Professor Paolo G. Radaelli OSI

Abstract:

The structural changes of stoichiometric LiNbO3 at temperatures up to 1470 K have been investigated by neutron powder diffraction. At Tc ≈ 1460 K, LiNbOi transforms from a paraelectric high-temperature (R3c) to a ferroelectric low temperature structure (R3c). The phase transition is of coupled order-disorder and displacive type. In the high-temperature phase, Li is highly disordered over two off-centre positions, whereas Nb takes a centrosymmetric position within O6 octahedra. An analysis of anisotropic displacement parameters shows that the probability density function of Li is extended along c, that of the oxygens is more or less perpendicular to the Nb-O bonds, and that of Nb is slightly extended in the a - b plane at high temperatures. An increase of the tilt angle of the NbO6 octahedra away from an h.c.p. arrangement towards a perovskite structure was observed. The octahedra become almost regular at high temperatures. Comparison with an earlier investigation of congruent lithium niobate shows a quite similar behaviour, however, all parameters of the latter are closer to those of the perovskite structure at same temperatures. Anomalies in the behaviour of the order parameters, a more complicated disorder and a high mobility of Li at high temperatures are explained by the competition of the order-disorder and displacive character of the phase transition. © 1997, Walter de Gruyter. All rights reserved.

Abstract:

Mn K-edge X-ray absorption near edge structure (XANES) measurements have been made to study the temperature-dependent lattice effects in the La0.75Ca0.25MnO3 system. The high-resolution XANES spectra recorded with high signal-to-noise ratio have allowed us to study the temperature dependence of the Jahn-Teller splitting indicated directly by the dipole forbidden pre-peaks corresponding to transition in Mn 3d states. The results show splitting of the eg state in its two orbitals (dx2-y2 and d3z2-r2). The energy splitting shows anomalous temperature dependence across the giant magnetoresistance transition temperature. © 1997 Plenum Publishing Corporation.

Abstract:

Neutron and synchrotron X-ray powder diffraction techniques have been used to refine the crystal and magnetic structures of the n = 2 Ruddlesden-Popper (RP) system Sr2LaMn2O7. The sample is shown to be biphasic, although both phases are of the RP type and have similar structural parameters. The majority phase (81%) adopts a collinear antiferromagnetic structure below ∼210 K whereas the minority phase is ferromagnetically ordered below ∼125 K. The ordered magnetic moments lie in the xy plane in both phases. The behavior observed is discussed in terms of the interplay between structural and electronic factors. Comparison with data obtained previously by other workers leads to the conclusion that our results have some general significance in the study of n = 2 RP systems.

Abstract:

This paper reports pressure dependent high resolution neutron diffraction work on N2-clathrates, which for the first time provides numbers on the compressibility as well as the location and degree of filling of the guest molecules in the small and large cages. N2-clathrates crystallize, at least at lower pressures and temperatures near 0 °C, in the Stackelberg type II structure. However, during the diffraction experiments we have observed the transient and partial formation of the von Stackelberg type I N2-clathrate at pressures exceeding several hundred bar. The filling of the small cages in the type II clathrate roughly follows a Langmuir isotherm. In contrast to most previous assumptions there is strong evidence that the large cages are doubly occupied in both type I and type II N2-clathrates. The observed filling can be fitted reasonably well by a two-constant Langmuir model.

Abstract:

The unusual magnetic properties of (Formula presented) were found to be associated with structural and magnetic ordering phenomena, resulting from the close interplay between charge, orbital, and magnetic ordering. Analysis of synchrotron x-ray and neutron powder diffraction data indicates that the anomalous and hysteretic behavior of the lattice parameters occurring between (Formula presented) ∼225 K and (Formula presented) ∼155 K is due to the development of a Jahn-Teller (J-T) distortion of the (Formula presented) octahedra, the (Formula presented) orbitals being oriented perpendicular to the orthorhombic b axis. We observed an unusual broadening of the x-ray Bragg reflections throughout this temperature region, suggesting that this process occurs in stages. Below (Formula presented), the development of well-defined satellite peaks in the x-ray patterns, associated with a transverse modulation with q=[1/2-ɛ,0,0], indicates that quasicommensurate (ɛ∼0) orbital ordering occurs within the a-c plane as well. The basic structural features of the charge-ordered low-temperature phase were determined from these satellite peaks. The low-temperature magnetic structure is characterized by systematic broadening of the magnetic peaks associated with the "(Formula presented) " magnetic sublattice. This phenomenon can be explained by the presence of magnetic domain boundaries, which break the coherence of the spin ordering on the (Formula presented) sites while preserving the coherence of the spin ordering on the (Formula presented) sublattice as well as the identity of the two sublattices. The striking resemblance between these structures and the structural "charge ordering" and "discommensuration" domain boundaries, which were recently observed by electron diffraction and real-space imaging, strongly suggests that these two types of structures are the same and implies that, in this system, commensurate long-range charge ordering coexists with quasicommensurate orbital ordering. © 1997 The American Physical Society.