The group V metal bismuth (Bi) is well-known for exhibiting a large number of polymorphic transitions within a low pressure and temperature region [1]. the ideal candidate for such studies. Within 3 GPa it undergoes the I-II and the II-III transition and the III-V transition above 7 GPa. At high temperature, between 2.5 and 6 GPa, phase III transforms into Bi-IV. This view of the Bi-phase diagram can be drastically altered by metastable effects in static compression experiments [2], with the appearance of phase V at pressures as low as 5 GPa for example. Moreover, it was recently shown in a static study that hydrostaticity conditions have an important effect on the melting line of bismuth, which in turn is found to be much higher (the I-II-liquid triple point is found around 2.2 GPa and 490 K instead of 1.65 GPa and 465 K) [3]. Synchrotron ns time-resolved X-ray diffraction has been performed on shocked bismuth along various compression and release paths, hence exploring the Bi phase diagram up to 8 GPa and 600 K. Marked departures from the equilibrium behavior are observed. The sequence of structural changes is different upon compression and release. Bi-III, the complex host-guest structure, is never observed. Instead Bi-V is observed over a large domain. Melting of Bi-V and crystallization of the fluid into Bi-I are clearly identified on stress release. These observations on a prototypical system underline the possible difficulties of stabilizing equilibrium structural transformations at high pressure by using dynamic compression.

[1] E. Yu Tonkov and E. G. Ponyatovsky, CRC Press

[2] E. Princip et al. PRB 74, 064101 (2006);

[3] C. Lin et al. Nature Comm. 8, 14260 (2017)