Crystal fields in REOF : Eu3+(RE = La, Gd and Y)

Abstract

The luminescence spectra of the Eu³⁺-doped rare-earth-metal oxyfluorides, REOF:Eu³⁺(RE = La, Gd and Y), have been measured at room and liquid-nitrogen temperatures. Simulation of the ⁷F_J(J= 0–5) Stark levels was carried out with the aid of the phenomenological crystal field (cf) theory according to C_3v and O_h symmetries. The cf parameters reproduced the experimental energy-level schemes with rms deviations between 8 and 11 cm^–1. The slight trigonal distortion from the cubic fluorite CaF₂-type structure resulted in the B²₀ values close to zero and both the B⁴₀/B⁴₃ and B⁶₀/B⁶_q(q = 3 and 6) ratios close to the ideal cubic ones. The strength of the cf effect increases from the LaOF host to YOF. The B^k_q parameters were obtained as products of the electrostatic lattice sum parameters, A^k_q, and the radial integrals, 〈r^k〉, with modified charges for ions and correction for expansion and shielding effects. The cf parameters of ranks four and six were found to be quite close to the experimental values, but the B²₀ parameters were too large, evidently owing to short-range covalency effects.