Professor Paolo G. Radaelli OSI

Abstract:

Total scattering based atomic pair distribution function (PDF) analysis, with the advent of high data throughput neutron powder diffractometers, helps understanding the nature of the Jahn-Teller (JT) phase transition in La 1-xCaxMnO3 colossal magnetoresistive (CMR) manganites. The JT distortion of the MnO6 octahedra, is long-range ordered in the orthorhombic (O) phase, but disappears in the pseudo-cubic (O') phase crystallographically. An anomalous unit cell volume contraction occurs at the transition. The PDF study indicates that the distortion persists locally deep in the O' phase, contrary to the crystallographic view. Simultaneously, local structural features observed in PDF at 10.3 Å, sensitive to the oxygen sublattice changes, evolve dramatically across the transition. The same effect is observed irrespective of the way the O-O' phase boundary is crossed: it is seen both in the temperature series data for x=0, and in the doping series data at 310 K and at 550 K. © by Oldenbourg Wissenschaftsverlag.

Abstract:

The local and intermediate structure of La1 - x Cax MnO3 has been studied in the orthorhombic (O) and pseudo-cubic (O^′) phases, both as a function of temperature (x = 0.00), and as a function of Ca content (at 310 K). Neutron powder diffraction-based high real space resolution atomic pair distribution function (PDF) analysis shows that the Jahn Teller distortion of the MnO6 octahedra persists locally deep in the O^′ phase in both cases studied, contrary to the average crystallographic view. The O to O^′ structural phase transformation does have a local structural signature evidenced as a dramatic change in a PDF peak at 10.3 Å sensitive to the rotations of the MnO6 octahedra, providing further evidence that the nature of the transformation is orbital order to disorder regardless of whether the phase boundary is crossed as a function of T or x. © 2006 Elsevier B.V. All rights reserved.

Abstract:

Texture analysis by time-of-flight neutron diffraction, carried out on a multidetector instrument, requires just a few sample orientations. Moreover, on a diffractometer like GEM at ISIS with sufficiently high detector coverage a quantitative bulk texture analysis can be performed even on a stationary sample in a matter of minutes. A 'single-shot', rapid texture measurement on a high-count-rate instrument can be of considerable benefit for studying materials at non-ambient conditions or for bulky samples that cannot be mounted on a goniometer. The capabilities of GEM for texture analysis are demonstrated with results on copper texture standards, which were studied to accompany diffraction analyses of archaeological objects. © 2006 Elsevier B.V. All rights reserved.