Issue Date: May 26, 2008
Sodium's Many Lattice Patterns
Sodium, a paradigm of simplicity highlighted in many textbooks, actually has extremely complicated crystal structures and may not be a metal at high pressures, according to new research (Science 2008, 320, 1054). Knowing the behavior of sodium can shed light on the chemistry and physics of hydrogen and other fundamental systems that can't be examined due to limits on experimental capabilities. Two simple crystal-lattice structures are known for sodium: body-centered cubic at ambient pressure becomes face-centered cubic at roughly 9 million psi. Eugene Gregoryanz at the University of Edinburgh, in Scotland, and colleagues used a single-crystal high-pressure X-ray diffraction technique to discover that seven more complex crystalline phases exist at various pressures up to about 17 million psi at 300 K. These lattice structures contain eight to 512 atoms per unit cell. No known crystalline phases of elements have more than 105 atoms in a unit cell, and the researchers suspect that sodium has very unusual properties at such high pressures.
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