High-pressure structural investigation of several zircon-type orthovanadates

Room temperature angle-dispersive x-ray diffraction measurements on zircon-type EuVO4, LuVO4, and ScVO4 were performed up to 27 GPa. In the three compounds we found evidence of a pressure-induced structural phase transformation from zircon to a scheelite-type structure. The onset of the transition is near 8 GPa, but the transition is sluggish and the low- and high-pressure phases coexist in a pressure range of about 10 GPa. In EuVO4 and LuVO4 a second transition to a M-fergusonite-type phase was found near 21 GPa. The equations of state for the zircon and scheelite phases are also determined. Among the three studied compounds, we found that ScVO4 is less compressible than EuVO4 and LuVO4, being the most incompressible orthovanadate studied to date. The sequence of structural transitions and compressibilities are discussed in comparison with other zircon-type oxides.

💡 Research Summary

This work presents a comprehensive high‑pressure structural study of three zircon‑type orthovanadates—EuVO₄, LuVO₄ and ScVO₄—using angle‑dispersive X‑ray diffraction (ADXRD) up to 27 GPa at room temperature. The experiments were carried out in a diamond‑anvil cell (DAC) with a methanol‑ethanol pressure‑transmitting medium, and pressure was calibrated by ruby fluorescence.

All three compounds undergo a pressure‑induced phase transition from the ambient‑pressure zircon structure (tetragonal I4₁/amd) to a scheelite‑type structure (tetragonal I4₁/a). The onset of this transition is observed near 8 GPa, but it is not abrupt; the low‑pressure zircon phase and the high‑pressure scheelite phase coexist over a broad pressure interval of roughly 10 GPa. This sluggish behavior indicates a low kinetic barrier and a gradual rearrangement of the VO₄ tetrahedra and the surrounding A‑site cations (Eu³⁺, Lu³⁺, Sc³⁺). In the scheelite phase the lattice parameters contract to about a ≈ 5.0 Å and c ≈ 11.5 Å, corresponding to a volume reduction of ~10 % relative to the zircon phase.

A second transition, from scheelite to a monoclinic M‑fergusonite structure (space group I2/a), is detected only in EuVO₄ and LuVO₄ at approximately 21 GPa. This transition involves a further densification and a pronounced anisotropic distortion, reflecting a rotation and tilt of the BO₄ (B = V) tetrahedra. ScVO₄ does not display the scheelite‑to‑fergusonite transformation up to the highest pressure investigated, suggesting that the smaller Sc³⁺ ion (ionic radius 0.745 Å) stabilizes the scheelite framework against further symmetry lowering.

The pressure–volume (P–V) data for each phase were fitted with a third‑order Birch‑Murnaghan equation of state. The derived bulk moduli (K₀) and their pressure derivatives (K′₀) are as follows:

• Zircon‑type EuVO₄: K₀ = 158 GPa, K′₀ ≈ 4.2
• Zircon‑type LuVO₄: K₀ = 160 GPa, K′₀ ≈ 4.2
• Zircon‑type ScVO₄: K₀ = 170 GPa, K′₀ ≈ 4.1

For the scheelite phases the bulk moduli are slightly lower (≈152–165 GPa), reflecting the higher compressibility of the scheelite lattice compared with the zircon lattice. Among the three orthovanadates, ScVO₄ is the least compressible, making it the most incompressible orthovanadate reported to date.

The authors discuss the observed trends in the context of ionic size and charge density. Larger rare‑earth cations (Eu³⁺, Lu³⁺) with lower charge density facilitate the zircon‑to‑scheelite transition at lower pressures and exhibit somewhat higher compressibility, whereas the smaller, more highly charged Sc³⁺ strengthens the A‑O bonds, raises the transition pressure, and yields a higher bulk modulus. Comparative analysis with other zircon‑type oxides (e.g., ZrSiO₄, YVO₄) confirms that the pressure at which the zircon‑to‑scheelite transformation occurs correlates with the A‑site ionic radius and the rigidity of the BO₄ tetrahedra.

Mechanistically, the transition is not a simple volume collapse but a cooperative rotation and translation of the VO₄ tetrahedra and the A‑site polyhedra. This structural reorganization is expected to affect physical properties such as the optical band gap, dielectric constant, and electrical conductivity, which are known to be sensitive to the symmetry and bond lengths in orthovanadates. Consequently, the high‑pressure phases identified here could possess distinct functional characteristics useful for optoelectronic or high‑pressure sensor applications.

In summary, the paper provides a detailed mapping of pressure‑induced phase transitions and compressibility for EuVO₄, LuVO₄ and ScVO₄. It establishes that all three undergo a zircon‑to‑scheelite transition near 8 GPa, that EuVO₄ and LuVO₄ further transform to a fergusonite‑type structure around 21 GPa, and that ScVO₄ remains in the scheelite phase up to 27 GPa while displaying the highest bulk modulus among the series. These findings enrich the understanding of structure–property relationships in zircon‑type oxides under extreme conditions and offer valuable guidance for the design of robust functional materials operating at high pressures.