New Molybdenum Sound Speed Measurements Near 2 Mbar

Previous sound speed measurements of molybdenum at high pressures suggested a bcc$\rightarrow$hcp and a solid$\rightarrow$liquid phase transition at 210 GPa and at 390 GPa, respectively [1]. These results played an important role in shaping the high pressure - high temperature phase diagram of molybdenum. Here we report recent measurements of the sound velocity of molybdenum under shock loading pressures between 190 and 240 GPa. We used a standard ``overtake'' method [2] and symmetric impacts. Our results, within the uncertainty of the measuring technique, do not support a previous interpretation of a solid-solid phase transition at 210 GPa. However, there is consistent agreement in sound speed at other pressures. We will also describe our analysis technique and interpretation of the new data. \\[0pt] [1] R.S. Hixson, D.A. Boness, J.W. Shaner, and J.A. Moriarty, Phys. Rev. Lett. 62, 637 (1989)\\[0pt] [2] R. G. McQueen, J.W. Hopson, and J.N. Fritz, Rev. Sci. Instrum. 53, 245 (1982)