A liquid-liquid transition can exist in monatomic transition metals with a positive melting slope

Electrical resistivity of liquid Fe to 12 GPa: Implications for heat flow in cores of terrestrial bodies

Electrical and thermal transport properties of liquid Fe under high pressure have important implications for the dynamics and thermal evolution of planetary cores and the geodynamo. However, electrical resistivity (ρ) and thermal conductivity (k) of liquid Fe at high pressure still remain contentious properties. To date, only two experimental investigations of ρ of liquid Fe in the pressure region below 7 GPa are reported in literature. Here we report the results of measurements of ρ for solid and liquid Fe (inversely proportional to k through the Wiedemann-Franz law) at pressures from 3 to 12 GPa, using a large multi-anvil press. We show that ρ of liquid Fe decreases as a function of pressure up to the δ-γ-liquid triple point at ~5.2 GPa, and subsequently remains invariant from 6 to 12 GPa, which is consistent with an earlier study on liquid Ni. Our results demonstrate an important effect of solid phase on the structure and properties of liquid Fe. Our values of ρ for solid and liquid Fe are used to calculate k in Mercury’s solid inner core and along the adiabat in the liquid outer cores of Moon, Ganymede, Mercury and Mars. Our robust values of thermal conductivity place the focus on uncertainties in thermal expansion as the cause of variation in values of core conducted heat. Except for Mercury, our adiabatic heat flux values in these terrestrial cores validate the use of similar values used in several previous studies. Our high values of core adiabatic heat flux in Mercury would provide a stabilizing effect on, and lead to an increase in thickness of, the thermally stratified layer at the top of the core.

Pressure-induced structural change in liquid GeI4

The similarity in the shape of the melting curve of GeI₄ to that of SnI₄ suggests that a liquid-liquid transition as observed in liquid SnI₄ is also expected to occur in liquid GeI₄. Because the slope of the melting curve of GeI₄ abruptly changes at around 3 GPa, in situ synchrotron diffraction measurements were conducted to examine closely the structural changes upon compression at around 3 GPa. The reduced radial distribution functions of the high- and low-pressure liquid states of GeI₄ share the same feature inherent in the high-pressure (high-density) and low-pressure (low-density) radial distribution functions of liquid SnI₄. This feature allows us to introduce local order parameters that we may use to observe the transition. Unlike the transition in liquid SnI₄, the transition from the low-pressure to the high-pressure structure seems sluggish. We speculate that the liquid-liquid critical point of GeI₄ is no longer a thermodynamically stable state and is slightly located below the melting curve. As a result, the structural change is said to be a crossover rather than a transition. The behavior of the local-order parameters implies a metastable extension of the liquid-liquid phase boundary with a negative slope.

Comparative study of crystallization process in metallic melts using ab initio molecular dynamics simulations

The crystallization process of liquid metals is studied using ab initio molecular dynamics simulations. The evolution of short-range order during quenching in Pb and Zn liquids is compared with body-centered cubic (bcc) Nb and V, and hexagonal closed-packed (hcp) Mg. We found that the fraction and type of the short-range order depends on the system under consideration, in which the icosahedral symmetry seems to dominate in the body-centered cubic metals. Although the local atomic structures in stable liquids are similar, liquid hcp-like Zn, bcc-like Nb and V can be deeply supercooled far below its melting point before crystallization while the supercooled temperature range in liquid Pb is limited. Further investigations into the nucleation process reveal the process of polymorph selection. In the body-centered cubic systems, the polymorph selection occurs in the supercooled state before the nucleation is initiated, while in the closed-packed systems it starts at the time of onset of crystallization. Atoms with bcc-like lattices in all studied supercooled liquids are always detected before the polymorph selection. It is also found that the bond orientational ordering is strongly correlated with the crystallization process in supercooled Zn and Pb liquids.

Pressure-induced structural change in liquid GaIn eutectic alloy

Synchrotron x-ray diffraction reveals a pressure induced crystallization at about 3.4 GPa and a polymorphic transition near 10.3 GPa when compressed a liquid GaIn eutectic alloy up to ~13 GPa at room temperature in a diamond anvil cell. Upon decompression, the high pressure crystalline phase remains almost unchanged until it transforms to the liquid state at around 2.3 GPa. The ab initio molecular dynamics calculations can reproduce the low pressure crystallization and give some hints on the understanding of the transition between the liquid and the crystalline phase on the atomic level. The calculated pair correlation function g(r) shows a non-uniform contraction reflected by the different compressibility between the short (1st shell) and the intermediate (2nd to 4th shells). It is concluded that the pressure-induced liquid-crystalline phase transformation likely arises from the changes in local atomic packing of the nearest neighbors as well as electronic structures at the transition pressure.