Behavior of materials under High Pressure
Most materials undergo phase transitions to new crystal structures
at high pressures. The determination of crystal structure is
the most fundamental subject and is best obtained by x-ray diffraction
techniques. A diamond anvil cell (DAC) is a device widely used
to apply ultla-high pressure to materials. Since the sample volume
in the DAC is extremely small,1pl, the intense synchrotron
radiation x-ray is required to measure weak diffraction signals.
In cooperation with the high-resolution and high-sensitive
capabilities of an Imaging Plate detector, we are now able
to determine crystal structures accurately and to discuss
high-pressure behavior of materials quantitatively.
Praseodymium, a rare earth metal, exhibits the fcc-rhombohedral
phase transition at 7 GPa. The figures show (a) atomic arrangement
of the rhombohedral structure determined at 13.8 GPa and (b) pressure
dependence of atomic displacements in this structure.
These results imply the presence of the softening and condensation
of a particular phonon mode at this phase transition.
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