A Solid Squeeze
Studying materials at terapascal (TPa) pressures may shed light on the state of matter deep within planets and on the stability of crystal phases under extreme conditions. Amy Lazicki from the Lawrence Livermore National Laboratory, California, and colleagues now report how a technique known as laser-ramp compression allowed them to squeeze solid tin up to a pressure of 1.2 TPa—about 30 times the pressure at the center of Mars—and to determine the unexpected crystal structure that the material adopts under such high stress.
The significance of their study is threefold. First, 1.2 TPa represents the highest pressure at which a crystal structure has ever been observed. Second, the results confirm that laser-ramp compression can prevent the melting that a metal would otherwise undergo at high pressure, allowing its structure to be probed with x-ray diffraction. The technique uses temporally shaped laser pulses to exert pressure on a target while minimizing its heating. Third, the findings provide access to a previously uncharted region of the pressure–temperature phase diagram of tin. The authors carried out x-ray measurements of the material at pressures between 0.12 and 1.2 TPa and temperatures below the melting point, showing that tin has a body-centered-cubic crystal structure over the entire pressure range. This observation is at odds with earlier studies, performed at room temperature and using diamond anvil cells to compress the target, which revealed a hexagonal-close-packed phase between 0.16 and 0.19 TPa—the highest pressure at which tin’s structure had previously been probed. Lazicki and co-workers reckon that the higher temperature of their experiments stabilizes the body-centered-cubic structure relative to the hexagonal-close-packed structure.
This research is published in Physical Review Letters.
–Ana Lopes