Pressurized Potassium Behaves Like A New Element


7-5-96
University Park, Pa. --- Potassium and nickel have, for the first time, bonded with each other to form a compound, according to research to be published in the July 5 issue of the journal Science.

Using pressures as high as those deep within the Earth to form the new potassium/nickel compound, Penn State Assistant Professor of Chemistry John V. Badding and his colleagues demonstrated that pressurized potassium functions like a new element with chemical properties like a transition metal. The research could further scientific understanding of fundamental chemistry as well as the chemistry and composition of the Earth's core.

"Alkali elements like potassium don't form compounds with transition elements like nickel at normal pressure because their size and electronic structure are so incompatible," Badding explains.

By making the potassium/nickel compound, the researchers demonstrated that pressurized potassium functions chemically like a new member of the transition-element family.

The chemists produced the compound by first compressing potassium and nickel powder in a diamond-anvil cell to 310 thousand times normal atmospheric pressure (31 Gpa), then heating it with a laser to about 4,000 degrees F (2,500 K). They confirmed the formation of the resulting compound with a powerful single-wavelength X-ray-diffraction device.

Potassium buckles under these pressures, collapsing by a factor of 5. Its single outermost valence electron, which controls bonding, deforms from the spherical "s" orbital shape typical of the alkali elements to the smaller-volume, four-leaf-clover-pattern, "d" orbital characteristic of the transition elements.

"A single d-electron is an extraordinary valence configuration that we just don't find in any of the other elements," Badding says. When potassium's outermost electron transforms to the d-orbital state, potassium sheds its alkali character and starts behaving like a transition element, making its bonding with nickel possible.

"Nickel's electron configuration changes much more slowly under pressure because it is relatively incompressible, so nickel stays in its primarily d-electron configuration while potassium changes completely," Badding explains.

Strictly speaking, an element is defined by the number of protons in its nucleus, but chemists also associate a certain electronic structure and characteristic chemistry with certain groups of elements.

"I think you can argue that when you change the chemistry of potassium from that of an alkali-like s-electron element to that of a transition-metal-like d-electron element, you've pretty much got a new element," Badding says.

The research may help geophysicists understand the composition of the Earth's core, which contains primarily iron or an iron/nickel alloy plus some unknown lighter element or elements.

"Our research shows it is possible that potassium could be incorporated as a compound into Earth's core," Badding says. The research also could help to explain the source of heat in the core, which could result from the radioactive decay of potassium that might be present there. "Our next step is to make potassium/iron compounds," Badding adds.

"We are fascinated now by the interesting chemistry of pressurized potassium and intrigued by its geophysical implications, as well as by a variety of other chemical situations in which alkali elements in a d-electron state could have interesting or important chemistry," Badding says. "The formation of the compound cesium difluoride, which would involve bonding to electrons in a noble-gas configuration octet, should be favored under high pressure.

"Also, the availability of the d-electron bonding state of the alkali metals should allow new chemistry with many of the nonmetallic elements," he explains.

Other members of the research team include graduate student Laura J. Parker and postdoctoral fellow Toshiyuki Atou, now a research associate at Tohoku University in Japan. This research was sponsored by the National Science Foundation, the Petroleum Research Fund of the American Chemical Society, and the David and Lucille Packard Foundation.

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EDITORS: John V. Badding is at (814) 863-7913 (office) or (814) 863-0556 (lab), or by email at diamond@chem.psu.edu

Contact:
Barbara K. Kennedy (814) 863-4682 (o) science@psu.edu