Abstract
Liquid-liquid transitions under high pressure are found in many elemental materials, but the transitions are known to be associated with either sp-valent materials or f-valent rare-earth elements, in which the maximum or a negative slope in the melting line is readily suggestive of the transition. Here we find a liquid-liquid transition with a positive melting slope in transition metal Ti from structural, electronic, and thermodynamic studies using ab-initio molecular dynamics calculations, showing diffusion anomaly, but no density anomaly. The origin of the transition in liquid Ti is a pressure-induced increase of local structures containing very short bonds with directionality in electronic configurations. This behavior appears to be characteristic of the early transition metals. In contrast, the late transition metal liquid Ni does not show the L-L transition with pressure. This result suggests that the possibility of the L-L transition decreases from early to late transition metals as electronic structures of late transition metals barely have a Jahn-Teller effect and bond directionality. Our results generalize that a phase transition in disordered materials is found with any valence band regardless of the sign of the melting slope, but related to the symmetry of electronic structures of constituent elements.
Highlights
The L-L transition in elemental transition liquid metals with positive melting slope is unlikely to happen
The increasing intensity and narrowing width of the peaks with pressure are a typical indication of a better atomic correlation given no change of the local order of the liquid
In addition to the transition evidenced in the slope changes in main peaks of g(r) (Fig. 1c), the number of atoms in the nearest distance (coordination number (CN)) changes around 30~40 GPa with pressure (Fig. 1d)
Summary
The L-L transition in elemental transition liquid metals with positive melting slope is unlikely to happen. Some early transition metals (e.g., Ti and Zr), unlike water and silicon that have LDA and HDA, have been strongly suspected to have a glassy phase under high pressure, which does not guarantee an observation of LDL-HDL13. Ab-initio molecular dynamics calculations at 2400 K reveal the evidence of the LDL-HDL transition on liquid Ti. It is found that the local order and the electronic structure of the liquid Ti substantially change by compressing. The LDL-HDL transformation in liquid Ti provides a clue for the lower melting slope of the early transition metals than that of late transition metals[18,19]. Since the high compressibility of the liquid Ti relative to liquid Ni reduces the volume change across the solid-liquid phase boundary at high pressure and temperature, and explains the lower melting slope of the early transition metals. A further study of other transition metal liquids may complete the understanding of phase transition phenomena, and may reveal unexplored high-pressure phases
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