The melting curve of vanadium at high pressure and temperature (P-T) is of great interest to our understanding of d-orbital transition metals with simple crystal structures at extreme P-T conditions. Here we have investigated the melting curve and crystal structures of polycrystalline vanadium at high P-T using synchrotron x-ray diffraction (XRD) in laser-heated diamond anvil cells (LH DACs) up to ∼100 GPa and ∼4400 K, a two-stage light-gas gun with in situ shock temperature measurements up to ∼256 GPa and ∼6200 K, and ab initio molecular dynamics (AIMD) with density functional theory computations up to ∼200 GPa. The occurrence of the diffuse scattering signals in high P-T XRD patterns is used as the primary criterion to determine the melting curve of body-centered cubic (bcc) vanadium up to ∼100 GPa in LH DACs. Analysis of thermal radiation spectra of shocked vanadium using a quasispectral pyrometer constrains the melting curve up to ∼246 GPa and ∼5830 K, which is consistent with our static results using the Simon equation. The present static and dynamic experiments on the melting curve of vanadium are consistent with our AIMD simulations with the two-phase melting modeling, and are overall consistent with other theoretical simulations using the Z method. The results reconcile the recently reported theoretical discrepancy, and refute a higher melting curve report given by self-consistent ab initio lattice dynamics calculations. The consistencies among our studies indicate that one does not have to invoke superheating as a hypothesis to describe the solid-liquid equilibrium boundary of vanadium as an explanation for static vs dynamic experimental results. Our static and dynamic results with in situ diagnostics of melting and two-phase AIMD simulation have implications for studying melting curves of other d-orbital transition metals and their alloys at extreme P-T conditions.
Youjun Zhang, Ye Tan, Hua Y. Geng, Nilesh P. Salke, Zhipeng Gao, Jun Li, Toshimori Sekine, Qingming Wang, Eran Greenberg, Vitali B. Prakapenka, Jung-Fu Lin, Melting curve of vanadium up to 256 GPa: Consistency between experiments and theory, (2020) Phys. Rev. B 102, 214104 abstract
Representative x-ray diffraction patterns of vanadium at high P-T. (a) Bcc-V phase transforms to the rh–V phase at pressures above ∼52 GPa. Vertical red, blue, and black lines are the Bragg diffraction positions of bcc V, rh V, and B2 KCl, respectively. (b) The rh-V phase transforms to the bcc V phase with increasing temperature at ∼52 GPa. The pressure of ∼52 GPa was determined at ambient temperature (∼300 K). LeBail refinements of x-ray diffraction patterns for the rh V phase are also plotted in (b) as insets (open blue circles with blue lines) to highlight the peak broadening feature. The wavelength of the incident x-ray source was 0.3344 Å. KCl in the B2 phase was used as the thermal insulator and pressure calibrant.