Local structural investigation of Eu3+-doped BaTiO3 nanocrystals

Abstract

A structural investigation of sub-15 nm xEu:BaTiO₃ nanocrystals (x = 0–5 mol%) was conducted to determine the distribution of the Eu³⁺ ion in the BaTiO₃ lattice. Pair distribution function analysis of X-ray total scattering data (PDF), steady-state photoluminescence, and X-ray absorption spectroscopy (XANES/EXAFS) were employed to interrogate the crystal structure of the nanocrystals and the local atomic environment of the Eu³⁺ ion. The solubility limit of the Eu³⁺ ion in the nanocrystalline BaTiO₃ host synthesized via the vapor diffusion sol–gel method was estimated to be ∼4 mol%. A contraction of the perovskite unit cell volume was observed upon incorporation of 1 mol% of europium, while an expansion was observed for nominal concentrations between 1 and 3 mol%. The average Eu–O distance and europium coordination number decreased from 2.46 Å and 9.9 to 2.42 Å and 8.6 for europium concentrations of 1 and 5 mol%, respectively. Structural trends were found to be consistent with the substitution of Eu³⁺ for Ba²⁺via creation of a Ti⁴⁺ vacancy at low europium concentrations (<1 mol%), and with the substitution of Eu³⁺ for both Ba²⁺ and Ti⁴⁺ at high europium concentrations (1–3 mol%). The significance of accounting for local structural distortions to rationalize the distribution of lanthanide ions in the perovskite host is highlighted.