Professor Paolo G. Radaelli OSI

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.

Abstract:

Geometrical frustration arises when geometrical constraints promote a locally degenerate ground state. A periodic system with this local geometry may "freeze" on cooling forming "ices" or remain liquid down to the lowest temperatures due to quantum effects. A third possibility is that of a structural phase transition that lowers the local symmetry and lifts the degeneracy. Two classic examples of geometrical frustration are the so-called pyrochlore lattice, which is also found in AB2X4 spinels, and the "J1-J2" model on a square lattice, which involves competing nearest- and next-near-neighbor magnetic interactions. We present recent results obtained by time-of-flight (TOF) neutron powder diffraction on orbital ordering in transition-metal spinels, leading to the concept of orbitally-driven Peierls state, and more recent data on MoOVO4, a realization of the J1-J2 model. A surge of interest in the so-called multiferroic materials has led to revisit the role of geometrical frustration in coupling different degrees of freedom. In this context, we present recent results on REMn2O5 obtained by neutron single-crystal and powder diffraction. © 2006 Elsevier B.V. All rights reserved.

Abstract:

Identification and imaging of crystallographic phases inside an object can be achieved by time-of-flight neutron diffraction, based on a correction formula that is usually used to account for a sample offset on a powder diffractometer. The procedure allows the distribution of crystallographic phases along the incident beam path through the thickness of the material to be reconstructed. Phase reconstruction is demonstrated on a benchmark object. © 2006 Elsevier B.V. All rights reserved.

Abstract:

The magnetoelastic coupling in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ and in Ca$_{1.5}$Sr$_{0.5}$RuO$_4$ has been studied combining high-resolution dilatometer and diffraction techniques. Both compounds exhibit strong anomalies in the thermal-expansion coefficient at zero and at high magnetic field as well as an exceptionally large magnetostriction. All these structural effects, which are strongest in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$, point to a redistribution of electrons between the different $t_{2g}$ orbitals tuned by temperature and magnetic field. The temperature and the field dependence of the thermal-expansion anomalies in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ yield evidence for a critical end-point lying close to the low-temperature metamagnetic transition; however, the expected scaling relations are not well fulfilled.