Professor Paolo G. Radaelli OSI

Abstract:

A quantitative determination of the statistical distribution of the Mn-O instantaneous bond lengths in the La0.75Ca0.25MnO3 system by Mn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy shows that the giant magneto-resistance (GMR) transition occurs at a crossover from a metallic phase with large (or intermediate) polarons to an insulating phase where small polarons coexist with large (or intermediate) polarons. In the GMR region, the two Mn polaronic domains coexist and share half of the space. © 1998 Published by Elsevier Science Ltd. All rights reserved.

Abstract:

We report Mn K-edge extended x-ray absorption fine structure spectra on La0.75Ca0.25MnO3 up to high momentum transfer across the metal-insulator (M-I) transition. The data show compelling evidence for (i) large or intermediate Jahn-Teller polarons (IJTP), characterized by an anomalous longer Mn-O bond (Δ R = 0.09 Å) in the metallic phase (T < 170K), and (ii) appearance of small JT polarons (SJTP) at T> 170K, characterized by a longer Mn-O bond (Δ R = 0.21 Å), which coexist with the IJTP above the M-I transition and has equal probability in the temperature range of colossal magnetoresistance. © 1998 The American Physical Society.

Abstract:

The structural properties of (Formula presented) were studied by x-ray synchrotron and neutron-powder diffraction as a function of temperature (Formula presented) K), and as a function of x-ray fluence at 15 and 20 K. The temperature evolution of the lattice parameters and of the superlattice reflections is consistent with the development of charge and orbital ordering below (Formula presented)180 K, followed by antiferromagnetic ordering below (Formula presented)140 K, similar to what was previously observed for (Formula presented). Below (Formula presented)120 K, the magnetic structure develops a ferromagnetic component along the (Formula presented) axis on the Mn ions. At low temperatures, a small ferromagnetic moment of 0.45(2)(Formula presented) oriented in the same direction appears on the Pr ions as well. The observation in (Formula presented) of significant lattice strain developing below (Formula presented) as well as the development of a ferromagnetic component to the magnetic structure at (Formula presented)120 K, can be interpreted in terms of the presence of ferromagnetic clusters with an associated lattice distortion from the average structure. At low temperatures, exposure to the x-ray beam produces a phase-segregation phenomenon, whereby the ferromagnetic droplets coalesce into larger aggregates. Further exposure results in a gradual melting of the charge-ordered phase and the formation of a second phase, recently shown to be a ferromagnetic metallic phase by Kiryukhin et al. [Nature (London) 386, 813 (1997)]. The ferromagnetic phase has a significantly smaller (Formula presented) lattice parameter and unit-cell volume (Formula presented) than that of the charge-ordered phase. © 1998 The American Physical Society.

Abstract:

The strucure of the fast proton conductor Cs3H(SeO4)2 was investigated by high resolution neutron powder diffraction at 5, 300, 400 and 483K. The aim of this study was a precise localization of the hydrogen position and the determination of the hydrogen bond network to get insight into the mechanism of the proton conductivity and the phase transitions.

Abstract:

The superconducting oxide BaPb(1-x)Bi(x)O3, discovered in 1975 (ref. 1), is an exotic system having an unusually high transition temperature (T(c)) of ~12K, despite a relatively low density of states at the Fermi level. The subsequent prediction that doping the electronically inactive barium donor sites, instead of the bismuth sites, might induce superconductivity with a higher T(c) led to the discovery in 1988 of superconductivity in the Ba(1-x)K(x)BiO3 system (T(c) ~30 K for x = 0.4). But it remains an open question why many of the superconducting properties of these materials are similar to those of the well-known copper oxide superconductors, despite their pronounced structural differences: the former have a three-dimensional bismuth-oxygen framework, whereas the structures of the latter are predominantly two-dimensional, consisting of copper-oxygen planes. Understanding of the copper oxide superconductors has gained immensely from the study of many different superconducting systems, and so it might be expected that the identification of bismuth oxide superconductors beyond the substituted BaBiO3 compounds will prove to be similarly fruitful. Here we report the synthesis of a second family of superconducting bismuth oxides, based on SrBiO3. We show that partial substitution of potassium or rubidium for strontium induces superconductivity with: T(c) values of ~12 K for Sr(1-x)K(x)BiO3 (x= 0.45-0.6) and ~13K for Sr(1-x)Rb(x)BiO3 (x = 0.5).