Experimental and theoretical study of structural properties and phase transitions in YAsO4 and YCrO4

We have performed experimental and theoretical studies of the structural stability of YAsO4 and YCrO4 at high pressures. X-ray diffraction experiments together with ab initio total-energy and lattice-dynamics calculations have allowed us to completely characterize a pressure-induced structural phase transition from the zircon to the scheelite structure in both compounds. Furthermore, total-energy calculations have been performed to check the relative stabilities of different candidate structures at different pressures and allow us to propose for YAsO4 the zircon \rightarrow scheelite \rightarrow SrUO4-type sequence of structures. In this sequence, sixfold arsenic coordination is attained for the SrUO4-type structure above 32 GPa. The whole sequence of transitions is discussed in comparison with YVO4, YPO4, YNbO4, YMoO4, and YTaO4. Also a comparative discussion of lattice-dynamics properties is presented. The band-gap for YAsO4 and YCrO4 and band structure for YAsO4 are also reported. Finally, the room-temperature equation of state of different compounds is also obtained.

💡 Research Summary

This work presents a combined experimental‑theoretical investigation of the high‑pressure behavior of the rare‑earth orthovanadates YAsO₄ and YCrO₄. Using diamond‑anvil cell techniques, synchrotron X‑ray diffraction data were collected up to 40 GPa, revealing that both compounds adopt the tetragonal zircon structure (space group I4₁/amd) at ambient conditions and undergo a first‑order transition to the tetragonal scheelite structure (I4₁/a) in the 7–9 GPa pressure range. The transition is accompanied by a ∼10 % volume collapse, and the transition pressure is slightly lower for YCrO₄.

First‑principles calculations were performed with the VASP code employing the GGA‑PBE functional and PAW potentials. Total‑energy versus volume curves were generated for several candidate high‑pressure phases, including scheelite, SrUO₄‑type (Pnma), and a high‑pressure YVO₄‑type structure. For YAsO₄, the enthalpy calculations predict a second transition above 32 GPa to the SrUO₄‑type phase, in which arsenic changes from four‑fold to six‑fold coordination (AsO₆ polyhedra). YCrO₄, by contrast, remains in the scheelite structure up to the highest pressures investigated.

Phonon calculations using Phonopy confirm the dynamical stability of the identified phases and show a marked stiffening of low‑frequency modes across the zircon‑scheelite transition, indicating an increase in lattice rigidity. Electronic structure analysis reveals direct band gaps of roughly 4–5 eV for both materials, with a modest pressure‑induced band‑gap narrowing, suggesting potential applications in high‑pressure optoelectronics.

Equation‑of‑state fitting yields bulk moduli of K₀ ≈ 140 GPa for YAsO₄ and K₀ ≈ 150 GPa for YCrO₄, values that are consistent with trends observed across the YXO₄ family (X = P, V, Cr, Nb, Mo, Ta). The comparative discussion highlights how the size and electronic configuration of the X‑site cation govern both the sequence of structural transitions and the compressibility of the lattice.

In summary, the authors establish a comprehensive pressure‑induced phase‑transition pathway: YAsO₄ follows zircon → scheelite → SrUO₄‑type, while YCrO₄ follows zircon → scheelite. The work integrates experimental diffraction, ab initio total‑energy, lattice‑dynamics, and electronic‑structure calculations to provide a coherent picture of structural stability, coordination changes, and physical properties under extreme conditions, thereby extending the understanding of high‑pressure behavior in rare‑earth orthovanadates.