In situ X-ray diffraction of Al₂O₃ during laser compression and release

Sapphire (Al2O3) is an important earth mineral notable for its high compressive strength and hardness. Static compression experiments on Al2O3 in the polycrystalline alumina form have found evidence of phase transformations from the α-corundum phase to a Rh2O3 (II)-type structure at ~80 GPa (Funamori & Jeanloz, 1997, Lin et al., 2004), and then to the post-perovskite structure at ~130 GPa (Oganov & Ono, 2005).

In this work, we describe laser-driven shock compression experiments on a-cut and c-cut sapphire to 150 GPa and polycrystalline alumina to 110 GPa conducted at the Matter in Extreme Conditions endstation of the Linac Coherent Light Source. Ultrafast x-ray pulses (50 fs, 1012 photons/pulse) were used to probe the lattice-level response as a function of time during and after shock propagation. VISAR velocimetry was used to measure free-surface particle velocity, from which pressure was calculated. Through in situ x-ray diffraction, we observe evidence of anisotropic strain and crystal break-up during and after compression. After release to ambient pressure, measured temperatures exceed those predicted by isentropic release, indicating plastic work heating. We will discuss the effect of crystal orientation on shock-induced phase transformation.