I.V. Kolbanev

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.

Mechanism of structure formation in samarium and holmium titanates prepared from mechanically activated oxides

We have studied the formation mechanism and phase transitions of samarium and holmium titanates prepared from mechanically activated oxide mixtures with the overall compositions Sm2(Ho2)Ti2O7 and Sm2TiO5. Mechanical activation of oxide mixtures leads to the formation of amorphous solid phases which crystallize in a distorted pyrochlore-like structure and contain OH groups on the oxygen site and structural vacancies up to 1000°C. In the range 800–1000°C, Sm2−xTi1−yO5−δ(OH)n (x < 0.02; y < 0.08; δ, n < 0.19) converts to a distorted orthorhombic phase as a result of the relaxation of internal stress and removal of OH groups. Above 1000°C, the phases studied have the compositions Sm2(Ho2)Ti2O7 and Sm2TiO5 and ordered pyrochlore-like and orthorhombic structures, respectively. The lattice parameters of the titanates have been measured in the range 800–1350°C. The internal stress produced by mechanical activation in the phases studied here fully relaxes by ∼1300°C.

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.