Mechanism of rapid compression-induced melt crystallization in selenium

Abstract

Amorphous selenium (Se) can be easily prepared by quenching the melt, which indicates that the Se possesses the good glass-forming ability. However, crystallization occurs after rapidly compressing the melt within about 20 ms. In this work, we investigate the mechanism of rapid compression-induced crystallization from Se melt. Compressing Se melt experiments are carried out at the following temperatures: 513, 523 and 533 K. The melt is rapidly compressed under 2.4 GPa for about 20 ms. Different holding times, i.e. 0, 30, 60 min after solidification are adopted. The samples are quenched to room temperature and then unloaded to ambient pressure. The X-ray diffraction analysis of the recovered sample indicates that the crystallization product is the <i>t</i>-Se. It is found that with the prolongation of holding time, the grain size increases due to the continuous aggregation growth of crystal grains. By comparing with the isothermal crystallization products of amorphous Se and ultrafine Se powder, it is suggested that the rapid compression-induced solidification product should be <i>t</i>-Se crystalline. The speculation that the solidification product is amorphous Se and it crystallizes in the cooling process does not hold true. The amorphous Se cannot be prepared through the rapid compression process on a millisecond scale. It is related to the thermal stability of amorphous Se under high pressure. It is reported that the dependence of crystallization temperature <i>T</i>_x on pressure i.e. d<i>T</i>_x/d<i>P</i> for amorphous Se is about 40–50 K/GPa in a range of 0.1 MPa–1 GPa. However, the <i>T</i>_x of amorphous Se is almost constant in a range of 2–6 GPa. It means that the thermal stability of amorphous Se against crystallization does not increase with increasing pressure after 2 GPa. In this work, the temperature of 513–533 K in the experiments is higher than the <i>T</i>_x of amorphous Se. Therefore, the<i> t</i>-Se crystal is the stable phase and amorphous Se is unstable. The Se melt tends to crystallize in the supercooled liquid state after rapid compression. It is interesting to investigate the mechanism of d<i>T</i>_x/d<i>P</i> curve discontinuous change at around 2 GPa in the future. Both the Se melt after rapid compression and the amorphous Se before crystallization are in supercooled liquid state. We speculate that high pressure may result in the microstructure transition in supercooled liquid state Se.