Theory of elastic phase transitions in metals at high pressures. Application to vanadium (original) (raw)
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Physical Review B, 2008
We present results from ab initio calculations of the mechanical properties of the rhombohedral phase (β) of vanadium metal reported in recent experiments, and other predicted high-pressure phases (γ and bcc), focusing on properties relevant to dynamic experiments. We find that the volume change associated with these transitions is small: no more than 0.15% (for β -γ). Calculations of the single crystal and polycrystal elastic moduli (stress-strain coefficients) reveal a remarkably small discontinuity in the shear modulus and other elastic properties across the phase transitions even at zero temperature where the transitions are first order.
Structural Phase Transition of Vanadium at 69 GPa
Physical Review Letters, 2007
A phase transition was observed at 63-69 GPa and room temperature in vanadium with synchrotron x-ray diffraction. The transition is characterized as a rhombohedral lattice distortion of the body-centeredcubic vanadium without a discontinuity in the pressure-volume data, thus representing a novel type of transition that has never been observed in elements. Instead of driven by the conventional s-d electronic transition mechanism, the phase transition could be associated with the softening of C 44 trigonal elasticity tensor that originates from the combination of Fermi surface nesting, band Jahn-Teller distortion, and electronic topological transition.
Theoretical confirmation of a high-pressure rhombohedral phase in vanadium metal
Physical Review B, 2007
Recent diamond-anvil-cell ͑DAC͒ experiments revealed a new phase in vanadium metal at high pressure. Here we present results from first-principles electronic-structure calculations confirming the existence of this phase. The structure corresponds to a rhombohedral distortion of the bcc ambient-pressure phase. The calculated transition pressure ͑0.84 Mbar͒ and density compare reasonably with the measured data. Interestingly, a reentrant bcc phase is discovered at ultrahigh pressures above 2.8 Mbar, close to the limit of DAC experimental capabilities. We show, extending prior work, that the phase transitions in vanadium are driven by subtle electronic-structure effects.
Melting curve and phase diagram of vanadium under high-pressure and high-temperature conditions
Physical Review B, 2019
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.
Landau theory for the phase transitions of interstitial hydrogen in strained vanadium
Physical Review B, 2014
A special version of the Landau theory of phase transitions has been developed, capturing the order-disorder phase transition in the bulk and elastically strained vanadium-hydrogen system. The equations for the solubility isotherms are obtained and the influence of biaxial strain on the critical temperature is determined. The critical temperature is found to change exponentially with strain. The calculated solubility isotherms agree well with experimental data for temperatures below, as well as above, the critical temperature. The influence of elastic strain on the solubility isotherm is analyzed and noticeable deviations from Sieverts' law, even at the extremely low concentrations, are obtained.
Phase Transitions, 2007
We use a combination of diamond anvil cell techniques and large volume (multi-anvil press, piston cylinder) devices to study the synthesis, structure and properties of new materials under high pressure conditions. The work often involves the study of structural and phase transformations occurring in the metastable regime, as we explore the phase space determined as a function of the pressure, temperature and chemical composition. The experimental studies are combined with first principles calculations and molecular dynamics simulations, as we determine the structures and properties of new phases and the nature of the transformations between them. Problems currently under investigation include structural studies of transition metal and main group nitrides, oxides and oxynitrides at high pressure, exploration of new solid-state compounds that are formed within the C-N-O system, polyamorphic lowto high-density transitions among amorphous semiconductors such as a-Si, and transformations into metastable forms of the element that occur when its ''expanded'' clathrate polymorph is compressed.
Elasticity of the superconducting metals V, Nb, Ta, Mo, and W at high pressure
Physical Review B, 2008
First-principles calculations have been performed for V, Nb, Ta, Mo, and W. The recently discovered bcc→ rhombohedral transition for vanadium ͓Phys. Rev. Lett. 98, 085502 ͑2007͔͒ was confirmed as the mechanical instability of c 44 was found at P = 80 GPa. Furthermore, the c 11 , c 12 , and c 44 constants for the group-V elements showed erratic behaviors whereas the constants for the group-VI elements were monotonically increasing with pressure. The metals were analyzed with Fermi surface calculations, showing shrinking nesting vectors with pressure for V, Nb, and Ta but were not seen for Mo and W. From electronic topological transition contributions, a critical energy closely situated to the Fermi level for vanadium could be the reason why the elastic constants of V and Nb were difficult to reproduce at ambient pressure.