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Giant suppression of dielectric loss in BaZrO3
(2019-11-01)
Kolodiazhnyi, T.
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Pulphol, P.
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Vittayakorn, W.
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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.
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Charoonsuk, T.
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Spreitzer, M.
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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 ∘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 Ce3+ ions as revealed by the magnetic susceptibility data. Therefore, we conclude that antimony is mainly present as Sb3+ 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 Sb3+ point defects.
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Giant magnetocapacitance in cerium sesquioxide
(2018-08-22)
Kolodiazhnyi, T.
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Sakurai, H.
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Avdeev, M.
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Charoonsuk, T.
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Lamonova, K. V.
We report structural, magnetic, magnetodielectric, and specific heat properties of hexagonal polymorph of Ce2O3. The A-type hexagonal Ce2O3 crystallizes in the P32/m1 space group and shows antiferromagnetic ordering at TN≈6.2 K as detected by magnetic susceptibility measurements. The magnetic ordering is accompanied by a λ-shape specific heat anomaly with a peak maximum at T≈6.1 K. An isotropic dielectric constant demonstrates a large magnetocapacitance effect at T=TN, which saturates at ∼80 kOe. In the magnetically ordered phase the relaxation rate of the dielectric polarization increases with the magnetic field and shows divergence as the temperature approaches TN. The magnetic calculations indicate that Ce2O3 undergoes "easy plane" AFM ordering, which is also supported by the neutron diffraction data.
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Disorder-induced f -electron localization in Nb and y co-doped CeO2
(2018-04-28)
Charoonsuk, T.
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Vittayakorn, N.
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Kolodiazhnyi, T.
We report the effect of the charge compensation on the electronic transport and optical properties of CeO<inf>2</inf> co-doped with donor, Nb, and acceptor, Y, ions. As expected, the concentration of Ce3+ decreases with an increase in the Y content in Ce<inf>0.992-</inf><inf>x</inf>Nb<inf>0.008</inf>Y<inf>x</inf>O<inf>2</inf>, where 0 ≤ x ≤ 0.008. More importantly, random electric fields generated by the Y ions bring additional disorder into the system. As a result, the high-temperature activation energy of conductivity increases significantly from 189 to 430 meV. A similar energy shift in the optical absorption peak centered at 1.3-1.5 eV is attributed to an increase in the energy gap separating the localized f-electrons from the empty Ce 4f band. The results underline the paramount importance of the disorder-induced Anderson localization of the f-electrons in ceria.
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Lattice evolution and point defect chemistry in Ta-doped ceria
(2017-02-25)
Charoonsuk, T.
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Vittayakorn, N.
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Kolodiazhnyi, T.
Point defects in donor-doped ceria impact the physical, chemical and electronic properties. This work reports on the point defects in Ta-doped ceria, relating the changes in the lattice parameters with the type and concentration of point defects. The accurate Ce3+ quantification using magnetization method is a key factor in solving the defect equilibria. Combining the experimental data and lattice expansion models it is demonstrated that the extra positive charge of Ta5+ donors is compensated by both the Ce3+ and the oxygen interstitials. An accurate quantitative analysis of the lattice expansion indicates that the theoretical calculations significantly overestimate the effect of the lattice response to oxygen interstitials. Furthermore, it is demonstrated that at high temperatures the anion Frenkel defects, whose concentration increases with the Ta substitution level, bring additional lattice expansion. The calculated enthalpy of formation of the anion Frenkel defects (H<inf>AF</inf> = 3.27–4.24 eV) for Ta-doped ceria is in good agreement with the ab-initio calculations and with the experimental values for the U-doped ceria. Furthermore, the accurate data of the lattice expansion caused by the interstitial oxygen reported here can be used to better understand and to predict the ‘swelling’ of the isomorphous UO<inf>2</inf>, PuO<inf>2</inf> and NpO<inf>2</inf> nuclear waste materials.
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Spin-flop driven magneto-dielectric effect in Co4Nb 2O9
(2011-09-26)
Kolodiazhnyi, T.
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Sakurai, H.
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Vittayakorn, N.
Co<inf>4</inf>Nb<inf>2</inf>O<inf>9</inf> becomes antiferromagnetic (AFM) below 27.4 K with a spin-flop transition at a critical field, H<inf>c</inf>, of 12 kOe. Room-temperature dielectric properties are dominated by finite electronic conductivity. Below 125 K, the charge carriers are frozen-out and the dielectric constant is controlled by the lattice phonons. A large (12%) spin flop-driven enhancement in dielectric constant is found in the very narrow temperature interval (ΔT = 1.6 K) in the vicinity of the AFM phase transition. Magneto-dielectric anomaly shows low-frequency dispersion; therefore, the H-induced changes in the phonon eigenfrequencies are unlikely. Other possible reasons for unusual magneto-dielectric effect in Co <inf>4</inf>Nb<inf>2</inf>O<inf>9</inf> are discussed. © 2011 American Institute of Physics.

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