Crystal-Bonding Principles Established for Compounds Such as Neodymium Distannide
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ed for each day of beam time, but only six bohrium nuclei were actually detected. The bohrium 267 compound was shown to be volatile at 180°C, behaving much like its periodic-table relatives technetium and rhenium.
Crystal-Bonding Principles Established for Compounds Such as Neodymium Distannide Researchers at Cornell University have established the principles of crystal-bonding of a group of thousands of compounds that carry significant electronic and magnetic properties. “This is an important step in understanding the bonding in alloys and intermetallic compounds,” said Roald Hoffmann, Nobel Laureate for chemistry (1981) and the Frank H.T. Rhodes Professor in Humane Letters at Cornell. Working with postdoctoral researcher Garegin Papoian, Hoffmann laid out a theory that extends the understanding of bonding in a particular class of alloys. As reported in the July 17 issue of Angewandte Chemie, the two researchers began by looking at the bonding of com-
Figure 1. A perspective view of the crystal structure of neodymium distannide, a compound of tin and the rare-earth metal neodymium. The small dark spheres are neodymium, and the large light spheres are tin. Credit: Garegin Papoian. © 2000 Cornell University.
pounds of antimony, tellurium, tin, and selenium. The compounds have names like europium, lithium antimonide, and neodymium distannide, and although they have been known for many decades, “experimentalists have said nothing
about what holds these compounds together,” said Hoffmann. These compounds consist of a melange of metallic, covalent, and ionic bonds that the researchers explain in a formula based on “magic numbers.” In their bonding formula, magic numbers are the electron counts that indicate whether a stable compound is linear or square: seven electrons per atom for a linear chain; six electrons per atom for a two-dimensional square lattice; and five electrons per atom for a simple cube lattice (see Figure 1). The crystal structures themselves can be seen in a series of computer-generated drawings—not based on theory, but on direct experimental work—that have an interlocking, architectural perfection. The molecular structures, ranging from simple geometries to complex lattices, reveal their bonding networks in a series of multidimensional building blocks. Hoffman said, “Some look terribly complicated, but take them apart and you can see square lattices with atoms above and below, and squares forming octahedrons.” Hoffman said that such structures reveal themselves sometimes as com-
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pounds of antimony and other times as tellurium or tin because the number of electrons determines the chemistry, less so the identity of the nucleus underneath.
Nobel Prizes for 2000 Announced The Royal Swedish Academy of Sciences has awarded the Nobel Prize in chemistry for 2000 jointly to Alan J. Heeger of the University of California—Santa Barbara, Alan G. MacDiarmid of the University of Pennsylvania, and Hideki Shirakawa of the
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