E. Burkel

Observation of Inelastic X-Ray Scattering from Phonons

We present the results of measurements of the inelastic scattering of X-rays with very high-energy resolution. Optical phonons in pyrolytic graphite with an energy transfer of ΔE = (171 ± 10) meV as well as an optical phonon in a Be single crystal with ΔE = (79 ± 10) meV were detected.

First measurement of a phonon dispersion curve by inelastic X-ray scattering

Inelastic scattering of 13.8 keV X-rays with very high energy resolution of ΔE=55 meV was used to measure the phonon dispersion curves for theLA andLO modes in the [00ξ] direction in Be. The results agree with inelastic neutron scattering data known from the literature. The X-ray scattering intensities of the phonon excitations for different momentum transfers are in very good agreement with the prediction from the scattering law.

Grazing incidence diffraction of X-rays at a Si single crystal surface: Comparison of theory and experiment

To test the grazing incidence diffraction scheme for applications to surface studies we have measured with high resolution the angular dependence (“rod profile”) of the [220] Bragg intensity of X-rays under the condition of total reflection from the [001] surface of a Si single crystal. The experimental data agree quantitatively with predictions of the dynamical scattering theory, which takes into account a natural oxide layer and a tilt of the [220] lattice planes.

Huang Diffuse Scattering of Neutrons

Huang diffuse neutron scattering was measured for the first time on niobium with interstitially dissolved deuterium as well as on MgO after neutron irradiation and Li7F afterγ-irradiation. With Huang diffuse scattering the strength and symmetry of the distortion field around lattice defects can be determined. Our results clearly demonstrate that this method is feasible with neutrons.The present results are compared with X-ray experiments and the advantages of using neutrons is discussed in some detail.

First observations of phonon dispersion curves with inelastic x‐ray scattering (invited)

The first direct observations of phonon excitations with x rays were carried out with the inelastic x‐ray scattering spectrometer ‘‘INELAX’’ at HASYLAB of DESY, Hamburg. A Be single crystal and a diamond were used to measure the dispersions of longitudinal phonons in the acoustic and optical range in the [00ξ] direction.

High-energy resolution in X-ray scattering with the spectrometer INELAX. I: The principles and the test instrument

Very high-energy resolution measurements using X-rays can be achieved by extreme backreflection (Bragg angle close to 90°) from perfect crystals. This technique, combined with the high intensity of X-rays emitted by synchrotron-radiation sources, allowed the development of the instrument INELAX for inelastic scattering experiments. The principles and test results are discussed.

Lattice Distortions, Orientational Correlations and Lattice Relaxation in (KBr)1-x(KCN)x

The coherent elastic diffuse scattering of thermal neutrons from (KBr)1-x(KCN)x single crystals with x = 0.07, 0.17 and 0.32, which strongly increases below the (orientational) freezing temperature, is described as Huang diffuse scattering caused by the lattice distortions around the CN- defects in the KBr matrix. A quantitative analysis reveals that at the lowest temperatures the CN- ions are frozen preferably in [111] directions with some also in [100]. The orientational pair correlation function of the CN- shows parallel (or antiparallel) alignment. At temperatures above the freezing temperature relaxation-time effects of the distortion field, due to the rapid reorientation of the defect, are apparent.

Near Surface Structure of Ion Implanted Si Studied by Grazing Incidence X-Ray Scattering

X-rays incident on a surface under grazing angle may undergo total external reflection and excite an interior wave field damped exponentially into the bulk. These evanescent waves are a sensitive probe for the study the real structure in the near surface region. We report results on the influence of implantation defects on Bragg diffracted and on diffuse intensities. By detailed comparison of Bragg intensities with predictions of dynamical scattering theory we detect the presence of amorphous layers and determine their thickness. For the first time defect induced diffuse scattering underconditions of grazing incidence andexit is observed and compared to recent theoretical results. Strength and symmetry of implantation induced defects can be determined as well as their depth distribution which is compared to results of a TRIM simulation: the defect distribution is found to agree with that of the deposited collisional energy.

Lattice dynamics and ordering effects in high-concentrated α′-NbD x -alloys

The phonon dispersion of NbDx (0.57≦x≦0.77) in [110]-direction has been determined by inelastic neutron scattering after in situ loading of the sample. Two quasioptical excitations with Γ-point frequencies of ν=3.8 THz and ν=4.2 THz, respectively, are observed along with a threefold splitting of the longitudinal acoustic branch. At the zone boundary two of these branches are getting soft when increasing the deuterium concentration. The elastic constants extracted from the initial slopes of acoustic phonon branches exhibit an anomalous behavior in the concentration range under consideration.

Location of interstitial deuterium in Niobium by Huang diffuse scattering

Huang diffuse scattering experiments were performed on NbD single crystals using neutrons. So far such experiments were used to determine the strength and symmetry of the distortion field around the deuteriuml. In the present study the interstitial site occupancy has also been determined and it is found that deuterium occupies tetrahedral sites in Niobium.