G. Krexner

Neutron holographic study of palladium hydride

A holographic image of the atomic arrangement in a PdH0.78 single crystal was recorded using spherical neutron waves generated by incoherent neutron-proton scattering (“internal source concept”). The resolution is sufficient to show the positions of single atomic nuclei on their respective lattice sites despite the delocalization of protons due to the weak bonding of hydrogen dissolved in palladium metal. Ways to overcome problems in the reconstruction of atomic positions related to the highly unfavorable sample shape are discussed. The technique offers possibilities for the investigation of various materials containing substantial amounts of hydrogen.

Aging effects on palladium pressure–composition isotherms during tritium storage

Abstract Aging effects on palladium pressure–composition isotherms have been studied for tritium storage up to 9 years. Storage properties are remarkably well preserved during aging, and only small modifications of P – c isotherms are observed: a decrease of plateau pressures, and shifts of α and β single phase regions towards greater stoichiometries. For the plateau pressures, the main changes are observed during the first half of the aging period (≈5 years) while phenomena slow down quickly to become almost negligible during further aging. By contrast, the displacements of α and β single phase solubilities are more continuous. These observations can all be correlated to the evolution of the (micro)structural properties. For instance, the 3 He generated by tritium decay and retained in the metallic lattice tends to segregate into bubbles, where it has only a weak influence on the structure and consequently on the storage properties. The bubble formation has been demonstrated by transmission electron microscopy (TEM) observations, and the global conservation of structural properties has been established by X-ray and neutron diffraction and by X-ray absorption.

Induced ferroelectric phases in TbMn2O5

The magnetostructural transitions and magnetoelectric effects reported in

Directional magnetoelectric effects in MnWO4: magnetic sources of the electric polarization.

{\text{TbMn}}_{2}{\text{O}}_{5}

Helium damage in long-aged metal–tritium systems

are described theoretically and shown to correspond to two essentially different mechanisms for the induced ferroelectricity. The incommensurate and commensurate phases observed between 38 and 24 K exhibit a hybrid pseudoproper ferroelectric nature resulting from an effective bilinear coupling of the polarization with the antiferromagnetic order parameter. This explains the high sensitivity of the dielectric properties of the material under applied magnetic field. Below 24 K the incommensurate phase shows a standard improper ferroelectric character induced by the coupling of two distinct magnetic order parameters. The complex dielectric behavior observed in the material reflects the crossover from one to the other transition regime. The temperature dependences of the pertinent physical quantities are worked out, and previous theoretical models are discussed.

Atomic resolution holography using advanced reconstruction techniques for two-dimensional detectors

The ferroelectric order and magnetic field induced effects observed in the spiral phase of MnWO4 are described theoretically. It is demonstrated explicitly that the Dzyaloshinskii-Moriya antisymmetric interactions contribute to the correlation between spins and electric dipoles in the incommensurate and commensurate ferroelectric phases of magnetic multiferroics. However, other single-site symmetric interactions are shown to be involved in the magnetoelectric process, suggesting the possible existence of an electric polarization originating from purely symmetric effects.

Theoretical consideration of the optimal performance of atomic resolution holography

Abstract The evolution of lattice damage in metal–tritium–helium systems is investigated by means of neutron scattering techniques. Polycrystalline samples of tantalum, yttrium and scandium have been loaded with tritium concentrations on the order of ten atomic percent and changes in the positions, intensities and line shapes of Debye–Scherrer lines have been determined over a period of about 15 years. The results are interpreted in terms of a steadily growing number of lattice defects due to helium production. Helium clustering and bubble formation induce self-interstitials and dislocation loops developing into extended dislocation networks at higher helium concentrations. No tritium loss from the samples could be observed even after aging times exceeding one half-life of tritium and giving rise to helium concentrations of several atomic percent. Differences in the results obtained for cubic (tantalum) and hexagonal (yttrium, scandium) host lattices will be briefly discussed. The present work complements results obtained earlier from investigating the same samples after aging times of about one and five years, respectively, and which were presented in Refs. [1] , [2] .

Investigation of the composition and structure of GP zones in - Ag by means of positron annihilation

Atomic resolution holography is based on two concepts. Either the emitter of the radiation used is embedded in the sample (internal source concept) or, on account of the optical reciprocity law, the detector forms part of the sample (internal detector concept). In many cases, holographic objects (atoms and nuclei) simultaneously adopt the roles of both source and detector. Thus, the recorded image contains a mixture of both inside source and inside detector holograms. When reconstructing one type of hologram, the presence of the other hologram causes serious distortions. In the present work, we propose a new method, the so-called double reconstruction (DR), which not only suppresses the mutual distortions but also exploits the information content of the measured hologram more effectively. This novel approach also decreases the level of distortion arising from diffraction and statistical noise. The efficiency of the DR technique is significantly enhanced by employing two-dimensional (2D) area detectors. The power of the method is illustrated here by applying it to a real measurement on a palladium-hydrogen sample.

Distortion induced by helium formation in tantalum-tritium systems

The recording of holograms with atomic resolution is usually affected by distortions, e.g., due to the finite size of the source and detector, twin images and various imperfections of the instrument some of which have already been analyzed in detail. Amongst others in the present paper we discuss the effect of statistical noise. It is shown that the level of statistical noise of the reconstructed image is not influenced by the number of data points in the hologram and their individual counting statistics but is determined by the total number of counts collected in the experiment. The effects of instrumental resolution, filtering methods and symmetry operations on the signal-to-noise ratio of the reconstructed image are investigated as well. These achievements form the basis of the optimization method for the signal-to-noise ratio of the reconstructed holographic image.

Investigation of the two martensitic phase transitions hcp-dhcp and dhcp-fcc in Co-0.85at%Fe by neutron scattering

Positron annihilation spectroscopy is used to obtain information about structures and concentrations of - and -GP zones in - Ag alloys. Measurements of positron lifetimes and Doppler broadening are reported. For the interpretation we present a method to estimate numerically the lifetimes of positrons trapped in spherical precipitates and give results for a series of possible concentrations and internal structures of the precipitated particles. From a comparison with the experimental data we conclude that the silver atoms are distributed homogeneously inside the -GP zones and that the silver concentration of both types of zone is .