Substitutional Ta-doping in Y₂O₃ semiconductor by sol-gel synthesis: experimental and theoretical studies

The Pechini sol-gel method has been employed for the synthesis of pure and (¹⁸¹Hf→)¹⁸¹Ta-doped Y₂O₃ nanopowders. We performed a structural characterization from the micro to the subnanoscopic scale by means of scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, and time-differential perturbed γ – γ angular correlation (PAC) spectroscopy. The results show the formation of the cubic bixbyite structure after a thermal treatment at 1473 K. For the synthesized ¹⁸¹Ta-doped Y₂O₃, the PAC experiments demonstrate that the impurities are mainly located at both substitutional cationic sites of the bixbyite structure. The experimental investigation was complemented by performing first-principles electronic structure calculations for Ta atoms localized at the two cationic sites of the Y₂O₃ semiconductor structure, which allow the study of the structural and electronic modifications induced in the host system when the impurities are introduced. These calculations confirm that the measured electric-field gradients for the synthesized ¹⁸¹Ta-doped Y₂O₃ correspond to double-ionized impurities located at substitutional defect-free cationic sites of the bixbyite host structure and indicate the site occupancy preference for ¹⁸¹Hf(→¹⁸¹Ta) doping. The behaviour of the site preference of ¹⁸¹Ta impurities with temperature is also discussed. In addition to an extensive structural and electronic characterization of pure and Ta-doped Y₂O₃ semiconductor, our results demonstrate that the Pechini sol-gel process is an affordable and effective way to successfully synthesize these PAC substitutional doped samples. (paper)