Glass shows 'memory' under severe pressure.

Glass shows 'memory' under severe pressure

Molten sand, if cooled quickly enough, will harden before its silicon dioxide molecules realign into an ordered crystalline arrangement. That's one way of making glass.

But two researchers find that an alternative method yields glasses that retain a "memory" of their original crystal structure and can actually revert to it. Geophysicist Raymond Jeanloz of the University of California, Berkeley, and graduate student Michael B. Kruger introduce their "memory glass" in the Aug. 10 SCIENCE.

The team's pressure-based technique involves a diamond-anvil cell that can subject materials to pressures thousands or even millions of times greater than external atmospheric pressures. First, Jeanloz and Kruger surround their crystals with a fluid to ensure even pressure distribution around the samples, in this case the berlinite form of aluminum phosphate.

When the pressure in the diamond-anvil cell reaches at least 150,000 atmospheres -- corresponding to the pressure about 250 miles below the Earth's surface -- the aluminum and phosphate ions edge away from their crystalline sites into an amorphous arrangement. At these pressures, the X-ray diffraction pattern indicative of crystalline order disappears. Changes in the way the material absorbs infrared radiation also suggest a crystal-to-glass transition, the researchers report.

But to their surprise, the ions snap back into their original crystalline places when the pressure in the cell falls below 500 atmospheres.

Jeanloz conjectures that the memory effect of aluminum phosphate, and of several other crystals tested, depends on the experiment's room-temperature conditions, which probably prevent the squeezed ions from wandering more than a smidgen from their original sites. So when the pressure goes down, the ions simply pop back into their thermo-dynamically favored places. The high temperatures used in conventional glass-making send the crystal's ions or molecules too far out of line to "remember" where they started, he says.

"The recrystallization of an amorphous phase upon decompression is very intriguing," remarks Russell J. Hemley of the Carnegie Institution of Washington (D.C.). Such studies may uncover mechanisms of molecular reorganization, he says. And that knowledge could enable engineers to tailor a variety of glasses from the same starting material for applications ranging from specialty windows to new semiconductor materials, Jeanloz says.