Masashi Yoshinaga

Observation of Two Kinds of Structural Phase Transitions in the Ba2In2 O 5 System

Structural phase transitions of Ba 2 In 2 O 5 at high temperatures were investigated by thermal gravimetric-differential thermal analysis (TG-DTA), dilatometry, and differential scanning calorimetry (DSC). It was revealed that there exist three phases in Ba 2 In 2 O 5 . At ∼910°C, a first-order structural phase transition was observed involving absorption of heat and discrete decrease of volume, which agreed well with the reported order-disorder transition of oxide-ion vacancy. At ∼1070°C, a second-order phase transition involving discrete increase of thermal expansion coefficient was observed. The crystal structures of these three phases of Ba 2 In 2 O 5 were investigated by means of high-temperature X-ray diffractometry (XRD). The XRD patterns at 900 and 1200°C could be indexed assuming brownmillerite-type orthorhombic and tetragonal crystal structures, respectively. The diffraction pattern obtained at 1000°C could not be explained as ideal cubic perovskite structure with disordered oxide-ion vacancy. High-temperature XRD also revealed that rate of the phase transition at 904°C was low, corresponding to high activation energy of the phase transition estimated by using a Kissinger plot of DTA curves.

Characterization of phase transition of Ba2-xSrxIn2O5 by thermal analysis and high temperature X-ray diffraction

The phase transitions of Ba2-xSrxIn2O5 were investigated with various thermal analyses and high-temperature X-ray diffraction. It was clarified that crystal structure of Ba2-xSrxIn2O5 with x=0.0~0.4 varies from brownmillerite through distorted perovskite to another distorted perovskite with increase of temperature. The phase transition from brownmillerite to distorted perovskite was revealed to be first order, whereas transition from distorted perovskite to another one was second order. The specimen with x≥0.5 showed only one first order phase transition from brownmillerite to distorted perovskite. The phase diagram of Ba2-xSrxIn2O5 was established and existence of tricritical point at ~1100°C with x=0.4~0.5 was suggested.

Electrical Conductivity and Crystal Structure of Ba2In2O5 at High Temperatures under Various Oxygen Partial Pressures

The dependence of the electrical conductivity, σ, of Ba 2 In 2 O 5 on oxygen partial pressures, p O 2 , was estimated. Independent log σ on log p O 2 , indicating oxide ion conduction, was observed at 850-1200°C under log(p O 2 /atm) of -0.7 to -3.0. Conduction by electrons generated from an oxide ion vacancy, the existence of which could be deduced by high-temperature X-ray diffraction (XRD), was detected in the brownmillerite phase below 900°C under log(p O 2 /atm) less than -3. The electrical conductivity under (logp O 2 /atm) < -3 decreased according to the p O 2 decrease at a temperature range of 1000-1100°C, suggesting that the vacancy decreased the oxide ion mobility in the distorted perovskite phase. At 1200°C, a constant log σ was observed in the log(p O 2 /atm) range of -0.7 to -4.0, indicating oxide ion conduction in the perovskite phase. The electrical conductivity of the specimen at more than 900°C drastically decreased with the decrease of log(p O 2 /atm) below -17, which can be attributed to the decomposition of Ba 2 In 2 O 5 revealed by high-temperature XRD under low log p O 2 .