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Sandia researchers uncovered secret lives of two elements

Unexpected differences recently discovered between the elements niobium and tantalum may lead to more optimized electronic materials and photocatalysts.

Sandia researcher May Nyman and colleagues reported on the new-found disparities in a cover story of the Dalton Transactions, an international inorganic chemistry journal published by the Royal Society of Chemistry, the British equivalent of the American Chemical Society. The research paper also received the distinction of being picked among “the hottest science” by referees of that journal on its web page.

Sandia is a National Nuclear Security Administration laboratory.

Tantalum oxides are used in medical implants, hypoallergenic surgical tools, and ceramics that hold nuclear waste because of their inertness in liquid media, says Nyman. They resist destruction by radioactivity and do not poison the body by deteriorating in its tissues, but are hard to work with because they precipitate out of solution in an uncontrolled and undesirable way.

Niobium is easier to work with, but less inert for reasons formerly not well understood.

“Tantalum and niobium are in the same column on the periodic table,” says Nyman. “Their electronic configurations are related and their ions are virtually identical in size. Generally, the prevailing belief has been that their characteristics are very similar. They are described that way in chemistry textbooks. But we found that tantalum oxides are considerably more inert and less soluble than niobium oxides, and we wanted to understand why.”

The unexpected difference, as well as a new “soft chemistry” method of forming compounds containing them, means that new materials with tailored properties may be formed more simply. The old method, called “the volcano method,” involved melting oxides together at very high temperatures. The “soft” method was published by Nyman and others this past summer and involves chemical finesse rather than brute force.

To explore differences between the two elements, Nyman’s group used the soluble Lindqvist “cluster” ions.

The ion is composed of tantalum and oxygen or niobium and oxygen, and contains only 25 atoms each. The predictable and repetitive structure of these collections of ions lends itself to study more than do tantalum or niobium oxide surfaces, which are formed of a disordered collection of oxygen and tantalum or niobium atoms. Therefore the ion was a possible model to study the surface — if they behaved the same way. ”Much to our surprise, the Lindqvist ions proved to be ideal models for both the structural features and the chemical reactivity of the tantalum and niobium oxide surfaces,” notes postdoc colleague and first author Travis Anderson.
“We did one of the few studies of both oxide surfaces and cluster ions where both behave the same way,” Nyman said, “and the study revealed unprecedented differences in the behavior of niobium and tantalum oxides.”

The difference was in the way that water exchanges with oxygen atoms in both the clusters and at the surfaces of these materials. In the tantalum materials it exchanges in a way that makes it unstable. Precipitating itself onto a surface is one way it stabilizes. In niobium materials, the reaction with water results in stable species that can stay in solution more easily.

Understanding how these oxides behave in aqueous media should lead to the production of new and better tantalum and niobium oxide materials.

Other authors on the paper include Sandia’s Mark A. Rodriguez and Todd Alam; Sandia summer student Joel Bixler from U.T. Austin; and Francois Bonhomme of the Ecole Centrale de Paris. The work was done in collaboration with Bill Casey, a professor at UC Davis.

The work was funded by Sandia’s Laboratory Directed Research and Development office.

In figure,
The cluster above is an ideal structural and reactivity model for niobate and tantalate surfaces, such as those used for metal surgical implants. In the cluster, the red spheres are oxygen, the blue spheres are niobium or tantalum. The white lines are pointing to the different types of oxygen present on the surface and cluster, and the yellow slab is the niobium or tantalum oxide surface.

Note for Niobium
Niobium or columbium is a chemical element that has the symbol Nb and atomic number 41. A rare, soft, gray, ductile transition metal, niobium is found in pyrochlore and columbite. It was first discovered in the latter mineral and so was initially named columbium; now that mineral is also called "niobite". Niobium is used in special steel alloys as well as in welding, nuclear industries, electronics, optics and jewelry.

Notable characteristics
Niobium is a shiny grey, ductile metal that takes on a bluish tinge when exposed to air at room temperature for extended periods. Niobium's chemical properties are almost identical to the chemical properties of tantalum, which appears below niobium in the periodic table.

When it is processed at even moderate temperatures niobium must be placed in a protective atmosphere. The metal begins to oxidize in air at 200 ° C; its most common oxidation states are +3, and +5, although others are also known.

Note for Tantalum
Tantalum is a chemical element in the periodic table that has the symbol Ta and atomic number 73. A rare, hard, blue-gray, lustrous, transition metal, tantalum is highly corrosion-resistant and occurs naturally in the mineral tantalite.

Characteristics
Tantalum is dark, dense, ductile, very hard, easily fabricated, and highly conductive of heat and electricity. The metal is renowned for its resistance to corrosion by acids; in fact, at temperatures below 150 °C tantalum is almost completely immune to attack by the normally aggressive aqua regia. It can be dissolved with hydrofluoric acid or acidic solutions containing the fluoride ion and sulfur trioxide, as well as with a solution of potassium hydroxide. Tantalum's high melting point of 3017 °C (boiling point 5458 °C) is exceeded only by tungsten and rhenium for metals, and carbon.

Release link: http://www.sandia.gov/