The discovery, which will add a third phase transformation mechanism to the solid state physics textbooks, also promises to yield significant insight into the origin of earthquakes.

For the past few years, a group of scientists at the NSLS, led by Professor Jiuhua Chen of Stony Brook's Mineral Physics Institute, has been developing a state-of-the-art x-ray diffraction system at high pressure.

Structure refinements based on the data collected from this system can reveal a phase transformation at the atomic scale. Using this system, the Stony Brook scientists successfully followed one phase transformation and discovered a new mechanism of phase transitions.

The group's findings were published in yesterday's issue of Physical Review Letters under the title, "Observation of Cation Reordering during the Olivine-Spinel Transition in Fayalite by In Situ Synchrotron X-Ray Diffraction at High Pressure and Temperature."

Phase transformations play an important role in the behavior of materials, since materials behave very differently in different forms, or phases.

When graphite changes its crystal structure into a cubic structure, it transforms into diamond, greatly increasing its value -- even though both graphite and diamond consist of the same element, carbon.

When iron-carbon steel is quenched from high temperature, the steel becomes much harder, increasing its industrial value.

Many other phase transformations happen every day, changing the properties of a material to increase or reduce its value or utility.

All phase transformations have been classified for decades into two groups: diffusional (e.g., graphite-diamond) and diffusionless (e.g., martensitic transition, named after the German metallurgist, Adolf Martens, in iron-carbon steel.)

But in their study, the Stony Brook scientists found that during a phase transition, a substructure of a material can transform by diffusionless transition -- even while the rest of the atoms in the material transform through short-range diffusion.

This type of transformation is distinct from either the diffusional or diffusionless transitions described in current solid state physics textbooks, and is named a pseudomartensitic transition.

This transformation mechanism may be operative when the upper-mantle mineral, olivine, transforms into its high-pressure polymorphs in the earth subduction slab (subducted lithosphere), and therefore influences the structure of the slab.

This particular transformation holds important implications about the origin of earthquakes in the deep mantle.

Co-authors of the paper are Jiuhua Chen, Donald J. Weidner, John B. Parise, Michael T. Vaughan and Paul Raterron of the Center for High Pressure Research, Department of Geosciences, State University of New York at Stony Brook.

(Reference: Physical Review Letters April 30, 2001 Volume 86, Issue 18 pp. 4072-4075.)

(Editor's Note: The abstract of the paper is available at this URL.)

Related websites:

National Synchrotron Light Source

State University of New York at Stony Brook

[Contact: Dr. Jiuhua Chen]

01-May-2001