Katsuyoshi Kakinuma

Electrical conductivity anomaly around fluorite–pyrochlore phase boundary

Abstract The relationship between electrical conductivity and crystal structure was investigated for Ln2Zr2O7 (Ln=La, Nd, Sm, Eu, Gd, Y, or Yb) and (Ln1−xLnx′)2Zr2O7 (Ln=Gd, Sm, or Nd; Ln′=Y, Yb, or Gd) systems. The crystal structure of both systems changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios, r(Ln3+)/r(Zr4+) or r(Lnav.3+)/r(Zr4+), were larger than 1.26, where r(Lnav.3+) is estimated from the ionic radius of the component ions and the composition using the following equation: r(Lnav.3+)=(1−x)r(Ln3+)+xr(Ln′3+). The lattice parameter increased linearly with increasing ionic radius ratios. The electrical conductivity at 800 °C in air for Ln2Zr2O7 systems showed the sharp maximum at the vicinity of the phase boundary between fluorite- and pyrochlore-type phases. The electrical conductivity of (Ln1−xLnx′)2Zr2O7 system also showed the maximum at the phase boundary for some combinations of Ln3+ and Ln′3+. The pyrochlore-type Eu2Zr2O7, which is located at the nearest position to the phase boundary, showed the highest conductivity of 8.3×10−3 S cm−1 at 800 °C. On the other hand, the activation energy for the conduction remarkably decreased with the increasing ionic radius ratios in the fluorite-type phase range and showed the minimum at the given compositions, at which the maximum electrical conductivities were observed and then increased.

Oxide-ion conductivity of (Ba1−xLax)2In2O5+x system based on brownmillerite structure

Abstract (Ba 1− x La x ) 2 In 2 O 5+ x , whose end member is Ba 2 In 2 O 5 , is an oxygen-deficient perovskite oxide showing high oxide-ion conductivity. In order to clarify the reason why the high oxide ion conductivity appeared in this system, the electrical conductivity was measured as a function of temperature and La content. With an increasing La content, the discontinuous jump of ion conductivity in the Arrhenius plot, which is related to the disordering of the oxygen vacancies, disappeared for the sample with x ≒0.2. Above x =0.12, the ion conductivity linearly increased with La content, while the activation energy remained constant with respect to the La content. Moreover, the conductivity for x =0.6 was 0.042 (S/cm) at 1073 K, which exceeded that of 8 mol% yttria-stabilized zirconia. The higher oxide-ion conductivity of this system could be dominated by the amount of mobile oxygen ions.

Oxide-ion conductivity of the perovskite-type solid-solution system, (Ba1−x−ySrxLay)2In2O5+y

Abstract The oxide-ion conductivity and crystal structure of the perovskite-type (Ba 1− x − y Sr x La y ) 2 In 2 O 5+ y solid-solution system, which was derived from Ba 2 In 2 O 5 with a brownmillerite-type structure, were investigated as a function of temperature and oxygen partial pressure. The oxide-ion conductivity was discussed from the viewpoint of the unit cell free volume. The oxide-ion transport number approached unity with increasing La content and the ion conductivity increased with the La or Sr content. The oxide-ion conductivity (1073 K) of (Ba 0.3 Sr 0.2 La 0.5 ) 2 In 2 O 5.5 exhibited a maximum value of 0.12 (S/cm), which exceeded that of yttria-stabilized zirconia. It was found that the oxide-ion conductivity of the present system was dominated by both the unit cell free volume and the amount of mobile oxide ion.

Electrical conductivity of the systems, (Y1-xMx)3NbO7 (M = Ca, Mg) and Y3Nb1-xMxO7 (M' = Zr and Ce)

Abstract In order to clarify a dominant factor occupying oxide-ion conductivity, electrical conductivity was measured for the fluorite-type solid–solution systems, (Y1−xMx)3NbO7 (M=Ca, Mg) and Y3(Nb1−xMx′)O7 (M′=Zr and Ce) by means of the ac two-probe method. A solubility limit of dopant was around x=0.1 for each system. Activation energy for the electrical conduction was estimated to be 117 kJ/mol for Y3NbO7 and decreased to the range 94–404 kJ/mol for the solid–solution samples. The compositional change in the conductivity could be interpreted neither by oxygen vacancy concentration nor by substituted ion radius. Resultantly, it was found that the oxide-ion conduction was strongly dependent on the unit cell free volume in the present study.

New Ion Conductor of (Ba1−xLax)2In2O5+x

The structural characterization, thermogravimetric analysis and electrical properties for solid solution system, (Ba1–xLax)2In2O5+x with perovskite-type structure were investigated. X-ray diffraction showed that the orthorhombic phase was in the range of 0.0<x≤0.3, the tetragonal phase 0.3<x≤0.5, and the cubic phase 0.5<x. The sharp transition of electrical conductivity shifted to a lower temperature with increasing x and disappeared at the phase boundary between the orthorhombic and tetragonal phases. This perovskite-related oxide exhibited a pure oxide-ion conduction over the oxygen partial pressure range of 1 atm to 10−3.5 atm, and the electrical conductivity reached the value of 1.6⋅10−1 (S cm−1) at 1073 K, which was nearly equal to that of the yttria stabilized zirconia. These properties were successfully explained in terms of disordered oxygen ions.

Oxide-ion conductivity of the oxygen deficient perovskite solid-solution system, (Ba0.5-xSrxLa0.5)2(In1-yMy)2O5.5 (M=Y or Ga)

In order to elucidate the mechanism of oxide-ion conductivity for (Ba0.5-xSrxLa0.5)(In1-yMy)2O5.5 (M=Y or Ga, 0<x<0.2, 0<y<0.2) solid-solution system, the electrical conductivity was measured as a function of oxygen partial pressure and temperature, and the results were investigated in terms of a dopant content and unit cell free volume. The system was confirmed to be an oxide-ion conductor from the oxygen partial pressure dependence on electrical conductivity. The oxide-ion conductivity increased with increasing the unit cell free volume at first. However, it showed a maximum at a value of free volume, and then decreased. The decreasing conductivity vs. the volume would be related to the crystal symmetry change.

The relationship between crystal structure and electrical conductivity in the LaY1−xInxO3 (x=0.0–0.7) system

Abstract The electrical conductivity of the LaY1−xInxO3 (x=0.0–0.7) system has been studied from the viewpoint of crystal chemistry. The high temperature form of LaYO3 (x=0.0) was ascertained to be the Sm2O3-type (B-type rare earth) structure, not perovskite-type one. The X-ray diffraction (XRD) experiments revealed that the samples with x=0.05 and 0.10 were the mixed phase of Sm2O3-type and perovskite-type structure, and changed to perovskite phase in the range of x≧0.20. From oxygen partial pressure dependence of the electrical conductivity, it was found that both the Sm2O3-type and the perovskite-type single phases showed hole conduction, but the mixed phase did oxide-ion one. The electrical conductivity of the LaY1−xInxO3 (x=0.0–0.7) system increased with increasing x, and showed the maximum value in the range of x=0.05–0.10, and then decreased with increasing x. The occurrence of oxide-ion conduction was discussed from the viewpoint of lattice distortion in the mixed phase.

Co-doping effect on electrical conductivity in the fluorite-type solid-solution systems Zr0.7(Sc1-xMx)0.3O2-δ (M = Ca, Mg, Al, Gd, Yb)

Electrical conductivity of the fluorite-type solid-solution system Zr0.7(Sc1-xMx)0.3O2-δ (M=Ca, Mg, Al, Gd, Yb) was systematically investigated from the viewpoint of a stabilization effect of the cubic phase. The rhombohedral phase of Zr0.7Sc0.3O1.85 transformed to the cubic phase by the partial substitution of M for Sc. At the same time, the electrical conductivity abruptly increased. The electrical conductivity of the cubic phase decreased linearly and gradually with increase in oxygen vacancy content, but nevertheless the kind of dopants, and the extrapolated value of the conductivity to δ=0.25 agreed with that of a pyrochlore-type La2Zr2O7. This fact was understood by considering that the partial substitution of M ions in place of Sc produced a micro-cluster having a pyrochlore-type structure with cubic symmetry.

The relationship between the mean dopant-ion radii and conductivity of co-doped ZnO systems, Zn1−x−y Mx M′y O (M, M′ = Al, In, Ga, Y)

The conductivities of the Zn1−x−y Mx M′y O (M, M′ = Al, In, Ga, Y) and Zn1−x Mx O (M = Al, In, Ga) systems were measured from room temperature to 1173 K in order to elucidate a dominant parameter of the conducting mechanism. The conductivity at 873 K first increased with the dopant content. However, it showed a maximum value at a given dopant content, and then gradually decreased. For the samples with the same dopant content, their conductivity at 873 K was strongly dependent on the mean dopant-ion radii, and reached a maximum value at around 0.51 Å of the mean dopant-ion radii. The results suggested that the conductivity of the system would be influenced not only by the dopant content, but also by the mean dopant-ion radii. It was found that the co-doped ZnO system of Zn0.995Al0.003In0.003O had a conductivity higher than that of the other usual mono-doped system.

High Oxide Ion Conductivity of (Ba1-x-ySrxLay)InO2.5+y/2 Members Derived from the Ba2In2O5 System

We have discovered a high oxide ion conductor within the perovskite-type (Ba1-x-ySrxLay)InO2.5+y/2 solid-solution system. The system was derived from brownmillerite-type Ba2In2O5, which possessed a ordered oxide ion vacancies. When we doped La3+ into the Ba site, the vacancy changed to a disordered state. The oxide ion conductivity increased with the amount of doped La3+, reaching a maximum value of 0.12 (S/cm) at 800 oC in (Ba0.3Sr0.2La0.5)InO2.75, a level exceeding that of yttria-stabilized zirconia. The oxide ion conductivity of this system was strongly dependent on the unit cell free volume, which appears to be the key parameter governing oxide ion mobility.