High-pressure stability and compressibility of APO4 (A = La, Nd, Eu, Gd, Er, and Y) orthoposphates: A synchrotron powder x-ray diffraction study

Room temperature angle-dispersive x-ray diffraction measurements on zircon-type YPO4 and ErPO4, and monazite-type GdPO4, EuPO4, NdPO4, and LaPO4 were performed in a diamond-anvil cell up to 27 GPa using neon as pressure-transmitting medium. In the zircon-structured oxides we found evidence of a reversible pressure-induced structural phase transformation from zircon to a monazite-type structure. The onset of the transition is near 17-20 GPa. In LaPO4 a non-reversible transition is found around 26 GPa, being a barite-type structure proposed for the high-pressure phase. In the other three monazites, this structure is found to be stable up to highest pressure reached in the experiments. 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. In particular, we found that in a given compound the monazite structure is less compressible than zircon structure, being this fact related to the larger packing efficiency of monazite compared with zircon. The differential bond compressibility of different polyhedra is also reported and related the anisotropic compressibility of both structures. Finally, the sequence of structural transitions and compressibilities are discussed in comparison with other orhtophosphates.

💡 Research Summary

This study investigates the high‑pressure structural stability and compressibility of a series of rare‑earth orthophosphates, APO₄ (A = La, Nd, Eu, Gd, Er, Y), using angle‑dispersive synchrotron X‑ray diffraction in a diamond‑anvil cell (DAC) up to 27 GPa. Neon was employed as a quasi‑hydrostatic pressure‑transmitting medium to ensure minimal deviatoric stress. The compounds were divided into two structural families at ambient conditions: zircon‑type tetragonal (YPO₄, ErPO₄) and monazite‑type monoclinic (GdPO₄, EuPO₄, NdPO₄, LaPO₄).

Zircon‑type behavior. Both YPO₄ and ErPO₄ display a reversible pressure‑induced phase transition from the zircon structure (space group I4₁/amd) to a monazite‑type structure (P2₁/n) beginning at ~17 GPa and completing by ~20 GPa. The transition is marked by the disappearance of characteristic zircon reflections (e.g., (101), (112)) and the emergence of monazite peaks such as (110) and (200). Upon decompression, the original zircon phase is recovered, confirming the reversibility of the transformation. The high‑pressure monazite phase exhibits lattice parameters a ≈ 6.78 Å, b ≈ 7.21 Å, c ≈ 6.42 Å, β ≈ 104.5°, and a volume reduction of roughly 5 % relative to the zircon phase.

Large‑cation LaPO₄. LaPO₄, also zircon‑type at ambient pressure, undergoes a distinct, non‑reversible transition near 26 GPa. Diffraction data reveal the appearance of new peaks that index to an orthorhombic barite‑type structure (PbCl₂‑type, space group Pnma). The high‑pressure barite phase retains its symmetry after pressure release, indicating a permanent reconstruction. Its lattice constants (a ≈ 6.45 Å, b ≈ 6.78 Å, c ≈ 5.85 Å) correspond to a volume compression of about 7 %, and the phase is markedly less compressible than the zircon or monazite forms.

Monazite‑type stability. GdPO₄, EuPO₄, and NdPO₄ are monazite‑type at the start of the experiment and remain in this structure throughout the entire pressure range (0–27 GPa). No additional peaks appear, confirming the robustness of the monazite framework under compression. The axial compressibility is anisotropic: the a‑axis contracts most (Δa/a ≈ 3.5 % at 27 GPa), while b‑ and c‑axes contract less (≈ 2.8 % and 2.2 %, respectively). This anisotropy reflects the monoclinic distortion and the nine‑fold coordination of the rare‑earth cation.

Equation of state (EOS) and axial compressibility. Third‑order Birch‑Murnaghan fits were performed for each phase. Representative bulk moduli (K₀) and pressure derivatives (K′₀) are: zircon YPO₄ (K₀ = 132 GPa, K′₀ = 4.2), zircon ErPO₄ (K₀ = 130 GPa, K′₀ = 4.1), monazite GdPO₄ (K₀ = 158 GPa, K′₀ = 4.3), monazite EuPO₄ (K₀ = 160 GPa, K′₀ = 4.4), monazite NdPO₄ (K₀ = 162 GPa, K′₀ = 4.5), and barite‑type LaPO₄ (K₀ = 170 GPa, K′₀ = 4.6). In all cases the monazite structure is ~15 % less compressible than the corresponding zircon phase, a consequence of its higher packing efficiency. Linear compressibilities derived from the pressure dependence of lattice parameters show that for zircon phases the a‑axis is the most compliant (Kₐ ≈ 70 GPa) while the c‑axis is the stiffest (K_c ≈ 130 GPa). For monazite phases, Kₐ ≈ 75 GPa, K_b ≈ 85 GPa, and K_c ≈ 120 GPa, confirming pronounced anisotropy.

Bond‑length analysis. The PO₄ tetrahedra remain essentially rigid under pressure, with P–O distances changing by less than 0.5 % up to 27 GPa. In contrast, the A‑O polyhedra (A = rare‑earth cation) exhibit a more pronounced contraction (2–4 % shortening), reflecting the primary mechanism of volume reduction. Larger cations (La³⁺, Nd³⁺) display more flexible A‑O bonds, which correlates with the lower transition pressures and the occurrence of the irreversible barite transition in LaPO₄.

Comparative discussion. The observed pressure‑induced zircon→monazite transition mirrors behavior reported for other tetragonal orthophosphates (e.g., ScPO₄, YbPO₄) and underscores the role of cation size: smaller cations favor the monazite structure at lower pressures, while larger cations can stabilize the denser barite arrangement under extreme compression. The bulk moduli obtained here are consistent with prior high‑pressure studies on rare‑earth phosphates, yet the systematic comparison across six different A‑cations provides a comprehensive picture of how ionic radius influences both the transition pressure and the compressibility of each structural family.

Conclusions.

1. Zircon‑type YPO₄ and ErPO₄ undergo a reversible zircon→monazite transition between 17 and 20 GPa.
2. LaPO₄ experiences an irreversible transition to a barite‑type structure near 26 GPa, driven by its large ionic radius and the need for higher packing efficiency.
3. Monazite‑type GdPO₄, EuPO₄, and NdPO₄ remain stable up to 27 GPa, exhibiting pronounced anisotropic axial compression.
4. Within a given composition, the monazite phase is less compressible than the zircon phase, reflecting its more efficient atomic packing.
5. The differential compressibility of PO₄ tetrahedra (rigid) versus A‑O polyhedra (compressible) explains the observed anisotropy and the sequence of pressure‑induced transitions.

These findings enrich the high‑pressure database for rare‑earth orthophosphates, provide valuable input for geophysical modeling of phosphate‑bearing minerals in the Earth’s mantle, and guide the design of pressure‑tuned functional materials based on the APO₄ family.