaller than that of a corresponding substitutional atom. The tetrahedral (T) interstitial position is more stable than an octahedral (O) position for both He and H interstitials. The nudged elastic band method has been used to study the migration of interstitial H and He atoms in hcp-Sc. It is found that the migration energy barriers for H interstitials in hcp-Sc are significantly different from those for He interstitials, but their migration mechanisms are similar. In addition, the formation energies of five different configurations of a H-H pair were determined, revealing that the most stable configuration consists of two H atoms located at the second-neighbor tetrahedral interstitial sites along the hexagonal direction. The formation and relative stabilities of some small He clusters have also been investigated.

High-energy x-ray and neutron diffraction measurements on polycrystalline La(2/3-x)Li(3x)TiO(3) (0.075x0.165) were performed. The total scattering structure factors were analysed by the reverse Monte Carlo (RMC) modelling technique, resulting in three-dimensional particle configurations. These configurations were then used for revealing the distributions of La and Li ions and to understand the relationship between these distributions and ionic conduction. An alternating arrangement of La-rich and La-poor layers along the c-axis was found in the x = 0.075 composition. Intriguingly, this arrangement has gradually disappeared in samples with higher Li concentration. Furthermore, RMC models exhibit disordered distributions of Li ions, situated mainly on the La-rich layer, and there is a significant probability of Li ions occupying the interstitial sites (T site) between the O-3 triangle plane of the TiO(6) octahedron and an La ion or its vacancy site. It was also found on the basis of the RMC models that the bond valence sum (BVS) for Li ions behaves differently on La-rich and La-poor layers at low Li concentration compositions, but they are similar at high Li concentration compositions. This is consistent with the behaviour of the alternating arrangement of La-rich and La-poor layers. It is also suggested that the Li ions around the bottleneck at (1/2, 0, 0) (bottom layer) can jump to an adjacent bottleneck at (0, 1/2, 0) through the T site and not only Li ions in the La-poor layers but also Li ions in the La-rich layers contribute to the bottleneck-bottleneck Li conduction.

The site occupancy, structure, and bonding properties of O in an NiAl grain boundary (GB) have been investigated by employing a first-principles total energy method based on density functional theory with the generalized gradient approximation and ultrasoft pseudopotential. The Σ5(310)/[001] tilt GB of NiAl has been chosen because (i) the Σ = 5 GB has been observed to be a higher fraction in NiAl experimentally, and (ii) the Σ5(310)/[001] is energetically favorable in comparison with the Σ5(210)/[001]. The NiAl GB is shown to favor the O segregation with a segregation energy of -1.75 eV, indicating that most of the O impurity will distribute in the NiAl GB thermodynamically. Moreover, O is shown to prefer occupying the interstitial sites rather than the substitutional sites in the GB according to the calculated formation energies. The O-Al bond is energetically favorable as compared with the O-Ni bond due to different electronegativity of Al and Ni in reference to O. Charge distribution and the density of states further indicate the intrinsic bonding properties of O-Al that contain obvious covalent characteristics. It is interesting to find that O is coplanar with the surrounding Al atoms in both interstitial and substitutional cases with lower formation energies, forming stronger coplanar O-Al bonding clusters. Such stronger bonding clusters in the GB can embrittle the NiAl intermetallics and thus are not beneficial to the plasticity of NiAl. Our results will provide a useful reference for improving the mechanical properties and for understanding the oxidation effect of the NiAl intermetallics.

The Ti40Zr40Ni20 hydrogen-absorbing alloy was prepared in the icosahedral and amorphous phases by controlling the rotation speed of the melt–spinning method of sample preparation, and the deuterium dynamics was investigated by 2H NMR dynamic lineshape and spin–lattice relaxation. The results were analysed by the lineshape and relaxation models that assume deuterium thermally activated hopping within a manifold of different chemical environments. The observed 8% larger activation energy for the deuterium hopping over the interstitial sites and the 10% larger static spectrum width of the amorphous phase, as compared to the icosahedral phase, can be accounted for by the larger deuterium content of the investigated amorphous sample. From the deuterium dynamics point of view, the icosahedral phase is not special with respect to the amorphous modification of the same material.

Zinc oxide nanoparticles were obtained by milling in a planetary ball mill with a zirconia milling assembly for up to 5 h in air. The samples were characterized by scanning electron microscopy, x-ray diffraction (XRD) and Raman spectroscopy methods. The deviation of the lattice parameters from single crystal values was related to defect creation and increase of strain inside the hexagonal lattice of milled ZnO nanoparticles. The observed redshift and peak broadening of the major first-order Raman modes were ascribed to the formation of intrinsic defects by mechanical milling combined with the effects of phonon confinement in nanosized powders. To investigate the type of intrinsic defects and impurities introduced during milling, it was necessary to analyze both milled and thermally treated ZnO. After thermal treatment, the intensity of the Raman spectra increased and the peak positions reverted to values similar to those in unmilled ZnO powder, pointing to defect annihilation. XRD patterns of sintered samples confirmed the existence of zirconia impurities and the Rietveld analysis revealed a small amount of zirconium introduced in the ZnO crystal lattice on the Zn sites or interstitial sites. The large influence of those impurities on the micro-Raman spectra of thermally treated samples was observed in this study.

Using first-principles calculations we have studied the effect of Al addition on the diffusion of interstitial oxygen atoms and ions in HfO2. Calculated results show that interstitial oxygen ions in the singly negatively charged state easily diffuse in HfO2 via exchange with lattice oxygen atoms, due to the lower diffusion barrier as compared to those for neutral interstitial oxygen atoms and interstitial oxygen ions in the doubly negatively charged state. The addition of Al raises the diffusion barrier for interstitial oxygen because the interstitial oxygen is strongly attracted by its neighboring Al atoms. The electronic structure analysis further reveals that the bonding between interstitial oxygen and lattice oxygen atoms is strengthened for each charge state of interstitial oxygen in HfO2 with the addition of Al.