Quantum diffusion of light interstitials in metals

A quantum theory of diffusion of self-trapped light interstitials in metals is presented. The theory encompasses both coherent and incoherent tunneling, but the approximation used neglects the dependence of the interstitial transfer matrix element on the vibrational state of the crystal. The coherent tunneling contribution is estimated by fitting the incoherent diffusion rate to experimental data for hydrogen and muon diffusion. It is predicted that coherent diffusion should be dominant below ～ 80 K for H in Nb and below ～ 190 K for μ⁺ in Cu. Experimental verifications of these predictions would require high purity strain free samples and low concentrations of the diffusing species.     

Production and migration of interstitials in deformed metals

An investigation was made of defect production during plastic deformation. Prior to cold-rolling, samples of Au were damaged and preannealed at temperatures in order to maximize site fractions of five different In-vacancy defect complexes, such as In-1V and In-2V. Evolutions of site fractions were monitored by perturbed γ-γ angular correlation spectroscopy after additional strains at 77 K. Observed transformations between sites are consistent with defect-antidefect annihilation reactions between the small complexes and mobile interstitials, and indicate a large production of interstitials during deformation. An extended defect complex attributed to a planar faulted loop became glissile under small strain.     

On the theory of diffusion of light interstitials in metals

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.     

Intrinsic point defects and light interstitials in metals

Clusters consisting of a radioactive probe atom and various point defects in metallic hosts were studied by means of NMR-ON and PAC. NMR resonance signals were observed for¹³¹IV₂ clusters in Fe and^114mInV₄ clusters in Ni. Decoration of¹¹¹InV₂ clusters by H atoms in W and Mo and by He atoms in W was monitored by PAC. The measurements yield the first reliable data on vacancy-hydrogen binding energies in these metals. The He-decoration results are in excellent agreement with the interpretation of He-desorption data and clearly show the existence of trap mutation.     

Strain-induced interaction of interstitials in IVA group hcp metals

Strain-induced (elastic) interactions of oxygen, nitrogen and carbon atoms in IVA group metals, α-Ti, Zr, and -Hf, are calculated in the framework of the microscopic Krivoglaz-Kanzaki-Khachaturyan theory. The experimental elastic constants, lattice spacing of the host metal, and concentration expansion coefficients are used as the input numerical parameters. The resulting interactions are stronger in α-Ti than in α-Zr and α-Hf. A comparative analysis of interactions in the hcp IVA group metals with those in bcc and fcc solid solutions reveals the crystal structure effect. In general, the strain-induced interactions of O, N, and C in hcp IVA group metals are weaker than in bcc solid solutions and are stronger than in fcc solid solutions.     

Spin-polarization in ccv Auger electron spectroscopy for transition metals

The contributions of core states and the valence band to spin-resolved ccv Auger electron spectra (AES) are discussed in terms of a relativistic, spin-polarized theory. The influence of the core-state spin polarization and of selecting particular valence bands via cross sections is shown for the surface layer of BCC Fe(1 0 0) as calculated by means of the Screened KKR ab-initio electronic structure method.     

The motion of light interstitials in metals: Recent experiments

Recent theoretical considerations suggest, that at low temperatures the transport properties of light interstitials in metals are dominated by the interaction with the conduction electrons. After briefly surveying the state of the theory, experimental examples for the low and high temperature cases are presented. We begin with the low T regime and display first neutron scattering experiments on H trapped at O-impurities in Nb where a coherent tunneling state is observed. Its dependence on temperature and its properties in normal and superconducting Nb are discussed. Thereafter muon diffusion results in Al are examined. In Al, muon diffusion is observed via diffusion limited trapping and local properties of the impurity as well as long range diffusion are playing a role. Using a large variety of impurities it was possible to evaluate the diffusion coefficient over a large T-range. Muon motion in Al at accessible temperatures falls entirely in the hopping regime and constitutes an example for the high T regime of the theory.     

Neutron scattering study of the ferroelectric phase transition of SbSI

The paraelectric-ferroelectric phase transition of SbSI has been studied using neutron scattering. Several low frequency phonon branches have been observed. In particular, it has been shown that the [100] and [010] transverse acoustic modes polarized along the [001] direction with the same symmetry as the soft mode, exhibit very anisotropic dispersion relations in reciprocal space. For large q values, an anomalous dispersion relation and a very large temperature dependence have been observed. All these effects are shown to be due to a coupling with a soft mode-central component entity. This entity was observed away from the zone center, but at the zone center, in contrast with the light scattering measurements, no well-defined soft mode response could be observed.     

Resonant modes due to anion interstitials and anion vacancies in fluorites

The resonant modes due to anion interstitials and anion vacancies in fluorites are computed on the basis of a Green's function formalism. The Green's functions are calculated on the basis of shell model fitted to the phonon dispersion curves of the respective crystals. The force constant changes are calculated by finding the relaxations of the different host atoms and then using the short range potential due to Catlow and Norgett. The calculated frequencies of the resonant mode due to anion interstitials in BaF₂; shows good agreement with the experimental results. The results discussed in a comparative fashion for the three crystals CaF₂, SrF₂; and BaF₂; along with the possibility of their experimental detection.     

Neutron scattering study of 49 K phase transition in TTF-TCNQ

Elastic neutron scattering measurements show the existence of three low temperature phase transitions in TTF-TCNQ and are interpreted within the framework of a recent theoretical investigation by Bak and Emery. Direct evidence for the new 49 K transition is presented in addition to the 38 and 54 K transitions reported previously.     

Neutron diffraction study of the structural phase transition in SnS and SnSe

The structural phase transitions in SnS and SnSe have been investigated by neutron diffraction in the temperature range 295–1000 K using a high temperature furnace. Accurate positional and thermal parameters have been obtained as a function of temperature both in the α- (GeS-type, B16) and β-phase (TlI-type, B33). These investigations indicate that the α → β phase transitions in SnS and SnSe are of second order displacive type, and consist mainly of the continuous movement of Sn and S/Se atoms almost entirely along the [100] direction, and suggest the soft-mode behaviour of a zone-boundary phonon of the β-phase.     

Neutron scattering study of the phase transition in s-triazine at high pressure

The elastic transition in s-triazine (C₃N₃H₃) from a trigonal ($\text{R}\text{3}\text{c}$) high temperature (low pressure) structure to a monoclinic (C2/c) low temperature (high pressure) phase has been investigated at pressures up to 5 kbar using neutron scattering techniques. Neutron diffraction was used to measure the pressure dependence of the order parameter and inelastic scattering to study the softening of the transverse acoustic phonon modes on three isotherms. In both cases the effect of pressure on the transition is found to be described primarily by that on the temperature of the transition.     

Determination of the local concentrations of Mn interstitials and antisite defects in GaMnAs

We present a method for the determination of the local concentrations of interstitial and substitutional Mn atoms and As antisite defects in GaMnAs. The method relies on the sensitivity of the structure factors of weak reflections to the concentrations and locations of these minority constituents. High spatial resolution is obtained by combining structure factor measurement and x-ray analysis in a transmission electron microscope. We demonstrate the prevalence of interstitials with As nearest neighbors in as-grown layers.     

Far-infrared study of impurity local modes in gallium-doped PbTe

We present far-infrared reflection spectra and results of galvanomagnetic measurements of PbTe single crystals doped with gallium between 10 and 300 K. The analysis of the far-infrared reflection spectra was made by a fitting procedure based on the model of coupled oscillators. Together with the strong plasmon–phonon coupling we obtain three local modes of gallium at about 122, 166 and 192 cm⁻¹. The position of these modes depends of impurity center charge, and their intensity depends of temperature and of gallium concentration. Persistent photoconductivity effect was registered in the sample with 0.4 at.% Ga by galvanomagnetic and far-infrared measurements.     

Electric quadrupole interaction due to vacancies and interstitials in cubic host metals

A procedure has been developed to evaluate from first principle the electric field gradient (efg) due to vacancies and interstitials in cubic host metals. For this purpose the screening charge distribution in the valence-effect has been constructed in scattering theory using appropriate muffin-tin potential. In the size-effect, the efg has been evaluated in a more realistic model of the lattice than the commonly used elastic continuum. Unlike other calculations, no adjustable parameter has been used in the present procedure.     

Neutron diffraction study of the smectic C-smectic a phase transition of TBBA

The smectic C-smectic A transition of terephtal bis butyl aniline (TBBA) is investigated using neutron diffraction. The critical exponent relating the tilt angle to the reduced temperature is found to be β = 0.44 ± 0.04. Comparison with results obtained by other methods and with theoretical predictions is presented.     

Neutron study of a displacive phase transition in N-nitrodimethylamine (DMN): High and low frequency vibrational modes; high pressure effect on the transition temperature; principal compressibilities

N-Nitrodimethylamine is known to undergo a displacive structural phase transition at T_t～107 K, (atmospheric pressure) associated with a soft-mode observed in the low temperature phase Raman spectrum.The soft-model has already been assigned to a lattice vibration although crystallographic observations of the symmetry breaking distortion suggest that a coupling with an internal vibration should not be ruled out. To clarify this point neutron inelastic spectra have been recorded. They lead to a better assignment of both the high and low frequency vibrations and to the conclusion that no softening of an intramolecular mode is visible.High pressure (up to 3.5 Kbars) neutron scattering experiments are also described. They give both the directions and magnitudes (k₁=0.33× 10^?2, k_b=1.17 × 10^?2, k₃ = 0.12× 10^?2Kbar^?1) of the isothermal principal compressibilities of DMN and the dependence of T_t on pressure ((dT_t/dP)_P=0 ～ + 4.3 Kbar^?1). Spectroscopic and crystallographic data now available on DMN allow us to discuss the mechanism of the transition. An extension of Samara's rule to molecular crystals is attempted     

Neutron scattering study of phonons associated with indirect band-gap transition in AgBr

Phonons in AgBr are studied by neutron inelastic scattering along the [111] direction of reciprocal lattice. From the phonon dispersion and the optical data, it is concluded that the valence band maximum is at the L-point and the phonons participating in the indirect band-gap transition are identified as TO(L) and LA(L).