Effects of high-pressure on the structural, vibrational, and electronic properties of monazite-type PbCrO4

We have performed an experimental study of the crystal structure, lattice-dynamics, and optical properties of PbCrO4 (the mineral crocoite) at ambient and high pressures. In particular, the crystal structure, Raman-active phonons, and electronic band-gap have been accurately determined. X-ray-diffraction, Raman, and optical-absorption experiments have allowed us also to completely characterize two pressure-induced structural phase transitions. The first transition is isostructural, maintaining the monoclinic symmetry of the crystal, and having important consequences in the physical properties; among other a band-gap collapse is induced. The second one involves an increase of the symmetry of the crystal, a volume collapse, and probably the metallization of PbCrO4. The results are discussed in comparison with related compounds and the effects of pressure in the electronic structure explained. Finally, the room-temperature equation of state of the low-pressure phases is also obtained.

💡 Research Summary

The authors present a comprehensive experimental investigation of the mineral crocoite (PbCrO₄), a monazite‑type oxide, under pressures up to 18 GPa at room temperature. Using powder X‑ray diffraction (XRD), Raman spectroscopy, and optical absorption measurements, they characterize the ambient‑pressure structure, lattice dynamics, and electronic band gap, and then follow their evolution with compression.

At ambient conditions the crystal adopts the monoclinic P2₁/n monazite structure (a = 7.098 Å, b = 7.410 Å, c = 6.779 Å, β = 102.4°) with nine‑fold coordinated Pb and tetrahedral CrO₄ units. Rietveld refinement yields a unit‑cell volume of 348 ų. Raman spectra display 26 active modes (A_g and B_g), including a strong ν₁ symmetric stretching at 840 cm⁻¹, and the optical absorption edge follows Urbach’s law with a direct band gap E_g = 2.3 eV and Urbach energy 60 meV.

High‑pressure XRD shows that the monazite structure persists up to 3.25 GPa, where an isostructural (second‑order displacive) transition occurs. This transition is marked by peak splitting and a pronounced change in the pressure dependence of the lattice parameters, especially a steep increase in the c‑axis length and monoclinic angle, but without a detectable volume discontinuity. The bulk modulus derived from a Birch‑Murnaghan fit (fixed B′ = 4) is B₀ ≈ 57 GPa, indicating that PbCrO₄ is considerably more compressible than monazite phosphates (B₀ > 100 GPa). The anisotropic compression is evident: the a‑axis is most compressible, while the c‑axis is the stiffest, reflecting the chain‑like arrangement of alternating PbO₉ and CrO₄ polyhedra along c.

Raman measurements under pressure reveal that most modes shift to higher frequencies, but three low‑frequency modes (≈327, 81, and 73 cm⁻¹) soften with pressure, signaling incipient structural instability. Around 5.3 GPa the spectra exhibit additional peaks and mode splitting that coincide with the isostructural transition observed in XRD. Above 9 GPa a new set of diffraction peaks appears, consistent with an orthorhombic P2₁2₁2₁ cell (a ≈ 6.95 Å, b ≈ 6.11 Å, c ≈ 6.63 Å, V ≈ 282 ų). This high‑pressure phase involves a ~5 % volume collapse, an increase of Pb coordination from 9 to 12, and a higher symmetry related to the barite‑type structure of BaSO₄.

Optical absorption under pressure shows a dramatic band‑gap collapse at the isostructural transition: the gap reduces from 2.3 eV to roughly 0.3 eV, indicating a strong pressure‑induced modification of the electronic structure. At pressures above the second transition (≈9 GPa) the absorption edge becomes increasingly smeared, suggesting the onset of metallization.

The authors discuss these findings in the context of other monazite‑type compounds. The presence of magnetic Cr³⁺ lowers transition pressures compared with phosphates, and the observed coordination increase and symmetry change follow crystal‑chemical expectations for AXO₄ systems under compression. The work provides the first complete equation of state for both low‑pressure monoclinic phases and the high‑pressure orthorhombic phase of PbCrO₄.

In summary, the study identifies two pressure‑induced phase transitions in PbCrO₄: (i) an isostructural, second‑order transition near 3.3 GPa that triggers a large band‑gap reduction and subtle lattice distortions, and (ii) a reconstructive transition above 9 GPa to an orthorhombic, higher‑symmetry phase accompanied by a 5 % volume collapse and possible metallization. The comprehensive structural, vibrational, and electronic data deepen our understanding of pressure effects in monazite‑type oxides and provide a valuable reference for designing materials whose properties can be tuned by pressure.