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July 23, 2003 Superhard Materials Under Pressure: Strength and Elasticity of StishoviteS.R. Shieh1, T.S. Duffy1 and B. Li2 Stishovite, a six-coordinated form of SiO2 and one of the hardest known oxides, was compressed in a diamond anvil cell to study its elastic and rheological behavior at extreme pressures. Our results show that the yield strength is surprising low at high pressures compared with other silicates. Furthermore, we provide experimental confirmation of the prediction that the transition from stishovite to the CaCl2-type structure near 50 GPa is accompanied by an elastic instability. These results show that application of high pressures can greatly modify expectations of material behavior based on low-pressure considerations.
Stishovite was loaded into a diamond anvil cell and compressed non-hydrostatically. Energy dispersive x-ray diffraction experiments were conducted at beamline X17C at the National Synchrotron Light Source (Figure 1). The sample was contained within an x-ray transparent (beryllium) gasket, which enabled us to measure the diffraction pattern at any angle with respect to the loading axis of the cell. The data were analyzed using lattice strain theory, which relates the anisotropy of the measured lattice strains to the yield strength and the elastic stiffness coefficients.
Figure 2 shows the yield strength of stishovite as a function of pressure from our x-ray diffraction data combined with theoretical predictions for the shear modulus. The yield strength was found to be nearly constant at pressures of 15-40 GPa and dropped sharply as the transition pressure was approached. The yield strength then increased rapidly in the CaCl2-type phase. In contrast to its ambient-pressure behavior, our measurements indicate that the yield strength of stishovite is surprisingly low at high pressures especially when compared with other silicates (e.g. ringwoodite).
Our data also allow for inversion to recover a partial elastic stiffness tensor at high pressure. In general, our values of C11 and C12 (Figure 3) lie below theoretical values, but they are qualitatively consistent with theory in that C11-C12 is markedly reduced near the transition pressure. This provides direct experimental support for the theoretical prediction of an elastic instability involving C11-C12 in stishovite near 50 GPa.
This study of stishovite provides an example of how recent developments in diamond anvil cell technology together with synchrotron x-ray diffraction techniques are yielding new advances in our understanding of fundamental quantities, such as elasticity and yield strength, of strong materials under extreme pressure conditions. BEAMLINE FUNDING PUBLICATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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Stishovite is likely to be an important constituent of the
Earth's deep mantle. It is also a prototype for the six-coordinated
silicates that are of fundamental importance in geophysics and
materials science. The nature of the transformation of stishovite
from the rutile structure to the CaCl2-type structure near 50 GPa
has been the focus of much recent interest. Stishovite is also among
the strongest known oxides, and understanding its elastic and
rheological properties is fundamental to the search for new
superhard materials. In this study, we use new compression
measurement techniques in a diamond anvil cell to examine the
elasticity and yield strength of dense SiO2 over a broad pressure
range for the first time. 
