Theoretical confirmation of a high-pressure rhombohedral phase in vanadium metal (original) (raw)

Elastic constants and volume changes associated with two high-pressure rhombohedral phase transformations in vanadium

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.

Phonon triggered rhombohedral lattice distortion in vanadium at high pressure

Scientific Reports, 2016

In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, niobium and tantalum, show strong interactions between the electronic properties and lattice dynamics. Further, these interactions can be tuned by external parameters, such as pressure and temperature. We used inelastic x-ray scattering to probe the phonon dispersion of single-crystalline vanadium as a function of pressure to 45 GPa. Our measurements show an anomalous high-pressure behavior of the transverse acoustic mode along the (100) direction and a softening of the elastic modulus C 44 that triggers a rhombohedral lattice distortion occurring between 34 and 39 GPa. Our results provide the missing experimental confirmation of the theoretically predicted shear instability arising from the progressive intra-band nesting of the Fermi surface with increasing pressure, a scenario common to all transition metals of group V. Although body-centered-cubic (bcc) metals have one of the simplest crystal structures in the periodic table, they display a rich variety of physical properties and thus provide an important benchmark for the validation of modern first-principle theory 1. In particular, the lattice dynamics of bcc transition metals have attracted great scientific attention. The Kohn anomaly in the phonon dispersion of bcc transition metals, and its dependence upon pressure and temperature, has been a challenge for first principle calculations to capture 2,3. The strong differences displayed by the phonon dispersion of the various elements of group V (vanadium, niobium and tantalum) suggest that there is a profound dependence of the phonon energies on the electronic structure and the topology of the Fermi surface 4,5. The high superconducting temperature (T c = 9.25 K for Nb and T c = 5.3 K for V) and its notable increase with pressure have also been suggested to be due to electron-phonon coupling and Fermi-surface properties 6-8. The stability at high pressure of the bcc structure is speculated to critically hinge on the topology of the Fermi surface as well, and an intra-band nesting is theoretically predicted to give rise to shear phonon instabilities 9. Focusing on vanadium, calculations of shear instabilities arising from phonon softening 9 have prompted the reinvestigation of the structural stability of V under high pressure. X-ray powder diffraction showed a transition from the bcc to a rhombohedral phase at 69 GPa 10 and subsequent calculations have confirmed the nature of the rhombohedral distortion-even though different transition pressures were proposed 5,11-13. Interestingly, under hydrostatic conditions the transition is hindered, and non-hydrostaticity helps in overcoming the energy barrier associated with the structural phase change 14. Irrespective of the exact pressure at which the transition occurs, the bulk of theoretical work points towards a common mechanism: the progressive intra-band nesting at the Fermi surface that eventually leads to an electronic topological transition (ETT) with a concomitant transverse acoustic phonon mode softening. Specifically, at a critical pressure, parts of the 3rd electronic, partially occupied, conduction band of d symmetry move into the close vicinity of the Fermi level. The nesting vector, already responsible for the Kohn anomaly in the transverse acoustic phonon mode along the (ξ, 0, 0) direction at ξ = 0.25 at ambient pressure 8 , reduces to zero and the ETT takes place, with instability in the shear elastic constant C 44 9. This anomalous softening of the elastic response causes an energy gain that counterbalances the standard elastic strain energy

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.

High-pressure studies of atomically thin van der Waals materials

Applied Physics Reviews

Two-dimensional (2D) materials and their moiré superlattices represent a new frontier for quantum matter research due to the emergent properties associated with their reduced dimensionality and extreme tunability. The properties of these atomically thin van der Waals (vdW) materials have been extensively studied by tuning a number of external parameters such as temperature, electrostatic doping, magnetic field, and strain. However, so far pressure has been an under-explored tuning parameter in studies of these systems. The relative scarcity of high-pressure studies of atomically thin materials reflects the challenging nature of these experiments, but, concurrently, presents exciting opportunities for discovering a plethora of unexplored new phenomena. Here, we review ongoing efforts to study atomically thin vdW materials and heterostructures using a variety of high-pressure techniques, including diamond anvil cells, piston cylinder cells, and local scanning probes. We further addres...

Theory of elastic phase transitions in metals at high pressures. Application to vanadium

Journal of Experimental and Theoretical Physics, 2011

Structural transformations in elementary metals under high pressures are considered using the Landau theory of phase transitions, in which the finite strain tensor components play the role of the order parameter. As an example, the phase transition in vanadium observed at a pressure of 69 GPa is analyzed. It is shown that it is a first order elastic phase transition, which is close to a second order transition.

Evidence of pressure induced compressibility enhancement in pure and Cr-doped vanadium dioxide

2011

We present structural studies of V1−xCrxO2 (pure, 0.7% and 2.5% Cr doped) compounds at room temperature in a diamond anvil cell for pressures up to 20 GPa using synchrotron x-ray powder diffraction. All the samples studied show a persistence of the monoclinic M1 symmetry between 4 and 12 GPa. Above 12 GPa, the monoclinic M1 symmetry changes to isostructural Mx phase (space group P 21/c) with a significant anisotropy in lattice compression of the b-c plane of the M1 phase.

Anisotropic compression in the high-pressure regime of pure and chromium-doped vanadium dioxide

Physical Review B, 2012

We present structural studies of V 1−x Cr x O 2 (x = 0.0, 0.007 and 0.025) compounds at room temperature in a diamond anvil cell for pressures up to 20 GPa using synchrotron x-ray powder diffraction. All the samples studied show a persistence of the monoclinic M 1 symmetry between 4 to 13 GPa. Above 13 GPa, the monoclinic M 1 symmetry changes to isostructural M x phase (space group P 2 1 /c) with a significant anisotropy in lattice compression of the b M 1 -c M 1 plane.

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.