Glass with a memory.

Traditionally, researchers have pictured a glass as a random network of chemically bonded atoms. However, some glassy materials don't fit this simple picture. Experiments show that sufficiently high pressures can convert a crystalline form of aluminum phosphate known as alpha-berlinite into an apparently disordered, or glassy, solid. But as soon as the pressure is lifted, the solid returns to its previous crystalline state. Instead of deforming permanently, the compressed material somehow retains a "memory" of its original crystal structure.

To determine why aluminum phosphate shows such a remarkable recovery when other crystalline materials do not, John S. Tse and Dennis D. Klug of the Steacie Institute for Molecular Sciences in Ottawa, Ontario, computed the effects of high pressure on an aluminum phosphate lattice. Initially, each aluminum and phosphorus atom is surrounded by four oxygen atoms in a tetrahedral arrangement, and the entire crystal consists of an orderly network of these tetrahedra. As revealed in the simulations, increasing the pressure distorts the tetrahedral units, opening them up and twisting them into the empty space within the lattice. This shift changes the relative positions of the atoms but forces no substantial rearrangement -- even though the resulting structure looks quite disordered. When the pressure is lifted, the atoms simply retrace their paths to their original locations.

"It's like winding up a coil, then letting it unwind," Tse says.

In contrast, both experiments and simulations show that a crystalline form of silicon dioxide known as alpha-quartz, which has the same tetrahedral atomic arrangement as aluminum phosphate, fails to recover from its pressure-induced, disordered state. Unlike phosphorus atoms in aluminum phosphate, which remain bonded to four oxygen atoms throughout compression, silicon atoms end up in an arrangement in which each one is strongly associated with five oxygen atoms instead of four. This change in bonding stabilizes the disordered form of silicon dioxide, and the material retains its glassy structure when the pressure decreases.

Tse concludes that any material displaying a memory effect must contain rigid units -- like the tetrahedral phosphate groups in aluminum phosphate -- that preserve their bonding characteristics at high pressures. Glassy materials can indeed exhibit markedly different degrees of disorder.