Melting curve and phase diagram of vanadium under high-pressure and high-temperature conditions

Abstract

We report a combined experimental and theoretical study of the melting curve and the structural behavior of vanadium under extreme pressure and temperature. We performed powder x-ray diffraction experiments up to 120 GPa and 4000 K, determining the phase boundary of the bcc-to-rhombohedral transition and melting temperatures at different pressures. Melting temperatures have also been established from the observation of temperature plateaus during laser heating, and the results from the density-functional theory calculations. Results obtained from our experiments and calculations are fully consistent and lead to an accurate determination of the melting curve of vanadium. These results are discussed in comparison with previous studies. The melting temperatures determined in this study are higher than those previously obtained using the speckle method, but also considerably lower than those obtained from shock-wave experiments and linear muffin-tin orbital calculations. Finally, a high-pressure high-temperature equation of state up to 120 GPa and 2800 K has also been determined.

Experimental and theoretical confirmation of an orthorhombic phase transition in niobium at high pressure and temperature

“…Above that T, we observe rapid changes in the relative intensities of the Bragg peaks in consecutive diffraction patterns, which are due to the recrystallization of the sample. This phenomenon has also been observed in other metals under HP conditions at several hundred degrees below T m [21][22][23][24][25] . It is probably caused by an increase in the grain size in polycrystalline material at HT.…”

“…However, their phase diagrams have remained virtually unknown over decades of research. More recently, advances in theoretical [1][2][3][4][5][6][7][8][9][10] and experimetal [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] techniques started offering the possibility to gain better understanding of the high-pressure (HP) and high-temperature (HT) behavior of transition metals and to elucidate the fundamental mechanisms responsible for the physical appearance of their HP-HT phase diagrams. Of special interest is HP-HT polymorphism in the bcc transition metals of Group 4B (Cr, Mo, and W) and Group 5B (V, Nb, and Ta), which is related to laser-heated diamond anvil cell (DAC) melting experiments bringing up a small slope (dT/dP) of the corresponding melting curves in the pressure-temperature (P-T) coordinates in the megabar P range [11][12][13][14][15][16][17]19 .…”

Pressure-induced bcc-rhombohedral phase transition in vanadium metal

“…Furthermore, finitetemperature computational studies by Yu et al have predicted that the strength of V will increase with increasing temperature due to unusual hardening of the C 44 shear modulus with temperature [14]. Finally, the high-temperature behavior of V, and its melting curve, have been the subject of two recent studies by Errandonea et al [15] and Zhang et al [16]. Using a DAC, Errandonea et al mapped the bcc-rhombohedral transition at elevated temperatures, and showed that only the bcc phase is observed above ∼1800 K at 60 and 120 GPa [15].…”

Ab Initio Phase Diagram of Tungsten

“…Fast recrystallization observed in HP-HT experiments on transition metals may imply either a microstructural transition (discussed above) at which the sample texture changes but its crystal structure remains the same, or a true solid-solid phase transition. The former was recently proven to be the case in Mo [17], and it is very likely the case in V [19] as well as in W. The latter is apparently the case in Nb [58] and likely in Re as well [25]. In either case, the corresponding fast recrystallization lines have been misinterpreted vs. experimental [37,44,46,47] and other theoretical [45] data.…”

“…Research interest in HP-HT polymorphism in the bcc transition metals has reemerged in connection with laser-heated diamond anvil cell (DAC) melting experiments in which melting curves with a small slope (dT /dP ) in the megabar pressure range have been determined [5][6][7][8][9][10][11][12]. These flat melting curves do not agree with the results of more recent experiments [13][14][15][16][17][18][19] and calculations [20][21][22][23][24][25]. Several hypotheses have been proposed to explain these apparent discrepancies.…”