Study of the structural, optical, and electronic properties of gadolinium orthophosphate: a combination of experimental and theoretical studies

Abstract

We present an in-depth study combining experimental techniques with ab initio density functional theory (DFT) calculations to elucidate the electronic structure and optical properties of gadolinium orthophosphate (GdPO₄). Experimentally, X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the synthesized pure phase of GdPO₄. Rietveld refinement analysis confirmed that GdPO₄ crystallizes in the monoclinic space group P2₁/n with a monazite structure. UV-visible diffuse reflectance spectroscopy was performed on synthesized GdPO₄ powders. The absorption coefficient was analyzed via the Kubelka–Munk transformed Tauc plot to explore the optical bandgap value of GdPO₄. In addition, the absorption coefficient derived from UV-visible measurement demonstrated two peaks at 275 and 303 nm, corresponding to the 4f–4f transitions ⁸S_7/2 → ⁶I_J and ⁸S_7/2 → ⁶P_7/2 of Gd³⁺, respectively. Theoretically, DFT+mBJ+SOC was employed to simulate the density of states, band structures, and complex dielectric functions. We also successfully identified both 4fⁿ–4fⁿ and 4fⁿ⁻¹5d¹ transitions by means of DFT. Our main findings include an accurate prediction of the energy of the possible emitted photon (3.96 eV), which aligns with the experimental measurement of luminescence. Additionally, the predicted electronic bandgap of 7.8 eV for the monazite crystal GdPO₄ is consistent with previous experimental data. This large gap is typical for rare-earth orthophosphate systems. The results of our computational study are also validated by magnetic moment estimation. The Gd atom shows a saturation magnetic moment of 6.86µ_B, which is congruent with the experimentally measured magnetic moment µ_exp = 7.2 ± 0.3µ_B.