DETERMINATION OF EQUATION OF STATE OF AMORPHOUS MATERIALS AT HIGH PRESSURE USING NANOSCALE TRANSMISSION X-RAY MICROSCOPY

The effect of pressure (P) on volume (V) has long been recognized as a fundamental thermodynamic relationship. Experimental determination of accurate P-V relationships is currently limited to well-crystallized solids where x-ray or neutron diffraction can yield accurate unit cell data at high pressures. However, for amorphous and poorly crystalline materials, we still lack this fundamental equation of state data. Development of synchrotron transmission x-ray microscopy (TXM) with 30 nm spatial resolution has enabled volume determination by imaging with accuracy rivaling x-ray diffraction of crystalline solids.

Group IV elements, including carbon, silicon, and germanium, both alone and in combination, exhibit a variety of amorphous forms that show unique behavior upon compression. Compressing glassy carbon above 40 GPa, we observed a new carbon allotrope with a fully sp³-bonded amorphous structure and diamond-like strength. Amorphous SiO₂ and GeO₂ are classic network-forming glasses which display rich structural polymorphism at high pressure. We used TXM for accurate P-V determination of glassy carbon, SiO₂ glass, and GeO₂ glass in a cross diamond anvil cell. The results provide guidance for understanding the mechanisms of the structure and bonding changes in these amorphous materials at high pressure.