Solid solutions and crystal fields in (La_1-xY_x)OBr:Eu³⁺

Abstract

The formation of solid solutions in the (La_1-xY_x)OBr (0 ⩽ × ⩾ 1) series has been studied by X-ray powder diffraction (XRD) and by ultraviolet excited luminescence spectroscopy. The ⁵D₀→⁷F_0–4 and ⁵D₁→⁷F₃ emission spectra of the Eu³⁺ ion are interpreted according to the C_4v point symmetry of the RE₃₊ site (RE ≡ rare earth) in REOBr. The ⁷F_0–4 level schemes are consequently simulated by a phenomen-ological crystal-field (CF) theory to yield a quantitative measure of the formation of the solid solutions.

The XRD analyses revealed the separation of different REOBr phases, but neither the coexistence of phases in the middle of the (La_1-xY_x)OBr series nor the composition of these phases could be disclosed. The separation of phases in both ends of the series, the existence of both the lanthanum-rich and yttrium-rich phases as well as the composition of the solid solutions could be determined by the photoluminescence spectroscopy. The CF analysis gave in each case satisfactory results with r.m.s. deviations between 3 and 7cm⁻¹. The evolution of the CF parameters through the (La_1-xY_x)OBr series corresponded to the formation of the solid solutions and to their composition. The theoretical calculation of the CF parameters by an electrostatic point-charge model yielded inaccurate results.