Professor Paolo G. Radaelli OSI

Abstract:

Phengite 3T (Dora-Maira massif, Italian western Alps), with chemical composition K0.96Na0.01Al1.44Mg0.56(Si3.59Al0.41)O10(OH)2 has been investigated using powder neutron diffraction, at the ILL (Institut Laue-Langevin) Facility, on the D2B diffractometer. Data sets were collected at 293 and 873 K, and the present results are compared with those obtained previously using the time-of-flight (TOF) technique, on the same compound. In the octahedral sheet, Al tends to order into the M2 site, in accordance with the measurements mentioned above, whereas the Al/Si tetrahedral ordering we observe (i.e. Si fully into T1 site) is at variance with that determined previously. These discrepancies are ascribed to the presence of talc impurities in the sample used previously (~6 wt.%), which affected the results obtained.

Abstract:

The structural refinement of YSr2Cu3O6.84 allows us to analyze why the chemical pressure applied to YBa2Cu3O6.84, by substituting Sr for Ba, results in a decrease of Tc (88 to 63 K). Even though the chemical pressure induces a volume decrease of 5.7%, the thickness along the c axis of the superconducting block increases by 3.0%. This behavior seems to be due to the incompressibility of the Y coordination polyhedron. The SrO layer becomes negatively strained since the Sr polyhedron does not reduce along with the Sr cation substitution. The calculated bond valence sum for the latter cation is 1.63 v.u. instead of the expected 2. The apical distance of the pyramid around Cu2 decreases drastically from 2.30 to 2.09 Å. All these features induce an instability within YSr2Cu3O6.84, which makes it difficult for the extra oxygen to go in and out of the structure as it does in its Ba counterpart. This instability would also reduce the hole transfer from the CuO chains to the superconducting CuO2 planes. © 2000 Elsevier Science B.V All rights reserved.

Abstract:

The phase segregation in the narrow-bandwidth manganite Pr0.7Ca0.3MnO3 were investigated using high-resolution time-of-flight neutron powder diffraction. Results demonstrate that the phase segregation in Pr0.7Ca0.3MnO3 is unrelated to electronic phase segregation but is primarily driven by chemical disorder and intra-granular strains, similar to martensitic phase transitions.