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September 9, 2002
Volume 80, Number 36
CENEAR 80 36 p. 13
ISSN 0009-2347


SURFACE SCIENCE

New Method Reveals Surface Structure Of Complex Crystals

MITCH JACOBY

Measuring just a few angstroms in thickness, the topmost atomic layers of solids hardly take up any space. But don't be fooled by their tiny size. Surface layers often govern the properties of solids such as heterogeneous catalysts and thin-film structures. Yet despite the importance of surfaces to many areas of science and technology, the surface crystal structures of only a few of the simplest materials are known.

COURTESY OF NORTHWESTERN UNIVERSITY
That list is set to grow now that researchers at Northwestern University have demonstrated a technique for determining the surface structure of complex materials. By coupling atomic resolution transmission electron microscopy methods with quantum mechanical calculations, graduate student Natasha Erdman, materials science professor Laurence D. Marks, chemistry professor Kenneth R. Poeppelmeier, and coworkers have determined the surface crystal structure of strontium titanate, SrTiO3 [Nature, 419, 55 (2002)].

Unlike SrTiO3's bulk structure, which is made up of alternating SrO and TiO2 layers (Sr, gray balls; O, blue; Ti not shown), the group notes that the crystal's surface is composed of two TiO2 layers in which the relative positions of titanium (red) and oxygen (green) differ significantly from the bulk.

"The results suggest that a new era of crystallography is beginning," says Michael O'Keeffe, a chemistry professor at Arizona State University. In a commentary appearing in the same issue of Nature, O'Keeffe points out that SrTiO3, which is an archetypical member of the perovskite family of oxides, is often used as a substrate for epitaxial growth for other perovskites, including ferroelectric materials and compounds that exhibit colossal magnetoresistance and superconductivity.



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