O. Blaschko

Pressure induced phonon frequency shifts in KBr measured by inelastic neutron scattering

Abstract Measurements of seven microscopic Gruneisen parameters in KBr by inelastic neutron scattering are reported and compared with several theoretical predictions.

A neutron scattering investigation of the early stages of guinier-preston zone formation in AlZnMg(Cu)-alloys

Abstract A diffuse neutron scattering investigation of the early stages of the Guinier-Preston zone formation in AlZnMg(Cu)-alloys has been made. All samples show at smaller wavevectors ( −1 ) elastic diffuse intensity which can be described by two Guinier radii. The smaller radius does not change with annealing time and may indicate the presence of small ‘nuclei’, unaffected by the further decomposition. The larger radius, however, shows a considerable time dependence and characterizes the size of the Guinier-Preston zones. For larger annealing times normal growth was found, but for times just after quench a decrease of the radius was observed. This decrease may be explained in the following way: just after quench greater but dilute Guinier-Preston zones form, which with time become smaller and denser. Only after this initial compactification the normal growth starts.

Pressure induced phonon frequency shifts in mixed valent CeSn3

Abstract Pressure induced phonon frequency shifts have been determined in mixed valent CeSn 3 . The results evaluated as mode Gruneisen parameters show rather normal positive values. There is no conclusive evidence for a pressure induced enhancement of the mixed valent character of this material.

Distortion induced by helium formation in tantalum-tritium systems

Two polycrystalline TaTx platelets, loaded with x = 4 and 11.5 at.% tritium (T), have been investigated by neutron scattering techniques for more than two years. The maximum 3He concentration resulting from radioactive tritium decay approaches 1.2 at.%. Changes of both positions and widths of the Debye-Scherrer lines are determined as a function of time. The lattice parameter is found to increase slightly with 3He concentration. The changes of the linewidths indicate the formation of growing dislocation networks. The data give evidence of an increasing anisotropic shear deformation of the bcc metal matrix in the TaT0.04 sample. Helium induced fracture of both platelets occurs already at concentrations below 1 at.%.

The splitting of the (1, 12, 0)-superlattice reflection in PdD0.73 near 50 K

Iso-intensity contours around the (1, 12, 0) point have been determined in PdD0.73 for three temperatures near 50 K. Under the experimental conditions no indication of a (45, 25, 0) reflection was found. The contour maps, however, reveal a new structural feature: i.e. a splitting of the (1, 12, 0) reflection along the [2, 1, 0] direction.

Stages of surface and pore volume evolution during sintering

Abstract Sintering in compacted powders of Mo, Nb, Ta, and alumina is investigated using small angle neutron scattering. The scattering data permits to follow the evolution of the total pore surface F together with the total pore volume V in the course of the entire densification process. Characteristic changes in the relation between F and V suggest that sintering proceeds in three different stages. Quantitative evidence is presented that, in particular, third stage sintering (taking place if the pores can be considered isolated from each other) is governed by a universal relation between F

Investigation of the “50 K-transition” in PdH0.73 by γ-ray and neutron diffraction

Abstract Rocking curves of the (111)-reflection in a PdH0.73 single crystal have been measured with 412 keV γ-rays during an ageing experiment at 50 K up to 260 h. The width (variance) shows an overall increase of 1.5% ± 0.3% and the integrated intensity a maximum at 100 h. An investigation at superstructure points in the same PdH0.73 crystal was made using neutron scattering. An enhancement of the elastic diffuse scattering has been found at the (1 1 2 0) and (110)-positions after an ageing at 54 K up to 300 h. The intensity ratio to the fundamental (002)-reflection is 10-4. It is concluded that below the anomaly temperature a local ordering of the hydrogen atoms occurs and the growth of the ordered zones slows down after 100 h ageing.

The influence of uniaxial deformation on the martensitic phase transformation in Li and LiMg

Abstract The influence of a uniaxial deformation on the low-temperature phase transformation in Li and in LiMg system has been investigated by neutron scattering techniques. The results of the experiments performed indicate that an elastic [001] compression of the BCC matrix promotes the phase transition. An investigation of the TA1[110] phonon branch in Li shows that the mild [001] compression of a few 0.1% decreases the phonon frequencies by several percent. Furthermore, a severe plastic deformation induces the formation of a disordered polytype phase instead of the 9R structure.

Comparative Investigation of Elastic Diffuse Neutron Scattering in C60 Powder

Abstract C60 powder samples of varying impurity content have been investigated by neutron scattering techniques. For Q > 2.5 A−1 (Q = transferred momentum) strong elastic diffuse scattering is found below Tc ~ 260 K providing evidence of static disorder in the low temperature phase. The elastic scattering increases on cooling from Tc to 150 K but remains virtually constant below this temperature. There is no marked dependence on the impurity level. For Q > 2.5 A−1 elastic diffuse scattering obeying a sin(QR)/(QR) law is observed both above and below the phase transition (R= radius of C60 molecule). This scattering is nearly independent of temperature, however, depends strongly on the amount of impurities in the sample. In order to account for the unusually strong elastic diffuse scattering intensity model calculations have been performed which relate orientational disorder in C60 to the elastic scattering observed in powder samples.

Influence of short range order on optical phonons in PdD0.74

Abstract A series of transverse and longitudinal optic phonons has been measured in PdD0.74 in the short range ordered state at 65 K and in the disordered state at 110 K by inelastic neutron scattering. A small change of the frequencies of the TO phonons were found at 65K in the [ζ00] and [ζζ0] directions. A LO phonon shows a narrowing of the peak in the short range ordered state.