In situ X-ray diffraction of Ce melting under shock loading

With 7 observed crystalline phases below 20 GPa, cerium exhibits a complex phase diagram. In particular, the γ-α phase transition exhibits a large volume collapse (13%-16%) resulting in a low melting pressure for a metal. Sound speed measurements have shown that Hugoniot intersects the melt boundary at 10 GPa with complete melting near 18 GPa; however, the processes and timescales involved are not yet known. This work presents experiments performed at the Dynamic Compression Sector examining the phase evolution of γ-Ce under shock loading. The results indicate that γ-Ce and α-Ce coexist with liquid Ce at pressures near the onset of melt. With increasing pressure, α-Ce is no longer observed; however, γ-Ce persists after impact at pressures beyond 18 GPa. The results indicate significant kinetics, with complete melting observed 200 ns after impact. An experiment performed below the melting pressure shows that the γ-α transition occurs over a similar timescale, with visible peaks for γ-Ce and α-Ce observed over 50 ns after impact. As a result, the γ-α transition appears to exhibit occurs over a relatively long timescale, which is surprising for an isostructural phase transition, and appears to be a significant barrier to melting.
LA-UR-18-30167