Monoclinic to cubic structural transformation, local electronic structure, and luminescence properties of Eu-doped HfO2.

We report the photoluminescence (PL) properties of Eu³⁺ doped HfO₂ nanoparticles prepared using co-precipitation method and annealed at 600 °C. X-ray diffraction results revealed the monoclinic phase in undoped HfO₂ and show mixed phase formation at lower concentration and a dominant cubic phase achieved at 5 mol% doping of Eu in HfO₂. The phase transition anticipated by the density functional theory is in excellent agreement with experimental findings. The oxygen K-edge XANES spectra clearly depicts the diverse hybridization of O 2p orbitals in M–O7 (for monoclinic) and M–O8 (for cubic) polyhedra of HfO₂. Hf L-edge XANES confirms Hf⁴⁺ ions in cubic and monoclinic structured HfO₂. The Eu³⁺ ions are dominantly present in the Eu-doped HfO₂ nanoparticles. PL study demonstrates the emission in red region with high color purity under different excitation wavelengths from near UV to blue light. PL emission spectra show four emission bands at 594 nm, 609 nm, 650 nm, and 716 nm corresponding to 4f–4f transitions of Eu³⁺ under excitation wavelengths of 361 nm, 383 nm, 394 nm and 465 nm. The reddish PL emission with high color purity under different excitation wavelengths from near-UV to blue region may be exploited in solid state lighting-based applications. [ABSTRACT FROM AUTHOR]