Preferential site substitution in sol-gel derived Eu3+ doped Lu2SiO5: a combined study by X-ray absorption and luminescence spectroscopies

Abstract

Eu³⁺ doped Lu₂SiO₅ (LSO) has been prepared by sol-gel chemistry. X-Ray diffraction showed that only the monoclinic phase corresponding to LSO is observed, even for doped samples. X-Ray absorption measurements were performed at both Lu and Eu L_III edges to investigate the substitution of Eu³⁺ for Lu³⁺ in LSO. Lutetium and europium oxides were used as references for XAFS signal extraction. The validity of XAFS measurements for studying the substitution of Eu³⁺ for Lu³⁺ at low doping levels (5at.%) was confirmed on a LuBO₃ sample which has a simpler crystalline structure. The XAFS spectrum recorded at the Eu L_III edge for Eu³⁺ doped LSO demonstrates the effective substitution of Eu³⁺ for Lu³⁺ and refined inter-atomic distances involving Eu³⁺ are consistent with a preferential substitution on the higher coordination site labelled A₂ [LuO₇]. Luminescence measurements performed on doped samples show the existence of an impurity phase identified as Lu₂O₃. Site selective excitation measurements can distinguish between the two crystallographic sites of (Lu,Eu) ions in LSO and the numbering of the Eu³⁺ emission lines is in good agreement with the expected C₁ symmetry. De-convolution of the ⁷F₀ → ⁵D₀ excitation spectrum recorded at room temperature allowed quantification of the populations of the two sites: 37% on A₁ [LuO₆] and 63% on A₂ [LuO₇]. This result agrees with structural considerations taking into account the difference in the ionic radii of Eu³⁺ and Lu³⁺.