Crystal fields in ROF:Tb (R = La, Gd)

Laser excited luminescence and site selective excitation spectra of trivalent terbium (Tb³⁺) in two rare earth oxyfluoride (ROF) matrices, LaOF and GdOF, were detected at temperatures down to liquid helium. From the analyses of spectra, the crystal field (c.f.) fine structure of the ⁷F_J (J=0–6) ground multiplet was obtained for the 4f⁸ electron configuration. The energy level schemes comprising 26 c.f. sublevels out of 33 allowed for the C_3v point symmetry of the R³⁺ site were simulated according to a phenomenological model taking into account only the c.f. effect.

The simulation including six c.f. parameters (B₀², B₀⁴, B₃⁴, B₀⁶, B₃⁶, and B₆⁶) reproduced the experimental energy level schemes in a satisfactory manner with rms deviations of 5 and 6 cm^?1 for the LaOF and GdOF hosts, respectively. The c.f. parameter sets are characterized by B₀² values close to zero while the fourth and sixth rank parameters assumed rather high values. The O_h to C_3v descending symmetry method revealed a slight trigonal distortion from the cubic fluorite CaF₂-type structure. The c.f. parameters vary only slightly as a function of the host matrix. The ⁷F₆ level was not simulated correctly probably because of the electron–phonon interaction.