X-ray diffraction of molybdenum under shock compression to 450 GPa

Abstract

Molybdenum (Mo) is a body-centered-cubic (BCC) transition metal that has widespread technological applications. While the BCC transition elements are used as test cases for understanding the behavior of metals under extreme conditions, the melting curves and phase transitions of these elements have been the subject of stark disagreements in recent years. Here we use X-ray diffraction to examine the phase stability and melting behavior of Mo under shock loading to 450 GPa. The BCC phase of Mo remains stable along the Hugoniot until 380 GPa. Our results do not support previous claims of a shallow melting curve for molybdenum.

Microstructures define melting of molybdenum at high pressures

“…5), with the exception of the study in reference3, which is close to our measurements. In fact, these MD results, except reference3, appear to be more consistent with the earlier higher P–T discontinuity point on the Hugoniot789. However, as discussed above, the most recent SW X-ray diffraction study9 has suggested a lower melting temperature—in better agreement with our result.…”

“…In fact, these MD results, except reference3, appear to be more consistent with the earlier higher P–T discontinuity point on the Hugoniot789. However, as discussed above, the most recent SW X-ray diffraction study9 has suggested a lower melting temperature—in better agreement with our result. The most recent MD study39 does point out that larger-scale MD calculations are needed to identify precisely the effects in Mo at the highest P–T conditions.…”

“…Consequently, the extrapolation of our melting curve to higher pressures of up to 400 GPa, in contrast to earlier LH DAC results, is in far better agreement with reports on Mo under shockwave compression891538. However, our result and the newer shockwave data891538 both point to a lower melting temperatures than those predicted by ab initio molecular dynamics (MD) calculations23456 (Fig. 5).…”

“…However, a number of more recent shockwave experiments failed to detect the first discontinuity7 at the lower pressure, suggesting that it could be an artifact in the earlier experiments89. The most recent shockwave-and-X-ray diffraction combined study (SW X-ray diffraction)9, based on the observation of the Mo X-ray diffraction (110) line broadening, has suggested9 that the melting point should be 7,700±1,500 K, at P ∼390 GPa, if a correction for the superheating of Mo prior to melting is applied. This result agrees with our extrapolated melting point within the experimental and extrapolation uncertainties (Fig.…”

“…Theoretical studies have predicted rapid increases of melting temperature in Mo with compression; at a pressure of 100 gigapascals (GPa), the majority of theoretical predictions23456 and acoustic velocity measurements under shock compression789 indicate melting temperatures nearly 3,000 K higher than those obtained by all of the previous static compression experiments10111213. This discrepancy is highly dissatisfactory and imposes intolerable limitations on the validation of theoretical models of high-temperature transition metal melting in general1.…”