Masatake Takahashi

Electrical Conduction Mechanism in Bi4Ti3O12 Single Crystal

To reveal the influence of lattice defects on electrical conduction of bismuth titanate (Bi4Ti3O12: BIT), one of the bismuth layer structured ferroelectrics, changes in lattice parameters and weight by high-temperature annealing and the dependence of conductivity on oxygen partial pressure (Po2) were examined for BIT powders and single crystals, respectively. Decrease in lattice parameters and weight loss by annealing suggested the bismuth vaporization and oxygen vacancy formation. The BIT single crystal showed ionic-p-type electronic mixed conduction along the a-axis and p-type conduction along the c-axis, with the conductivity along the c-axis being lower than that along the a-axis, in the Po2 region of 1 to 3×10-4 atm at 700°C. This indicates that oxygen vacancies are formed in the perovskite blocks leading to ionic-predominant conduction along the a-axis and low oxygen concentration in the bismuth oxide layer induces low and p-type electronic conductivity along the c-axis.

Oxygen Stability and Leakage Current Mechanism in Ferroelectric La-Substituted Bi4Ti3O12 Single Crystals

We have investigated the effects of La substitution on the oxygen stability, defects, leakage current and polarization properties in ferroelectric bismuth titanate (Bi4Ti3O12) by measuring the properties of single crystals and by ab initio electronic structure calculations. It is shown that electron holes arising from the incorporation of oxygen at oxygen vacancies act as detrimental carriers for leakage current at room temperature for the crystals of BiT and La-substituted BiT. La substitution is effective for reducing oxygen vacancies and thus electron holes, which is attributed to an excellent insulating property observed for La-substituted crystals. High-pressure oxygen annealing is demonstrated to improve the remanent polarization of La-substituted crystals.

Effects of V-Doping on Mixed Conduction Properties of Bismuth Titanate Single Crystals

Electrical conduction behaviors of bismuth titanate (BiT) and vanadium-doped bismuth titanate (V-BiT) single crystals were investigated by the complex impedance method at high temperatures. Oxygen partial pressure dependence of the electrical conductivity revealed that both BiT and V-BiT showed oxide ionic and p-type mixed conduction properties. In both BiT and V-BiT, ionic conductivity was larger than hole conductivity along the a(b)-axis, while hole conductivity was much larger than ionic conductivity along the c-axis. Vanadium doping largely decreased the ionic and hole conductivities along the a(b)-axis. In contrast, the ionic and hole conductivities of V-BiT along the c-axis were almost the same as those of BiT. It was suggested that bismuth and oxygen vacancies preferentially exist in the pseudo-perovskite blocks in BiT, and vanadium substitution effectively decreases the concentration of oxygen vacancies and holes in the pseudo-perovskite blocks.

Effect of Thermal Strain on Domain Fraction in a-/b-axis-oriented Epitaxial Bi 4 Ti 3 O 12 Films

a-/b-axis-oriented epitaxial Bi 4 Ti 3 O 12 and neodymium-substituted Bi 4 Ti 3 O 12 films with a different a-domain fraction, V (100) /[V (100) +V (010) ], were grown by metalorganic chemical vapor deposition above the phase transition temperature. It was demonstrated that the saturation polarization observed for the a-/b-axis-oriented film is proportional to the a-domain fraction estimated by x-ray diffraction. The liner relationship passing through the origin revealed that the 90° domain switching by an external electric field hardly occurred. The extrapolation gave spontaneous polarization of 58 μC/cm 2 for a pure a-axis-oriented (Bi 3.5 Nd 0.5 )Ti 3 O 12 film. The domain fraction was investigated as a function of thermal strain originated from a difference in thermal expansion coefficient between the film and substrates. The domain fraction of the films changed with the thermal strain along the in-plane [010] in tetragonal a-axis-oriented films as well as epitaxially grown tetragonal Pb(Zr,Ti)O 3 films.