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NANOSCALE CRYSTALLOGRAPHY REVEALS HIDDEN STRUCTURAL DETAILS
"Without a structure, you are without a road map," said Thomas Vogt, a physicist at the U.S. Department of Energy's Brookhaven National Laboratory. Vogt and scientists from Michigan State University, led by MSU physicist Valeri Petkov, have just demonstrated a technique that allows them to decipher such fine-level nanostructures. This material is also the first example of a room-temperature inorganic "electride," a stable separation of positively charged cations and electrons with properties determined by the topology of the pores in the host matrix. Nanoscale structures, made of particles 1,000 times smaller than the diameter of a human hair, are so difficult to decipher because they lack the long-range order and symmetry of perfectly crystalline materials. "This work shows that, even with a very low degree of order, at synchrotrons like the National Synchrotron Light Source at Brookhaven, using the right techniques, we can determine nanoscale structures. And with structural understanding, we can begin to predict properties, and perhaps begin to manipulate them for useful applications," said Vogt. In traditional crystallography, long-range order and symmetry, specifically the repeating 3-D patterns in the crystals, give rise to sharp "Bragg peaks" in the x-ray powder diffraction pattern, a pattern produced by examining how x-rays scatter as they bounce off the sample. These peaks are what enable scientists to determine the atomic structures. Materials constructed at the nanoscale, however, lack this long-range order and often accommodate a large number of defects and local disorder. The result, says Petkov, is that the diffraction patterns of nanocrystals are much more diffuse with few if any Bragg peaks. "This poses a real challenge to the traditional techniques for structure determination," Petkov said. To overcome this problem, the team of scientists employed a nontraditional experimental approach, called the "atomic pair distribution function technique," to "read between the Bragg peaks" of data produced by traditional x-ray powder diffraction experiments at the NSLS at Brookhaven. Using this technique, the scientists have shown direct structural evidence that cesium can be intercalated in the nano-sized pores of a silicon-oxide zeolite in the form of positively charged cesium ions arranged in short-range order zigzag chains. This verifies that CsxSi32O64 is a room-temperature stable inorganic electride. "Electrides are novel materials that are just beginning to be studied," said Petkov. First results show that they could be used in synthesis as reducing materials, and that they have useful electronic properties such as low-energy electron emission.
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