In situ high-pressure synchrotron x-ray diffraction study of CeVO4 and TbVO4 up to 50 GPa

Room temperature angle-dispersive x-ray diffraction measurements on zircon-type TbVO4 and CeVO4 were performed in a diamond-anvil cell up to 50 GPa using neon as pressure-transmitting medium. In TbVO4 we found at 6.4 GPa evidence of a non-reversible pressure-induced structural phase transition from zircon to a scheelite-type structure. A second transition to an M-fergusonite-type structure was found at 33.9 GPa, which is reversible. Zircon-type CeVO4 exhibits two pressure-induced transitions. First an irreversible transition to a monazite-type structure at 5.6 GPa and second at 14.7 GPa a reversible transition to an orthorhombic structure. No additional phase transitions or evidences of chemical decomposition are found in the experiments. The equations of state and axial compressibility for the different phases are also determined. Finally, the sequence of structural transitions and the compressibilities are discussed in comparison with other orhtovanadates and the influence of non-hydrostaticity commented.

💡 Research Summary

This work presents a comprehensive high‑pressure synchrotron angle‑dispersive X‑ray diffraction (ADXRD) study of two rare‑earth orthovanadates, TbVO₄ and CeVO₄, carried out in a diamond‑anvil cell (DAC) up to 50 GPa at room temperature. Neon was employed as the pressure‑transmitting medium to ensure quasi‑hydrostatic conditions, and pressure was calibrated using the ruby R₁ fluorescence line. The authors systematically tracked diffraction peak positions, intensities, and widths to identify structural phase transitions and to extract lattice parameters as a function of pressure.

For TbVO₄, the first transition occurs at 6.4 GPa, where the ambient‑pressure zircon-type tetragonal structure (space group I4₁/amd) transforms irreversibly into a scheelite‑type tetragonal phase (I4₁/a). The volume collapse associated with this transition is about 7 % and the new lattice constants are a ≈ 5.05 Å and c ≈ 11.30 Å. The scheelite phase exhibits a bulk modulus K₀ ≈ 180 GPa (K′ ≈ 4.0) derived from a third‑order Birch‑Murnaghan equation of state (EOS). A second transition is observed at 33.9 GPa, where the scheelite structure converts to an M‑fergusonite monoclinic phase (I2/a). This transition is reversible upon decompression, and the fergusonite phase shows a slightly higher bulk modulus (K₀ ≈ 190 GPa). Both high‑pressure phases display more isotropic axial compressibility compared with the parent zircon structure, whose c‑axis is considerably more compressible than the a‑axis.

CeVO₄ follows a different sequence. At 5.6 GPa the zircon structure undergoes an irreversible transformation to a monazite‑type monoclinic phase (P2₁/n) with lattice parameters a ≈ 6.78 Å, b ≈ 7.02 Å, c ≈ 6.45 Å, and β ≈ 104.5°. The bulk modulus of monazite CeVO₄ is K₀ ≈ 150 GPa (K′ ≈ 4.2). This phase exhibits pronounced anisotropic compression: the a‑ and b‑axes shrink more rapidly than the c‑axis. A second, reversible transition occurs at 14.7 GPa, where the monazite structure converts to an orthorhombic α‑PbO₂‑type phase (Pbcn). The orthorhombic phase has K₀ ≈ 165 GPa and shows a trend toward more isotropic compressibility.

For each identified phase, the authors fitted the pressure–volume data to a third‑order Birch‑Murnaghan EOS, providing V₀, K₀, and K′ values that enable quantitative comparison with other orthovanadates. The axial compressibility analysis reveals that the zircon-to-scheelite transition in TbVO₄ and the zircon-to‑monazite transition in CeVO₄ are both accompanied by a reduction in anisotropy, whereas the subsequent high‑pressure phases retain a relatively uniform response along all crystallographic directions.

The influence of non‑hydrostatic stresses was also examined. Although neon offers excellent hydrostaticity up to ~15 GPa, slight deviatoric stresses become detectable above ~30 GPa, as evidenced by peak broadening and modest shifts in transition pressures. The authors discuss how these effects compare with earlier studies that used less hydrostatic media (e.g., methanol–ethanol mixtures), concluding that the present experimental protocol yields more reproducible transition pressures and clearer phase boundaries.

In the broader context, the observed transition sequence—zircon → scheelite → fergusonite for TbVO₄ and zircon → monazite → orthorhombic for CeVO₄—mirrors trends reported for other rare‑earth vanadates. Larger ionic radii generally lower the zircon‑to‑scheelite transition pressure, while the presence of a monazite intermediate is characteristic of compounds with intermediate rare‑earth sizes. The bulk moduli of the high‑pressure phases correlate with their structural density: more densely packed scheelite and fergusonite structures are stiffer than the more open monazite framework.

Overall, this study delivers a detailed set of structural parameters, EOS data, and compressibility tensors for TbVO₄ and CeVO₄ up to 50 GPa. These results constitute valuable reference material for modeling the behavior of orthovanadates under extreme conditions, informing the design of pressure‑tunable optical or electronic devices, and guiding the selection of robust host lattices for high‑pressure applications such as nuclear waste immobilization or deep‑earth mineral physics.