O. K. Karyagina

Order–Disorder Transformations in Ln2Ti2O7(Ln = Lu, Yb, Tm, Gd)

The ordering processes in the rare-earth titanates Ln2Ti2O7 with Ln = Lu, Yb, Tm, and Gd are studied by x-ray diffraction, thermal analysis, IR spectroscopy, electron microscopy, and electrical conductivity measurements. The compounds are prepared via hydroxide coprecipitation, followed by freeze-drying and heat treatment in the temperature range 350–1700°C. The compounds Ln2Ti2O7 with Ln = Lu, Yb, and Tm are found to have the fluorite structure between 600 and 800°C. Above 800°C, they undergo a fluorite-to-pyrochlore transformation, with cation disordering and the formation of LnTi + TiLn antistructure pairs. Gd2Ti2O7 has the pyrochlore structure over the entire temperature range studied and contains no antistructure defects. In contrast to Gd2Ti2O7 , the compounds Ln2Ti2O7 with Ln = Lu, Yb, and Tm undergo a high-temperature pyrochlore-to-fluorite phase transition around 1700°C. The 750°C conductivity of Ln2Ti2O7 (Ln = Lu, Yb, Tm) samples sintered at 1700°C is 5 × 10–3 to 10–2 S/cm, which is two orders of magnitude higher than that of ceramics of the same composition prepared at lower temperatures (950 or 1400°C). The conductivity of the Gd2Ti2O7 ceramic prepared at 1500°C is two orders of magnitude lower than that of Ln2Ti2O7 with Ln = Lu, Yb, and Tm.

STRUCTURE AND ELECTRICAL CONDUCTIVITY OF LN2+XHF2-XO7-X/2 (LN = SM-TB; X = 0, 0.096)

Data are presented on the evolution of the pyrochlore structure in the Ln2+xHf2−xO7−δ (Ln = Sm, Eu; x = 0.096) solid solutions and Ln2Hf2O7 (Ln = Gd, Tb) compounds prepared from mechanically activated oxide mixtures. Sm2.096Hf1.904O6.952 is shown to undergo pyrochlore-disordered pyrochlore-pyrochlore (P-P1-P) phase transformations in the temperature range 1200–1670°C. The former transformation leads to a rise in 840°C conductivity from 10−4 to 3 × 10−3 S/cm in the samples synthesized at 1600°C, and the latter leads to a drop in 840°C conductivity to 6 × 10−4 S/cm in the samples synthesized at 1670°C. The reduction in the conductivity of Sm2.096Hf1.904O6.952 is accompanied by the disappearance of the assumed superstructure. In the range 1300–1670°C, Eu2+xHf2−xO7−δ (x = 0.096) and Ln2Hf2O7 (Ln = Gd, Tb) have a disordered pyrochlore structure. The highest 840°C conductivity is offered by Eu2.096Hf1.904O6.952, Gd2Hf2O7, and Tb2Hf2O7 synthesized at 1670°C: 7.5 × 10−3, 5 × 10−3, and 2.5 × 10−2 S/cm, respectively.

Effect of heterovalent substitution on the electrical conductivity of (Yb1-xMx)2Ti2O7 (M = Ca, Ba; x = 0, 0.05, 0.1)

Abstract(Yb1−xCax)2Ti2O7 and (Yb1−xBax)2Ti2O7 (x = 0, 0.05, 0.1) have been synthesized using hydroxide coprecipitation and mechanical activation of oxide mixtures, and their electrical conductivity has been measured from 350 to 1000°C. The pyrochlore titanate (Yb0.9Ca0.1)2Ti2O7 synthesized at 1400°C from a mechanically activated oxide mixture has the highest conductivity, ∼0.1 S/cm at 1000°C, among the oxygen-ion-conducting pyrochlores studied so far. The (Yb0.95Ca0.05)2Ti2O7 and (Yb0.9Ca0.1)2Ti2O7 samples prepared by reacting coprecipitated powder mixtures at 1400°C have a lower conductivity, as do the (Yb1−xBax)2Ti2O7 (x=0.05, 0.1) samples prepared using mechanical activation.

Synthesis and high-temperature electrical conductivity of Ln2Ti2O7 and LnYTi2O7 (Ln = Dy, Ho)

Ho2Ti2O7 and LnYTi2O7 (Ln = Dy, Ho) pyrochlores have been synthesized using hydroxide coprecipitation, mechanical activation, and firing at 1600°C. The bulk and grain-boundary components of their conductivity have been determined for the first time by impedance spectroscopy. The 740°C bulk conductivity of Ho2Ti2O7 is 4 × 10−4 S/cm, and that of HoYTi2O7 is 1 × 10−3 S/cm, with activation energies Ea = 1.01 and 1.17 eV, respectively, suggesting that these materials are new oxygen-ion conductors. The bulk conductivity of DyYTi2O7 (3 × 10−4 S/cm at 740°C, Ea = 1.09 eV) is almost one order of magnitude lower than that of HoYTi2O7 (1 × 10−3 S/cm at 740°C, Ea = 1.17 eV).

Oxygen Interstitial and Vacancy Conduction in Symmetric Ln 2 ± x Zr 2 ± x O 7 ± x/2 (Ln = Nd, Sm) Solid Solutions

We have compared (Ln2 − xZrx)Zr2O7 + x/2 (Ln = Nd, Sm) pyrochlore-like solid solutions with interstitial oxide ion conduction and Ln2(Zr2 − xLnx)O7 − δ (Ln = Nd, Sm) pyrochlore-like solid solutions with vacancy-mediated oxide ion conduction in the symmetric systems Nd2O3-ZrO2 (NdZrO) and Sm2O3-ZrO2 (SmZrO). We have studied their structure, microstructure, and transport properties and determined the excess oxygen content of the (Sm2 − xZrx)Zr2O7 + x/2 (x = 0.2) material using thermal analysis and mass spectrometry in a reducing atmosphere (H2/Ar-He). The Ln2 ± xZr2 ± xO7 ± x/2 (Ln = Nd, Sm) solid solutions have almost identical maximum oxygen vacancy and interstitial conductivities: (3–4) × 10−3 S/cm at 750°C. The lower oxygen vacancy conductivity of the Ln2(Zr2 − xLnx)O7 − δ (Ln = Nd, Sm; 0 < x ≤ 0.3) solid solutions is due to the sharp decrease in it as a result of defect association processes, whereas the interstitial oxide ion conductivity of the (Ln2 − xZrx)Zr2O7 + x/2 (Ln = Nd, Sm; 0.2 ≤ x < 0.48) pyrochlore-like solid solutions is essentially constant in a broad range of Ln2O3 concentrations.

Oxygen Ion Conductivity of (Yb 0.9 - x Tb x Ca 0.1 ) 2 Ti 2 O 7 - δ Solid Solutions

We have studied terbium substitution for ytterbium in (Yb0.9 − xTbxCa0.1)2Ti2O7 − δ (x = 0.1, 0.2, 0.3, 0.4) pyrochlore solid solutions synthesized through coprecipitation followed by firing at 1550°C. The results indicate that only a small amount of terbium (less than 10%) can be incorporated into the pyrochlore structure of (Yb0.9Ca0.1)2Ti2O6.9 because of the large difference in ionic radius between the terbium and ytterbium cations: Δr = r(TbCN 83+) − r(YbCN 83+) = 0.055 Å. The oxygen ion conductivity of the (Yb0.9 − xTbxCa0.1)2Ti2O7 − δ solid solutions has been determined by impedance spectroscopy in air in the temperature range 300 to 900°C. At high temperatures (t > 640°C), their bulk conductivity was essentially independent of the Yb/Tb ratio. The observed decrease in density and microstructural changes were insignificant. At relatively low temperatures (t < 640°C), the bulk conductivity decreased slightly, and the decrease depended little on terbium concentration.

Acceptor doping of Ln{sub 2}Ti{sub 2}O{sub 7} (Ln = Dy, Ho, Yb) pyrochlores with divalent cations (Mg, Ca, Sr, Zn)

New LANTIOX high-temperature conductors with the pyrochlore structure, (Ln1� xAx)2Ti2O7� d (Ln = Dy, Ho, Yb; A = Ca, Mg, Zn; x = 0, 0.01, 0.02, 0.04, 0.07, 0.1), have been prepared at 1400-1600 8C using mechanical activation, co-precipitation and solid-state reactions. Acceptor doping in the lanthanide sublattice of Ln2Ti2O7 (Ln = Dy, Ho, Yb) with Ca 2+ ,M g 2+ and Zn 2+ increases the conductivity of the titanates except in the (Ho1� xCax)2Ti2O7� d system, where the conductivity decreases slightly at low doping levels, x = 0.01-0.02. The highest conductivity in the (Ln1� xAx)2Ti2O7� d (Ln = Dy, Ho, Yb; A = Ca, Mg, Zn) systems is offered by the (Ln0.9A0.1)2Ti2O7� d and attains maximum value for (Yb0.9Ca0.1)2Ti2O6.9 and (Yb0.9Mg0.1)2Ti2O6.9 solid solutions:� 2 � 10 � 2 and 9 � 10 � 3 Sc m � 1 at 750 8C, respectively. Ca and Mg are best dopants for Ln2Ti2O7 (Ln = Dy, Ho, Yb) pyrochlores. Using impedance spectroscopy data, we have determined the activation energies for bulk and grain-boundary conduction in most of the (Ln1� xAx)2Ti2O7� d (Ln = Dy, Ho; A = Ca, Mg, Zn) materials. The values obtained, 0.7-1.05 and 1-1.4 eV, respectively, are typical of oxygen ion conductors. We have also evaluated defect formation energies in the systems studied.