Professor Paolo G. Radaelli OSI

Abstract:

The structural and magnetic properties of the hexagonal four-layer form of SrMn O3 have been investigated by combining magnetization measurements, electron diffraction, and high-resolution synchrotron x-ray and neutron powder diffraction. Below 350 K, there is subtle structural phase transition from hexagonal symmetry (space group P 63/mmc) to orthorhombic symmetry (space group C 2221) where the hexagonal metric is preserved. The second-order phase transition involves a slight tilting of the corner-sharing Mn2 O9 units composed of two face-sharing Mn O6 octahedra and the associated displacement of Sr2+ cations. The phase transition is described in terms of symmetry-adapted displacement modes of the high symmetry phase. Upon further cooling, long range magnetic order with propagation vector k= (0,0,0) sets in below 300 K. The antiferromagnetic structure, analyzed using representation theory, shows a considerably reduced magnetic moment indicating the crucial role played by direct exchange between Mn centers of the Mn2 O9 units. © 2007 The American Physical Society.

Abstract:

Total scattering based atomic pair distribution function (PDF) analysis, with the advent of high data throughput neutron powder diffractometers, helps understanding the nature of the Jahn-Teller (JT) phase transition in La 1-xCaxMnO3 colossal magnetoresistive (CMR) manganites. The JT distortion of the MnO6 octahedra, is long-range ordered in the orthorhombic (O) phase, but disappears in the pseudo-cubic (O') phase crystallographically. An anomalous unit cell volume contraction occurs at the transition. The PDF study indicates that the distortion persists locally deep in the O' phase, contrary to the crystallographic view. Simultaneously, local structural features observed in PDF at 10.3 Å, sensitive to the oxygen sublattice changes, evolve dramatically across the transition. The same effect is observed irrespective of the way the O-O' phase boundary is crossed: it is seen both in the temperature series data for x=0, and in the doping series data at 310 K and at 550 K. © by Oldenbourg Wissenschaftsverlag.

Abstract:

The local and intermediate structure of La1 - x Cax MnO3 has been studied in the orthorhombic (O) and pseudo-cubic (O^′) phases, both as a function of temperature (x = 0.00), and as a function of Ca content (at 310 K). Neutron powder diffraction-based high real space resolution atomic pair distribution function (PDF) analysis shows that the Jahn Teller distortion of the MnO6 octahedra persists locally deep in the O^′ phase in both cases studied, contrary to the average crystallographic view. The O to O^′ structural phase transformation does have a local structural signature evidenced as a dramatic change in a PDF peak at 10.3 Å sensitive to the rotations of the MnO6 octahedra, providing further evidence that the nature of the transformation is orbital order to disorder regardless of whether the phase boundary is crossed as a function of T or x. © 2006 Elsevier B.V. All rights reserved.