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

A comparative study of Eu 3+ ion luminescence in Y X O 4 ( X =P, As, V) with the tetragonal zircon structure is conducted in relation to the intensity of the hypersensitivity 5 D 0 → 7 F 2 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 Ω 2 intensity parameter increases in the order YPO 4 < YAsO 4 < YVO 4 . This correlation is interpreted qualitatively in terms of the covalency and polarizability of ( X O 4 ) 3− , which increases in the order (PO 4 ) 3− < (AsO 4 ) 3− < (VO 4 ) 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 YAsO 4 is calculated to probe the covalence of As–O bonding. The increasing oscillator strength of the Eu 3+5 D 0 → 7 F 2 transition in going from YPO 4 to YAsO 4 to YVO 4 is consistent with the expectation of ligand dipolar polarization model for hypersensitivity which states that the oscillator strength of the 5 D 0 → 7 F 2 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.