Pressuring potassium to react with nickel - Brief Article

Science News, July 6, 1996 by I Peterson

Under immense pressures, chemical elements can adopt new identities, displaying characteristics that differ radically from those seen at normal atmospheric pressure.

Researchers have already found that high pressure can force alkali metals, such as potassium, rubidium, and cesium, into behaving more like transition metals, such as iron, cobalt, and nickel. Now, a team of chemists has used a high-pressure apparatus to induce the formation of various nickel-potassium compounds.

Alkali metals and transition metals normally do not react, says chemist John V. Badding of Pennsylvania State University in University Park. Badding and his coworkers Laura J. Parker and Toshiyuki Atou report their achievement in the July 5 Science.

These results not only pioneer a new type of high-pressure chemistry but also have implications for geophysics. They suggest that Earth's core, made up largely of iron or an iron-nickel mixture, could also contain potassium. The researchers used an apparatus called a diamond anvil cell, which consists of a pair of gem-quality diamonds separated by a metal gasket with a small hole that serves as a sample chamber (see diagram). Applying a force to the diamonds squeezes the sample.

Badding and his coworkers compressed a mixture of potassium and nickel powder to 310,000 times atmospheric pressure. They then used an infrared laser to heat the sample to a temperature of about 2,500 kelvins.

The high pressure forced potassium atoms to collapse to roughly one-fifth their usual volume, packing the electrons of each atom more closely together. The pressure also forced the outermost electron of each atom into a type of energy level characteristic of transition metals. Nickel, on the other hand, changed much less under the same pressure, retaining its transition-metal electronic structure.

Measurements of the angles at which the resulting material deflected X rays revealed the emergence of a new pattern of deflections, indicating the formation of a compound or alloy after the sample was heated. "Potassium and nickel are chemically reacting," Badding says. "Some sort of bond formation is taking place."

The observation that potassium can combine with nickel at pressures comparable to those deep within Earth suggests that potassium is a constituent of Earth's core. Seismological measurements indicate that the core is less dense than would be expected for pure iron or an iron-nickel combination, and researchers have postulated that the presence of lighter elements, such as potassium, hydrogen, or carbon, would account for the discrepancy.

The researchers are now preparing to do the same experiment with potassium and iron, as well as trying to characterize the types of nickel-potassium compounds created under high pressure.