Oxide electronics

Abstract:

The magnetic superstructure in La0.333Ca0.667MnO3 has been solved using neutron powder diffraction analysis and two models (a `Stripe' and a `Wigner crystal' one) have been proposed for the charge and orbital ordering associated with it. The observed antiferromagnetic superreflections can be described by a collinear model where the spins are predominantly oriented along the a axis. However a considerable improvement in the Retvield refinement is yielded by a non-collinear model within the `Wigner' arrangement of the charges.

Abstract:

In order to unveil the mechanism responsible for the large decrease of Tc (ΔTc ≈ -30 K) found in YSr2Cu3O6+x with respect to YBa2Cu3O6+x, we compare structural and electronic properties of these two cuprates. We report X-ray, neutron diffraction, resistivity and ac susceptibility data and results of ab initio electronic structure calculations carried out within the local density approximation. Main structural differences between the two phases are: 1) the absence of long CuO chains in YSr2Cu3O6+x possibly due to a large orthorhombic distortion predicted by calculations; 2) the strong compression of the CuO5 pyramids along the z-direction, which reduces the metallic character of the bond between the chain copper and the apical oxygen. This is expected to hinder the hole transfer from the CuO chains to the superconducting CuO2 planes, thus reducing the effective doping of the planes with respect to the optimum doping level. Both the disorder associated with short CuO chains and the reduced hole transfer would qualitatively account for the observed reduction of Tc.

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.