Professor Paolo G. Radaelli OSI

Abstract:

We report on the temperature dependence of the crystal and magnetic structure of the layered colossal magnetoresistive manganite, La2-2xSr1+2xMn2O7, x = 0.3. Neutron-diffraction measurements show that the insulator-metal (IM) transition (TIM) at 100 K is accompanied by a ferromagnetic (FM) ordering of spins within MnO6 bilayers (intrabilayer coupling), but with an antiferromagnetic coupling between neighboring bilayers (interbilayer coupling). Below TIM, the Mn spins rotate from 45° inclination to the c axis until they are almost parallel to the c axis at 5 K. Coincident with this spin reorientation, a FM c axis component develops below 75 K. Evidence from both neutron and synchrotron x-ray-diffraction experiments suggest that the FM c-axis magnetic moment results from a second layered manganite phase with composition 0.3≲x≲0.32. This observation emphasizes the need for thorough examination of the homogeneity when measuring bulk properties (e.g., magnetization, transport) of nominally x=0.3 samples. Associated with the electronic and magnetic transitions, a pronounced lattice response along the c axis (observed in both phases) signals a transfer of charge into dx2-y2 orbitals in the low-temperature phase. That the lattice effects here are opposite in sign to those observed in the x=0.4 layered manganite points to the sensitivity of the spin-lattice-charge coupling to dopant concentration in these reduced-dimensionality manganites. © 1999 ThAmerican Physical Society.

Abstract:

Neutron Rietveld refinement of a synthetic low silica ferrierite in deuterium form was performed in the Immm space group. This space group is different from that of as-synthesized form (which is P21/n), and its cell volume (V=1977Å3) is slightly smaller (V=1984Å3). These differences do not cause noticeable modifications in the framework. Two Brønsted acid sites were recognized. The first was on framework oxygen O4, which bridges T1 and T3 tetrahedron cations, and headed towards the center of the ferrierite cage. The other was on the framework oxygen O6, which bridges two T4 tetrahedron cations, and headed towards the 10-ring channel parallel to [001], not far from the center of the 8-ring of the ferrierite cage facing towards the channel. Both deuterium sites were occupied in about 15% of cases. Three other extraframework sites, at great distances from the extraframework oxygens and weakly occupied, were recognized in D-ferrierite, and were attributed to residual ND4 groups. © 1999 Elsevier Science B.V.

Abstract:

Few recent examples are discussed where neutron diffraction had a major impact in rapidly developing areas of materials science and that illustrate advances in experimental technique and data analysis. Emphasis is put on imaging structural information across a multidimensional parameter space. High-speed diffractometers with low background have enabled impressive structural studies on small samples previously thought to be beyond the capability of neutron diffraction. Precision measurements of structural parameters have provided critical tests of theoretical models for the behavior of novel materials, and extreme sample environments have opened new experimental frontiers. The impact of neutron diffraction on condensed-matter science is highlighted.

Abstract:

The structural, magnetic and transport phase diagrams of the manganese perovskites (A1-xA'xMnO3) are characterized by several phenomena, including high-temperature polaronic behavior, charge-orbital and magnetic ordering and colossal magnetoresistance (CMR). These properties can be tuned by changing the doping level, the electronic bandwidth, and the A-site disorder. To demonstrate this, the recent x-ray synchrotron and neutron diffraction data on the crystallographic and magnetic modulation in La0.33Ca0.67MnO3 are presented.

Abstract:

Neutron powder diffraction measurements of perdeutero durene in the temperature range from 1.5 K to 290 K have been performed. The lowest temperature structure is the starting point for calculations of the methyl group tunneling and librational dynamics. Ab initio methods and atom-atom potentials are used to determine rotational single particle and coupling potentials. Tunneling splittings and librational bands are calculated by numerical solution of Schrödinger's equation for a system of many coupled methyl groups. High-resolution inelastic neutron scattering measurements of methyl tunneling and molecular vibrations have been repeated, the tunneling results resolving an inconsistency with earlier NMR work. Quantum molecular dynamics provide a stringent test of the numerical methods and the data are ultimately well reproduced. These results are also discussed in the context of optical measurements of dye molecules in a host lattice of durene. © 1999 American Institute of Physics.