Daily University Science News
A new mechanism
of phase transitions has been discovered by Stony Brook scientists
at Brookhaven National Laboratory, using a bright synchrotron
x-ray from the National Synchrotron Light Source (NSLS).
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.
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