Disordered Materials

M.-L. Saboungi, D. L. Price, G. Mao, Y. Ren, H. Schnyders, J. Enderby, R. Fernandez Perea, S. Tamura; in collaboration with L. A. Curtiss and L. E. Iton, Computational Materials Chemistry Group, and M. Grimsditch, Magnetic Thin Films Group

Present work is focused on silver chalcogenides, nanomaterials, glasses for waste storage, homogeneous and composite polymer electrolytes, and levitated liquids in both normal and supercooled states.  The objective is to study the structural and dynamical properties of disordered matter, measure electrical, optical and magnetic properties of interest, and derive a basic understanding of the inter-relationships.  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.

Unusual magnetoresistance in silver chalcogenides. Following up our previous discovery of unusual positive magnetoresistance (MR) in off-stoichiometric Ag2+xSe and Ag2+xTe, systematic studies of the effect have been carried out as a function of composition and temperature in both silver-rich and silver-deficient material.  In collaboration with LANL, extension of the measurements into higher magnetic fields is underway to determine if saturation exists for the MR. The results are being compared with A. A. Abrikosov's theory of quantized Landau levels.

Nanocrystalline materials. Nanomaterials and materials in confined geometry continue to be the center of our activities with a special emphasis on the relationship between transitions (thermal, electronic and electrical) and size.  To that effect, AXS continues to be our most appropriate structural technique along with small-angle neutron scattering, Raman scattering (including resonant Raman scattering for appropriate elements), transmission electron microscopy and spectrophotometry. Most recently, Se encapsulated in a variety of zeolites has been investigated showing (i) the formation of chains, radicals and rings such as the elusive Se8 rings and (ii) the presence of interactions leading to an unusually long intrachain bond length and a remarkably small band gap.
Glasses for nuclear waste storage. The impact of additive such as uranium dioxide on the intermediate and short-range order of iron-based phosphate glasses is being investigated by complementary techniques, neutron diffraction and AXS.  The extent of the preservation of local order is being followed as a function of UO2 content.  This work is done in collaboration with a DOE-funded program at the University of Missouri.  We maintain a continuing collaboration with the waste storage program in Chemical Technology Division.  Recently we measured the structure of the world's only sample of amorphous zirconium, amorphized by radiation damage at PNL.

Polymer electrolytes. We are studying the atomic structure and dynamics of polymer electrolytes made by dissolving lithium salts with large cations [perchlorate, triflate, TFSI) in polyethylene oxide. These are technologically important materials but surprisingly little is known about the details of the lithium conduction mechanism, except that it appears to take place principally in the amorphous, as opposed to crystalline, phases of the materials. We have established the lithium environment in the amorphous polymer electrolyte by NDIS and investigated the profound effects of the cross-linking by the lithium ions on the polymer dynamics by QENS measurements with back-scattering and spin-echo spectrometers. It appears that the lithium ions are coordinated to about five ether oxygens on two neighboring chains and that lithium transport is closely tied to segmental motions of the polymer chains.  Our experimental work is closely coordinated with the ab initio quantum chemistry and molecular dynamics of L. A. Curtiss and J. W. Halley.

Levitated liquids.  We have combined conical nozzle levitation with x-ray measurements at synchrotron sources to measure the structure of corrosive and refractory liquids that cannot be contained.  With these techniques we made the first diffraction measurements of molten Al2O3 and Y2O3 and liquid boron.  The lack of a container removes the possibility for heterogeneous nucleation and makes it possible to study the liquids down into the deeply supercooled state (typically by a few hundred K).  In this way we were able to make the first structural measurements on supercooled silicon and establish a drop in coordination number with cooling consistent with the transition proposed by Angell and others from a metallic to a semiconducting liquid.  In parallel with the diffraction measurements we have developed a method for electrodeless measurement of electrical conductivity and magnetic susceptibility of the levitated specimen.