Wang and colleagues used small angle X-ray diffraction and wide-angle X-ray diffraction to observe changes in the molecular structure of wurtzite crystal under pressure.
It may come as a bit of a surprise, but the strongest material in the world isn’t very strong. Subject it to ultra-high pressure, though, and graphite becomes the hardest substance known.
Most materials that transform under high pressure revert to their original structure when the pressure is lifted, losing any useful properties they may have gained when the pressure was on.
Now, by understanding the process behind the transformation itself, researchers have taken a step toward creating a new class of exceptionally strong, durable materials that maintain their high-pressure properties – including strength and superconductivity – in low-pressure environments.
Cornell University reported that staff scientist at the Cornell High Energy Synchrotron Source Zhongwu Wang and his team focused on wurtzite, a cadmium-selenium crystal in which atoms are arranged in a diamond-like structure and molecules are bonded on the surface. When thin sheets of wurtzite are squeezed under 10.7 gigapascals of pressure, or 107,000 times the pressure on the Earth’s surface, their atomic structure transforms into a rock salt-like structure.
As pressure was applied, Wang and colleagues integrated two X-ray diffraction techniques (small- and large-angle X-ray diffraction) to characterize changes in the crystal’s surface shape and interior atomic structure, as well as the structural change of surface-bonded molecules.
They first discovered that the nanosheets required three times the pressure to undergo the transformation as the same material in larger crystal form.
And adding a bonding molecule called a soft ligand to the surface of the high-pressure nanosheets, the researchers observed the effect of that bonding to the nanosheets’ internal structure, transformation pressure, and spacing.
They also tested the material’s yield strength, hardness and elasticity during the transformation. Understanding how those properties change as the molecules interact could help researchers design stronger, tougher materials, Wang says.
“Now we know how the atoms move. We understand the intermediate procedure,” says Wang. His experimental process could hold promise for understanding the transformation pathway for other compounds as well.
The research appears in the Proceedings of the National Academy of Sciences.