Impurities give crystals that special glow

Kids know that the easiest way to create a miniature light show is to munch on wintergreen candy in the dark. When a piece is cracked, the sugar emits light that the wintergreen flavoring intensifies. Many materials exhibit this phenomenon, called triboluminescence.

 In a new study, Linda M. Sweeting of Towson (Md.) State University and her colleagues try to deduce what makes sparks fly from some materials but not others. Their findings, reported in the May Chemistry of Materials, support the idea that crystal structure and impurities are central to whether a material is triboluminescent.

 For years, scientists suspected that only materials with an asymmetrical crystal structure would flash when crushed. Splitting such a crystal into two pieces puts positive charges on one face and negative ones on the other. The charges immediately recombine, crackling through the air like tiny lightning bolts.

 However, several materials with symmetrical crystal structures also show the characteristic sparks, Sweeting says. To study this discrepancy systematically, the researchers synthesized a group of 12 related compounds, coaxed them to grow into crystals, and identified their structures with X-ray diffraction. They tested the crystals for triboluminescence by mashing them with a glass rod in a test tube and watching carefully for light.

 The very first compound Sweeting synthesized sparked in the dark, which makes it "the first compound designed to be a triboluminescent molecule," she says. All of the substances with asymmetrical structures glowed, whereas only half of the symmetrical ones did.

 Moreover, those symmetrical crystals lost their triboluminescence once they were purified. "Impurities in crystals can reduce the symmetry that you think you have," says Bart E. Kahr, a chemist at the University of Washington in Seattle. "As you get down to the microscale, the differences between the pure and impure crystal can be extreme."

 Those local structural asymmetries could explain why materials that are symmetrical overall can still exhibit triboluminescence, Sweeting says.

 A surprising finding underscored the significance of impurities: One of the asymmetrical compounds also lost its ability to flash when purified, indicating that impurities may be required to generate light even in asymmetrical materials. In past experiments, some ostensibly pure asymmetrical compounds may have contained enough impurities to create triboluminescence, Sweeting notes.

 Scientists are still far from a full understanding of the effect, which not only explains wintergreen candy but may account for other mysterious lights observed in nature, such as deep-sea luminescence. Arnold L. Rheingold, a crystallographer at the University of Delaware in Newark who collaborated on the study, calls triboluminescence "a beautiful phenomenon in search of an application."

 Sweeting imagines that one day, triboluminescent coatings could be used in remote sensing applications to signal mechanical failure.