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    Giant suppression of dielectric loss in BaZrO3
    (2019-11-01)
    Kolodiazhnyi, T.
    ;
    Pulphol, P.
    ;
    Vittayakorn, W.
    ;
    Vittayakorn, N.
    We report the effect of precursor's purity and heterovalent substitution on the microstructure and dielectric properties of BaZrO<inf>3</inf> ceramics. We find that independent of the purity or raw materials, the dielectric loss of BaZrO<inf>3</inf> at microwave frequencies is rather high with a Q factor in the range of 1000–3000 at 10 GHz. All stoichiometric ceramics studied show up to three types of low-T dielectric relaxations in the 2–250 K range. All these dielectric anomalies can be suppressed by partial substitution of Zr for Nb. By alloying of BaZrO<inf>3</inf> with a hypothetical BaGa<inf>1/2</inf>Nb<inf>1/2</inf>O<inf>3</inf> phase the Q × f value of ceramics at microwave frequency can be improved significantly. The origin of this remarkable improvement is attributed to the effect of the donor ions on the random electric fields and antiferrodistortive instability. Ceramic with a chemical composition of BaZr<inf>0.96</inf>Ga<inf>0.02</inf>Nb<inf>0.02</inf>O<inf>3</inf> sintered at 1600 °C shows dielectric constant, ε′ = 36.7, temperature coefficient of the resonance frequency, τ<inf>f</inf> = +110 ppm/°C and Q × f = 172 THz.
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    Analysis of Sb-doped ceria: Magnetism, conductivity, dielectric, specific heat and optical properties
    (2019-02-01)
    Kolodiazhnyi, T.
    ;
    Charoonsuk, T.
    ;
    Spreitzer, M.
    ;
    Vittayakorn, N.
    We report on magnetism, charge transport, dielectric properties and specific heat of Ce<inf>1−x</inf>Sb<inf>x</inf>O<inf>2</inf> ceramics sintered at 1650 <sup>∘</sup>C with a final antimony content of 0 ≤ x ≤ 0.0017. In contrast to other donor dopants, such as Nb, Ta, U and W, charge compensation of antimony in Ce<inf>1−x</inf>Sb<inf>x</inf>O<inf>2</inf> does not involve the formation of the Ce<sup>3+</sup> ions as revealed by the magnetic susceptibility data. Therefore, we conclude that antimony is mainly present as Sb<sup>3+</sup> ion and acts as an acceptor dopant in Ce<inf>1−x</inf>Sb<inf>x</inf>O<inf>2</inf>. This conclusion is also supported by a very low electrical conductivity of the Sb-doped ceria that shows an activation energy E<inf>σ</inf> ∼ 0.97 eV. This activation energy is close to that observed in oxygen conducting acceptor-doped ceria and is significantly higher than the typical E<inf>σ</inf> ∼ 0.1–0.3 eV values reported for n-type CeO<inf>2</inf>. Below 10 K, both an anomaly in the dielectric loss and a small specific heat surplus in Sb-doped ceria indicate a low-energy dipolar relaxation probably associated with a local dynamics of the off-centered Sb<sup>3+</sup> point defects.