Orbital Hybridization and Hypersensitivity of Eu³⁺ in YXO₄ (X=P, As, V)

A comparative study of Eu³⁺ ion luminescence in YXO₄ (X=P, As, V) with the tetragonal zircon structure is conducted in relation to the intensity of the hypersensitivity ⁵D₀ → ⁷F₂ Δ J = 2 transition. Both the asymmetry ratio, R = I 5 D 0 − 7 F 2 I 5 D 0 − 7 F 1 , and the Judd-Ofelt Ω₂ intensity parameter increases in the order YPO₄ < YAsO₄ < YVO₄. This correlation is interpreted qualitatively in terms of the covalency and polarizability of (XO₄)³⁻, which increases in the order (PO₄)³⁻ < (AsO₄)³⁻ < (VO₄)³⁻. The trend is supported by the results of electronic band structure calculations of the three compounds which establish the strength of hybridization between the X cation and the oxygen 2p states. The electronic structure of YAsO₄ is calculated to probe the covalence of As-O bonding. The increasing oscillator strength of the Eu³⁺⁵D₀ → ⁷F₂ transition in going from YPO₄ to YAsO₄ to YVO₄ is consistent with the expectation of ligand dipolar polarization model for hypersensitivity which states that the oscillator strength of the ⁵D₀ → ⁷F₂ transition is proportional to the square of the ligand dipolar polarizability. The connection between the mechanism of hypersensitivity and second harmonic generation (SHG) is presented.