Comment on “Molybdenum sound velocity and shear modulus softening under shock compression”

In this Comment we discuss recent results presented by Nguyen et al. on shock compressed molybdenum up to 438 GPa [Phys. Rev. B 89, 174109 (2014)]. The aim of Nguyen's article is to show that there is no phase transition near 210 GPa. We propose instead an interpretation of their data that this material shows the onset of partial melting along the Hugoniot at $240\left(20\right)\mathrm{GPa}$, which is evident from abrupt changes in the pressure dependence of the shear modulus. This interpretation may solve the significant controversy in the melting slopes derived from shock and static experiments.

Figure 1

Pressure dependence of the sound velocity. Squares are from Ref. [1], and circles are from Ref. [2]. Solid lines are linear fits to the results of Hixson et al. [2], and dashed lines are linear fits to the results of Nguyen et al. [1] in different pressure ranges. The dotted lines indicate the 99% confidence level band of the fit to the results of Nguyen et al. [1].

Figure 2

Pressure dependence of shear modulus. The squares represent the values calculated from the data of Nguyen et al. [1]. Diamonds represent the shear modulus calculated from the data of Zhang et al. [8], and the circles represent the results reported by Liu et al. [9]. The solid line below 240 GPa is the shear modulus calculated using the model developed by Burakowsky et al. [10]. The solid lines above 240 GPa are a guide to the eye.