Local lattice dynamics and the origin of the relaxor ferroelectric behavior.

Relaxor ferroelectricity is observed in many strongly disordered ferroelectric solids. However, the atomistic mechanism of the phenomenon, particularly at high temperatures, is not well understood. In this Letter we show the local lattice dynamics as the origin of relaxor ferroelectricity through the first use of the dynamic pair-density function determined by pulsed neutron inelastic scattering. For a prototypical relaxor ferroelectric, Pb(Mg(1/3)Nb(2/3))O(3), we demonstrate that the dynamic local polarization sets in around the so-called Burns temperature through the interaction of off-centered Pb ions with soft phonons, and the slowing down of local polarization with decreasing temperature produces the polar nanoregions and the relaxor behavior below room temperature.

Damping of antiferromagnetic spin waves by valence fluctuations in the double layer perovskite YBaFe2O5.

Inelastic neutron scattering experiments show that spin dynamics in the charge-ordered insulating ground state of the double layer perovskite YBaFe(2)O(5) is well described in terms of e(g) superexchange interactions. Above the Verwey transition at T(V)=308 K, t(2g) double exchange-type conduction proceeds within antiferromagnetic FeO(2)-BaO-FeO(2) double layers by an electron hopping process that requires a spin flip of the five-coordinated Fe ions, costing an energy of 5S(2) approximately 0.1 eV. The hopping process disrupts near-neighbor spin correlations, leading to massive damping of zone-boundary spin waves.

Stabilization of charge ordering in La1/3Sr2/3FeO3-delta by magnetic exchange.

The magnetic exchange energies in charge ordered La1/3Sr2/3FeO3-delta (LSFO) and its parent compound LaFeO3 (LFO) have been determined by inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic exchange between Fe3+-Fe5+ pairs (JF) and antiferromagnetic exchange between Fe3+-Fe3+ pairs (JAF) fulfills the criterion for charge ordering driven by magnetic interactions (|JF/JAF|>1). The 30% reduction of JAF as compared to LFO indicates that doped holes are delocalized, and charge ordering occurs without a dominant influence from Coulomb interactions.

Chemical applications of neutron scattering.

Neutron scattering represents a versatile technique for chemists, as it explores the structure and dynamics of materials at the atomic scale. This review gives an outline of the neutron scattering formalism and describes diffraction and inelastic and quasielastic scattering techniques. Applications to chemistry are illustrated by examples from recent work on crystalline and liquid organic materials, including heterogeneous systems, bulk polymers and polymer solutions. There is particular emphasis on systems where hydrogen bonding plays a significant role. With more powerful sources and improved instrumentation in prospect, there is considerable potential for future extension of these methods to increasingly complex materials.