Research Interests of the Miller Group at ISU

The major goal of our research is to identify new inorganic materials that will show potentially interesting chemical and physical properties by coupling theoretical efforts with experimental approaches. During the past 12 years, we have concentrated on intermetallic compounds because not only are they well suited for combined theoretical/experimental investigations, but they still offer fundamental challenges towards fundamental understanding of the relationships among chemical composition, atomic structure, physical properties and chemical bonding. We study a variety of compound-types, but the primary theme is to elucidate the factors influencing the distributions and arrangements of chemical elements in complex structures: the so-called "coloring problem." That is, we want to answer the question, "Where are the atoms?". We have developed a successful algorithm, based on tight-binding theory, to predict patterns of atomic distributions in intermetallic compounds, and have demonstrated the need to combine careful synthesis with thorough diffraction and spectroscopic methods to characterize complex materials. A related goal is to understand how these structural features affect physical properties, e.g., magnetic transitions, which we are currently developing in the class of extraordinary magnetoresponsive materials, Ln5(Si,Ge)4 (Ln = rare-earth elements).

Chemical Bonding Electronic Structure

Our theoretical efforts range from semi-empirical to first principles electronic structure calculations. We utilize Extended Hückel, LMTO ( linear muffin- tin orbital), LAPW ( linear augmented plane wave) and modified pseudopotential (VASP) calculations to investigate patterns of atomic arrangements, solid-solid phase transitions, and magnetic order. Our experimental endeavors currently involve high-temperature synthesis, e.g., chemical transport, arc-melting or eutectic mixtures as reaction fluxes (solvents), but we have utilized low-temperature routes as well using metal-organic precursors to produce new materials. We are also developing methods to use diffuse X-ray scattering to provide hints towards structural solutions of problems involving mixed occupancies of sites or vacancies. Students and research associates in my group will become exposed to both experimental and theoretical aspects of solid state chemistry as they prepare for their professional careers. We receive generous support from the National Science Foundation (Division of Materials Research), the Ames Laboratory and the Department of Energy, the American Chemical Society - Petroleum Research Fund, and Iowa State University.

Accomplishments

New Polar Intermetallics (Current Project: Support from NSF and ACS-PRF)
To investigate fundamental factors influencing the transition from metallic to semiconducting materials, we are probing compounds with 3-4 valence electrons per element. We discovered new icosahedral compounds in the alkaline earth-(Cu,Ag,Zn)-aluminum system. Surprisingly, these are poor metals and nonmagnetic (temperature-independent paramagnetic but with negligible magnetic response). Electronic structure calculations suggest these phases to behave as modified Zintl phases (i.e., like valence compounds). Au can replace Al in these polar intermetallics, but that the structural chemistry is significantly different than with Cu and Ag. From electronic structure calculations, there is a complex interplay between site energies and bond energies (two-atom pair potentials) that controls the atomic arrangement of Au and Al in these structures.
NaZn13-Type Structure
The Ba/Zn/Al system has allowed us to probe changes in network dimensionality with valence electron concentration and one of the new structures we have discovered in this system shows nearly planar eight-membered rings of Zn atoms. Studies of the Mg/Zn/Al system directed us toward quasicrystalline approximants and their atomic distributions. We discovered new, quaternary compounds in Li/Mg/Zn/Al and atomic arrangements and physical properties were studied theoretically and experimentally, which required a combination of X-ray and neutron diffraction with elemental analysis to reveal the actual compositions and site preferences. In recent years, we have begun to explore rare-earth polar intermetallics to study the interplay between structure and magnetic behavior. Ce-Ni-Al systems show that Al-rich phases exhibit mixed valent or heavy-fermion behavior, whereas Ni-rich systems do not. Eu-M-Ge (M = Zn, Ga, Ge) is an important series for elucidating the relationship between conductivity and magnetic order. Three new structures were discovered in the Eu-Ga-Ge system. We are also investigating the Si-rich RE-Fe-Si systems, and have discovered an important superstructure in an intergrowth system between double FeSi,2 layers and "RE2Si3" planes, which may have important electronic and spintronic implications.
GD1.2Fe4Si10 + diff
Relationships between Quasicrystalline and Incommensurate Structures (Current Project: Support from USDOE/Ames Lab)
Two recent projects identify this growing topic in our group. (1) Careful examination of the GaM (M = Cr, Mn, Fe) systems has shown these to be possible approximants to quasicrystals based on one-dimensional chains of icosahedra and dodecahedra. Furthermore, the different transition metals give rise to different low temperature magnetic behavior, ranging from antiferromagnetic to metamagnetic to ferromagnetic. The change in magnetic behavior can be understood from spin-polarized electronic structure calculations. There is atomic ordering in the pseudobinary GaCr1-xFex series. Attempts to explore other ternaries led to the discovery of a new superstructure of the Heusler-alloy system in Ti-Ni-Ga. (2) The system Zn1-xPdx (0.15 < x < 0.25) reveals a beautiful series of structures based on Pd-centered, Zn-icosahedra that may show quasiperiodicity in one-dimension. We have developed an algorithm for predicting structure and composition, which ties very well with theoretical studies.
Zn1-xPdx: 'Farey Tree'
Solid-Solid Phase Transitions; Magnetoresponsive Materials (Current Project: Support from USDOE/Ames Lab and ACS/PRF)
Our most recent efforts have been with the giant magnetocaloric materials, Gd5(SixGe1-x)4, which show magnetic and structural transitions near room temperature. We have provided accurate, temperature-dependent single crystal structural solutions for various materials, and have demonstrated an unprecedented, reversible covalent bond breaking/making process that accompanies the magnetic transition. Electronic structure calculations together with an application of the Ruderman-Kittel theory for magnetic exchange mediated by conduction electrons provide a rationale for the observed behavior. We have discovered that ferromagnetic behavior is "linked" to the number of electrons in the conduction band (this electron count is coupled with the structure of the Si,Ge Zintl ions) in model compounds Gd2MGe2 (M = Na, Mg, Al) and have examined the correlation between magnetic interactions and chemical bonding in these solids. Chemical substitutions using Ga or mixed rare-earth metals have uncovered interesting site distribution results and an important enlightening of the structure-property relationships. As part of this effort, we also developed a technique for performing single crystal diffraction at high temperatures (up to ca. 700 K).
R5T4 Series
Previous accomplishments included a blend of Landau theory (with Prof. H. F. Franzen) and electronic structure theory to characterize the structural transition in CaAl4 at 600 ° C. We have applied the Jahn-Teller theorems to propose a mechanism for the superconducting transitions in the rare earth borocarbides LnNi2B2C, as well as in dicarbides. Band theory allowed us to identify important electronic components that control the trend in superconducting transition temperature, and provide an isolobal analogy between superconducting carbides and these borocarbides. We have predicted that LnM2Si2C and LnNiBN (experimentally verified!) may show similar behavior.
Itinerant Magnetism: Effective Exchange Parameters and Bonding Analyses (Current Project: Support from NSF)
Magnetic order in 3d metal systems is coupled with the unpaired electrons that are also involved in chemical bonding. A recent collaboration with Prof. Richard Dronskowski and Dr. Boniface Fokwa (RWTH-Aachen) involves a study of complex borides that show magnetically active 3d metals (Mn, Fe, Co, Ni) in low-dimensional environments, such as wires and ladders. We have shown relationships between valence electron count and magnetic order (ferromagnetism vs. antiferromagnetism), and are using both COHP ("overlap population") strategies as well as evaluation of effective two-center exchange parameters to rationalize and predict magnetic order in these complex solids.
Complex Borides
Low-Dimensional Solids with Transition Metal Clusters (On hold; Supported from ACS/PRF, USDOE/Ames Lab and ISU)
Materials with layered morphologies have spaces available for intercalated or inserted chemical species. If the "substrate" contains metal-metal contacts, then such systems have tremendous potential to affect chemical reactions involving the inserted moieties. We completed the synthesis and structural characterization of a series of layered ternary niobium and tantalum chalcogenide halides, M3XY7 (M=Nb, Ta; X=S,Se,Te; Y=Cl,Br,I), all of which contain M3 triangles; and have discovered an interesting structural and magnetic chemistry in the Nb-S-I and Nb-Se-I systems. The Ta compounds represent the first triangular six-electron clusters of Ta in extended solids. Characterization of these materials has included magnetic susceptibility measurements, neutron powder diffraction at Argonne National Laboratory, photoelectron spectroscopy, as well as scanning tunneling and atomic force microscopy in collaboration with Prof. H.-J. Cantow in Freiburg, Germany. We have also discovered a new compound in the Ta/S/I system: Ta4SI11, which contains a mixture of Ta3 clusters and Ta4+ ions in a crystal with interesting vacancy and site substitution patterns. Transmission electron microscopic studies and diffuse X-ray scattering are leading to a proper interpretation of the crystal structure. Mixed metal clusters, e.g., NbxTa3-xSeI7 and NbxTa3-xTeI7 were also prepared and characterized by diffraction and microscopic techniques. The types of metal clusters occurring in these solids depends on the composition. X-ray diffuse scattering (using CCD data) is providing models and insights into the distribution of clusters and stacking faults observed in these inherently disordered materials.
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Recent Advances in the Electronic Structures of Solids (Support from NSF)
We are examining magnetic intermetallic alloys, e.g., Fe(Si,P), Ni(In,Sn), Ni(In, Sb) for relationships among local coordination, structure, magnetism, and chemical bonding. Another problem involves U2MSi3 (M = Ru, Rh, Pd) materials, in which the arrangement of M and Si atoms on a graphite net lead to very different physical properties. Previous efforts include using electronic structure to understand the distribution of elements and vacancies in Tl5Te3, Sn1-xTl5+xTe3, and Rh1+xTe2 (0.14 < x < 0.86), which suggests an inherent preference for linear Rh-Rh-Rh units in these solids. In recent years, we have investigated interstitial hydrides in intermetallics with respect to site occupations, effects on physical properties and electronic structure, and composition. These studies include the superconductivity of Ti3SbHx (0 < x < 3), hydrogen compositions in Ti3IrHx, Zr2Fe, Zr2Al, Zr6FeAl2, and the structures of CsCl-based hydrides such as MgRhHx and FeTiHx.
Organometallic Precursors to Inorganic Materials (On hold; Support from USDOE)
We attempted to synthesize reduced niobium and molybdenum chalcogenides and chalcogenide halides as either thin films via MOCVD or in high surface area powder forms using low-valent organoniobium or organomolybdenum precursors. We encountered initial successes using Nb(mesitylene)2, Nb(2,2'-bipyridine)3 and Mo(mesitylene)2. Reactions with iodine, sulfur, triphenylphosphine telluride, and diethylsulfide produced amorphous and crystalline reduced niobium chalcogenides and halides. We successfully obtained NbS2, NbSe2, and NbTe4 at -70°C via these routes. We isolated the compound Nb2(SMe)4(mesitylene)4 -- an organometallic compound with an NbS2 kernel, which can serve as a single molecular precursor for the deposition of NbS2 thin films. We began reactions between Nb(mesitylene)2 and hydrazines, diboranes, and Co2(CO)8 in attempts to produce precursors to metal nitride, boride, and intermetallic thin films. Nb(mesitylene)2 reacts with hydrazines to form trimers as seen in mass spectrometric studies, and it reacts with Mn, Fe, Co, and Ni carbonyls to form intermetallic powders. Preliminary structural characterization revealed an intermediate [(mesitylene)2Nb][Co(CO)4] to form in the Co reaction. This project is now on hold.