Research Overview

The primary goal of the neutron and x-ray scattering group is to ensure that the division can pursue strong multidisciplinary research programs of the nature that are only possible if scattering capabilities are combined with other experimental work.  Members of the group play lead roles in several of these programs.  The primary research areas currently are in:

• High-Temperature Superconductors
• Colossal Magnetoresistive Materials
• Ceramic Membrane Reactor Materials
• Negative Thermal Expansion Materials
• Magnetic Response of Strongly Correlated Electron Systems
• Disordered Materials
• Magnetism in Thin Films and Multilayers

A second important goal of the group is to provide the technical expertise required to continue the development of a future advanced pulsed neutron source.  Staff in the group continue to play a lead role in the development of new instrumentation and user-based research programs at the Intense Pulsed Neutron Source (IPNS) and serve as instrument scientists, with the help of scientific support staff from the IPNS Division, for four IPNS instruments.  Members of the group are becoming involved in the development of instrumentation for the Spallation Neutron Source at Oak Ridge National Laboratory (ORNL) and are also active in developing new x-ray scattering instrumentation and techniques for use at the Advanced Photon Source (APS) and other synchrotron x-ray sources, working in collaboration with the Synchrotron Radiation Science program.

Approach

High-Temperature Superconductors. Our work on high-temperature superconductors focuses on the relationship among chemical composition, crystal structure, and superconducting properties.  At the present time, we are investigating what features of the chemistry and structure, beyond the use of defects to control the carrier concentration, are critical to superconductivity.  This work has led to a hypothesis for the ideal structure for a high-temperature superconductor and has revealed new aspects of the chemistry that are a reflection of the novel underlying physics of these materials.

Perovskite Colossal Magnetoresistive Materials.  In collaboration with B. Dabrowski (Northern Illinois University, we have studied the relationship among crystal structure, magnetic structure, and transport in materials of the general formula La1-xMxMnO3+d (M=Ca, Sr).  The goal of this work has been to establish comprehensive phase diagrams as a function of metal-site substitution (x) and oxygen defect concentration (d).

Ceramic Membrane Reactor Materials. The present work is focused on understanding and optimizing a ceramic material that exhibits both electronic and oxygen ionic conduction for use as an oxygen separator in a reactor that is used to convert natural gas to synthesis gas.  The target material, called SFC-2, was developed a few years ago at Argonne and found to perform well in the application, but its crystal structure and the basis for its performance (from a fundamental point of view) were not known.  Over the last three years we have characterized this material with the aim of optimizing its properties.  We first learned that SFC-2 was a mixed phase material consisting of a layered compound (not previously known) with fixed composition and a perovskite compound with variable composition.  Because the key to controlling the transport properties is the perovskite compound (by varying its composition) we have focused on this material during the last year.

Negative Thermal Expansion Materials. This work was begun as a collaboration with A. W. Sleight (Oregon State Univ.) and eventually grew to produce a number of publications.  A renewed interest in negative thermal expansion (NTE) occurred when Sleight showed that ZrW2O8 exhibits a very large isotropic NTE over the temperature range 4-1050 K.  Previously studied materials exhibit bulk NTE over a limited temperature range or NTE along one crystallographic axis.  Our work began with the study of ZrW2O8 under pressure to determine what kinds of structural changes could give rise to volume reduction.  This work was extended to studies as a function of temperature and studies of chemically modified compounds.

Magnetic Response of Strongly Correlated Electron Systems. The work on strongly correlated systems has concentrated on understanding the origin of the unusual bulk properties of non-Fermi liquid uranium compounds. We have now performed neutron measurements using full polarization analysis of a compound with x = 2.  In previous years, we have established the existence of universal scaling of the dynamic magnetic susceptibility of f-electrons in the non-Fermi liquid compounds UCu5-xPdx that we attributed to quantum critical scattering. Exploiting the "negative chemical pressure" produced by doping CeAl3 with lanthanum is providing new insights into puzzling low-temperature anomalies in this canonical heavy fermion compound.

Disordered Materials. Present work is focused the study of levitated liquids in both normal and supercooled states. Principal techniques include neutron diffraction with isotope substitution (NDIS), quasielastic (QENS) and inelastic (INS) neutron scattering, total and anomalous (AXS) x-ray scattering, four-probe and electrodeless measurements of conductivity and susceptibility, and optical spectrophotometry. Complementary and related work on other materials is described in Dynamics, Energetics and Structure of Ordered and Metastable Materials (57502).

Magnetism in Thin Films and Multilayers. We are using a polarized neutron reflectometer (POSY 1) in order to study the magnetism of thin films and multilayers. The research is conducted in close collaboration with the thin film group in MSD and with leading universities. The main object is to map out the configuration of the magnetic moments in thin films and multilayers and to link them with expectations due to micromagnetic calculations and transport phenomena such as magnetization, magnetic anisotropy and magnetoresistance.

Relationships to other Projects

This work is carried out through important collaborations with the Argonne Materials Science programs on Naturally Layered Manganites, Magnetic Thin Films, Emerging Materials, Condensed Matter Theory, Synchrotron Radiation Studies, and Computational Materials Chemistry, and the Intense Pulsed Neutron Source.  The Ceramic Membrane research has also involved a close collaboration with scientists in the Energy Technology Division and a CRADA with Amoco.  There are also extensive collaborations with scientists at Universities and, to a lesser extent, industry.  Until February 2000, significant support for research on High-Temperature Superconducting Materials came from the NSF through the Science and Technology Center for Superconductivity, with major  collaborations at Northwestern University and Northern Illinois University.