Pyrochlore-type oxide solid solutions: (Bi1−xGax)2−dTi2O7−3d/2†
Polycrystalline samples of pyrochlore-type oxide solid solutions, (Bi1−xGax)2−dTi2O7−3d/2 ((x = 0.04, d = 0.19) – (x = 0.18, d = 0)), were prepared by heating binary-oxide mixtures at 1100 °C in air. Single crystals of a solid solution with x = 0.08 and d = 0.24 (Bi1.62Ga0.14Ti2O6.64) were obtained from a sample melted at 1250 °C and solidified by cooling from this temperature. Broad peaks with tails and diffuse streaks between the fundamental spots were observed in X-ray diffraction patterns for the polycrystalline solid solutions and for a single crystal of Bi1.62Ga0.14Ti2O6.64. A statistical average model in which the Ga site having four-fold coordination is placed near the Bi site having eight-fold coordination was proposed based on the single-crystal structure analysis with the non-centrosymmetric space group F4132. The relative dielectric constant and dielectric loss (tan δ) measured at 100 kHz and 20 °C for a sintered polycrystalline sample of (Bi1−xGax)2−dTi2O7−3d/2 (x = 0.06, d = 0.15) were 126 and 0.003, respectively. Relative dielectric constants of 120–118 with a temperature coefficient of −166 ppm °C−1 were obtained between 20 and 160 °C at 1 MHz. Electrical resistivities of 2.9 × 10−5 S cm−1 at 703 °C and 2.7 × 10−5 S cm−1 at 692 °C were determined using the alternate current impedance and two-terminal direct current methods, respectively, indicating that the major carriers were holes and/or electrons at high temperature. The optical band gap energy estimated from the diffuse reflectance spectrum by postulating direct allowed transition was 2.90 eV, which was close to the value of 2.96 eV calculated from the activation energy for the conduction.