Structural Phase Transition of Vanadium at 69 GPa

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-centered-cubic 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 $s\mathrm{\text{−}}d$ 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.

Figure 1

(a) The diffraction pattern of vanadium recorded on an image plate at 65 GPa under nonhydrostatic conditions. The indexing is based on a bcc lattice. (b) The enlarged image of the shadowed area in (a). (c) The diffraction pattern of vanadium at 155 GPa. (d) The enlarged image of the shadow area in (c).

Figure 2

Nonhydrostatic x-ray diffraction patterns of vanadium at (a) 65 GPa, (b) 90 GPa, and (c) 155 GPa. The inset in (b) demonstrates the transformation of vanadium from cubic to the rhombohedral structure. The insets in (c) demonstrate the resolved splitting peaks according to refinement of rhombohedral structure model. $E$ denotes the energy of x-ray photon used for diffraction.

Figure 3

The diffraction pattern of vanadium at 73 GPa under quasihydrostatic conditions recorded on an image plate. The diffraction rings are spotty because the sample grinding was minimized in order to avoid the introduction of residual strain. From inside outward, the four rings are originally 110, 200, 211, and 220 reflections of the cubic phase. Insets are zoomed images showing the splitting of 110, 211, and 220 rings after rhombohedral transition.

Figure 4

(a) The pressure-volume data of vanadium. The lines represent the fitted results using 3rd order Birch-Murnaghan equation of state. (b) The plot of order parameter $⟨Q⟩$, defined as $(\alpha /{\alpha }_0)-1$ where ${\alpha }_0=109.47°$, vs pressure. The phase transition pressures determined from nonhydrostatic and quasihydrostatic experiments are 69(1) GPa and 63(1) GPa, respectively, using equation $⟨Q⟩=C(P-P_c{)}^{1/2}$.