X-ray and neutron scattering

Abstract:

In 1947 Kathleen Lonsdale conducted a series of experiments on X-ray diffraction using a divergent beam external to a crystal sample. Unlike the Kossel technique, where divergent X-rays are excited by the presence of fluorescing atoms within the crystal, the use of an external divergent source made it possible to study non-fluorescing crystals. The resulting photographs not only illustrated the complexity of X-ray diffraction from crystals in a truly beautiful way, but also demonstrated unprecedented experimental precision. This long-forgotten work is repeated here using a synchrotron radiation source and, once again, considerable merit is found in Lonsdale's technique. The results of this experiment suggest that, through the use of modern 'third-generation' synchrotron sources, divergent-beam diffraction could soon enjoy a renaissance for high-precision lattice-parameter determination and the study of crystal perfection.

Abstract:

We report on the extension of resonant magnetic diffraction into the soft X-ray region (500-1500eV). We conducted soft X-ray diffraction from a bulk single crystal of La2-2xSr1+2xMn2O7 using an in-vacuum diffractometer. We demonstrate that the resonant enhancement diffraction of the (001) magnetic peak at the manganese L-III and L-II absorption edges is extremely large. Measurements taken on samples with different strontium dopings demonstrate that the shape of the energy resonance is also dependent on the sample stoichiometry. We have applied this technique to the study of other materials such as UAs1-xSex where incommensurate magnetic diffraction has been observed at the uranium N-IV and N-V edges. (C) 2003 Elsevier B.V. All rights reserved.

Abstract:

We report measurements of the phonon dispersion relations in nonsuperconducting, oxygen-deficient PrBa2Cu3O 6+x (x ≈ 0.2) by inelastic neutron scattering. The data are compared with a model of the lattice dynamics based on a common interatomic potential. Good agreement is achieved for all but two phonon branches, which are significantly softer than predicted. These modes are found to arise predominantly from motion of the oxygen ions in the CuO2 planes. Analogous modes in YBa2Cu3O6 are well described by the common interatomic potential model.