Energetics of the oxygen vacancy order-disorder transition in Ba 2 In 2 O 5
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The heat capacity and the enthalpy associated with the reported oxygen vacancy order-disorder transition in Ba2ln2Os were measured by high temperature step scanning calorimetry. The transition temperature is 1205 ± 2 K. The transition appears first order or nearly so. The enthalpy and entropy of transition are 1.3 kJ/mol and 1.1 J/mol K, respectively. The latter is only 4.8% of the configurational entropy, arising from mixing one vacancy and five oxygens per formula unit, 22.5 J/mol K. This suggests that the transition involves only a small fraction of the oxygen vacancies and implies extensive short-range order, SRO, in the high temperature phase.
The defect perovskite oxide Ba 2 In 2 0 5 has been reported to be an oxide ion conductor at high temperatures.1 At room temperature Ba 2 In 2 0 5 is isostructural with brownmillerite.1 In the brownmillerite structure, the oxygen vacancies are ordered along alternate [110] rows on alternate (001) BO 2 planes of the cubic perovskite structure (see Fig. I). 2 There is a slight shift of the fourfold oxygen coordinated cations leading to tetrahedral coordination for In. The layer sequence along the c-axis can be described as alternate layers of octahedra and tetrahedra (OTOT'OTOT'---) where T and T' represent the two different orientations of the tetrahedra (Fig. I). 2 At 1203 K, Ba 2 In 2 0 5 is reported to undergo a first order transition and becomes an oxide-ion conductor.1 It is speculated that the oxygen vacancies disorder and become mobile, resulting in high oxide-ion conductivity. As the oxygen vacancies disorder, the structure is expected to change from the brownmillerite to defect perovskite. The transition is nonquenchable and no high temperature diffraction studies are reported. To study the nature of this transition, the specific heat and the enthalpy of transition were obtained by high temperature step scanning calorimetry. Ba 2 In 2 0 5 was synthesized by mixing appropriate amounts of BaCO 3 and In 2 O 3 . The powder was calcined at 1273 K for 24 h. After two intermediate grindings and firings, it was pressed into a pellet and sintered at 1573 K for 12 h. The pellet was slowly cooled to 1073 K over 3 h at 3 K/min and then quenched to room temperature. The surface of the pellet was darker in color, suggesting it had excess oxygen. This darker coating was scraped off. The remaining sample was pale yellow in color. X-ray diffraction of the remaining powder revealed a single phase with brownmillerite structure. Heat capacity measurements were performed in air in a commercial HT 1500 Setaram calorimeter which 1484
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J. Mater. Res., Vol. 8, No. 7, Jul 1993
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has been modified in this laboratory to operate in step scanning mode. 3 The calorimeter consists of two chambers linked by a thermopile, with upper thermocouples surrounding the sample crucible and the lower thermocouples surrounding a reference crucible. The calorimeter is suspended in an alumina tube which is heated in a graphite furnace under flowing argon
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