Metal flux growth of new intermetallics

Molten metals have been used as solvents for the growth of a variety of compounds, ranging from large single crystals of silicon to exploratory synthesis of new intermetallic materials. Reactions in metal fluxes can be carried out at lower temperatures than those typically required for traditional solid state synthesis, allowing for the formation of metastable or kinetically stabilized products. Flux reactions are also solution-state reactions, allowing for crystal growth with slow cooling of the melt.

Low melting metals such as gallium, indium, and tin have been well-explored as fluxes. We are investigating combinations of different metals to create unique reaction solvents for new materials. In many cases, combining two metals at specific ratios results in a lowered melting point (for instance, eutectic formation); this enables high-melting metals that have not been previously considered as fluxes to be used. Our investigation into metal flux reactions includes the production of new intermetallic phases with interesting electronic and magnetic behavior or with potential use as optical or thermoelectric materials. This research is funded by the National Science Foundation.

--Magnetic intermetallics from Ln/T eutectics

The hard magnetic materials used in computer hard drives are intermetallics such as SmCo5 and Nd2Fe14B. Their strong ferromagnetism is derived from the coupling of the local moments of the unpaired electrons on the lanthanide ion with the itinerant magnetism of the transition metal valence electrons. The binary phase diagrams of early lanthanides (Ln = La, Ce, Pr) combined with late transition metals (Ni, Fe) all feature low melting eutectics in the lanthanide-rich region which may act as fruitful synthesis media for magnetic compounds containing two paramagnetic elements. We have reported on the synthesis of new materials containing clusters and layers of iron separated by lanthanum ions from La/Ni eutectic (melting point 532 C). Even more complex phases are now being formed in reactions of metals and metalloids in Ce/Co and Pr/Co eutectic fluxes (melting points 424 and 541 C, respectively)! The magnetic properties of these phases are characterized using techniques such as temperature and field dependent magnetic susceptibility measurements, mossbauer spectroscopy, and neutron diffraction.

See Dr. Latturner's presentation on synthesis in La/Ni flux here!

Synthesis in La/Ni Flux

--Complex tetrelides, nitrides, and hydrides from alkaline-earth based fluxes

We have found that flux mixtures of alkaline earth metals with lithium (such as Ca/Li in 1:1 mole ratio, mp 300 C) are excellent solvents not only for metals and metalloids, but also for ionic compounds such as CaH2 and Ca3N2. This allows for the synthesis of complex hydrides (such as LiCa2C3H), nitrides (such as Ca6Te3N2), and carbides (such as Ca11Sn3C8) which are charge-balanced semiconductors featuring isolated hydride, nitride, and carbide anions. The metalloid elements accept electrons from the Ca/Li flux and are reduced to anions. However, if similar reactions are carried out using more electropositive metals (for instance, using indium instead of tin), those metals will not be completely reduced, and the regions of the product structure containing the indium atoms remain metallic, bordered by ionic hydride regions. This separation of metallic and ionic regions was confirmed in Ca54In13H27 (grown from Ca/Li/In/CaH2 reactions) using NMR spectroscopy and band structure calculations.

Compared to calcium-based melts, magnesium-based fluxes are poorer solvents for hydride, carbide, and nitride synthesis. However, Mg/Al mixtures (melting at around 460 C) are excellent reaction media for the growth of complex tetrelides (Tt = Si, Ge, Sn). Reactions of heavy divalent metals (A = Ca, Sr, Ba, Eu, Yb) with silicon in Mg/Al melts produce a variety of AxMgySiz semimetals. Compounds such as CaMgSi, Eu5Mg18Si13, and Ba2Ca2Mg10Si7 can be grown as large crystals, facilitating the study of their electronic properties. Their complex structures, substitutional chemistry, and semi-metallic behavior make them potential thermoelectric materials.

Flux-grown crystals. Left: Ba2Ca2Mg10Si7 grown from Mg/Al flux. Right: dichroic crystals of Ca6Te3N2 grown from Ca/Li flux.


Synthesis of uranium intermetallics

As part of the DOE-funded Center for Actinide Science and Technology at FSU, our group is exploring the flux synthesis of new uranium intermetallics. We are particularly interested in complex uranium borides, carbides, and silicides, which are likely to be refractory and of potential use as nuclear waste forms. They may also exhibit unusual electronic behavior such as the Kondo effect or superconductivity. Metal fluxes ranging from aluminum and gallium to mixtures such as U/Co and U/Fe eutectics are being investigated as solvents for growth of these materials.

Crystals of uranium intermetallics grown from aluminum flux.


Synthesis in Sulfur-Halogen melts

Metal sulfides are of interest for a wide variety of applications, including use as hydrodesulfurization catalysis, and as photovoltaic materials. Many transition metal sulfides have complex structures due to the combination of variable oxidation state on the transition metal, and the fact that sulfur can exist as S2 or S3 units in addition to isolated monoatomic anions. Some of these compounds are only stable at low temperatures, converting to simpler, more thermodynamically stable phases at higher temperatures. This makes synthesis somewhat difficult.

Low temperature sulfur flux chemistry enables the isolation of metastable metal sulfide phases. Reactions of a variety of metals and metalloids in sulfur/halogen fluxes are being carried out to explore the chemistry in these solvents. The metal sulfide and metal sulfide halide products range from metallic to semiconducting and their optical and magnetic properties will be investigated. Products of reactions of bismuth in S/I2 flux are shown below, yielding crystals of the metastable subvalent phase Bi13S18I2.

Our flux reactions are typically carried out in programmable furnaces. However, the fact that the sulfur/halogen flux is made of non-polar molecules allows for an alternative heating method: microwave heating. This selectively heats the metal powder reactants (large bulk metals...or metal fluxes...cannot be used for the same reason that you can't stick a metal fork in your microwave oven). Fine metal particles absorb microwaves and heat rapidly, melting the surrounding sulfur, and allowing it to act as a solvent and a reactant. This is a very rapid and greener method of heating; it allows for very different reaction times and temperatures compared to furnace heating. Different metal sulfide phases are obtained from the different heating methods; they are characterized by X-ray diffraction and optical spectroscopy.