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1 2009

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Synteza, właściwości optyczne i elektryczne nanokrystalicznych materiałów BaTiO_3 domieszkowanych jonami ziem rzadkich

Pązik R., Stręk W., Nitsch K.

Wiadomości Chemiczne

2009

[Z] 63, 9-10

878-931

The main goals of the presented paper were focused on synthesis of nanocrystalline BaTiO^3 materials doped with chosen rare earth ions (Eu^3+ and Yb^3+) utilizing sol-gel technique. Structure, morphology, optical and electrical properties were characterized. Size effects and their influence on the mentioned physicochemical properties were discussed. Strong effect of the dopant concentration on the structure of BaTiO_3 was found. In case of the Eu3+ ions doping results in stabilization of tetragonal phase of BaTiO^3 whereas addition of the Yb^3+ ions favors formation of the cubic structure. Besides that all introduced ions could be considered as inhibitors for the grain growth process. Moreover resulted powders showed co-existence of the tetragonal (ferroelectric) and cubic (paraelectric) phases. The amount of the tetragonal phase depends on the heat treatment temperature and concentration of the incorporated rare earth ions. The structural changes of a matrix were studied using the Eu^3+ and Yb^3+ ions as optical probes. Their luminescence properties were dependent on the sintering temperature. In case of the Eu^3+ ions we have found that the intensities of D→7F transitions increased with the increasing temperature. It means higher Eu3+ ions symmetry associates with higher temperature [1–4]. On the other hand in nano-crystals doped with Yb3+ ions a higher intensity of SHG (second harmonic generation) was observed in the tetragonal phase, whereas the isotropic cubic phase was detected by the absence of a SHG signal. Cooperative emission, indicating the presence of Yb3+ ion pairs, was easily detected in the cubic phase and not seen in the ferroelectric tetragonal structure [5]. BaTiO_3 nanoceramics were fabricated from the nanopowders using LTHP (low-temperature high pressure) sintering process [6, 7]. The impedance spectra indicated the complex nature of the effects appearing in the analyzed BaTiO3 nanoceramics. Four types of physical mechanisms contributing to the overall conductivity were found, namely hopping, diffusion, two relaxation mechanisms and the dc conduction. The studied samples show a ferroelectric-paralelectric phase transition observed for the higher frequency regions.