Molecular Dynamic Simulation and Electrical Properties of Ba 2 In 2 O 5
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an oxygen-deficient perovskite
structure [1]. In the structure, oxygen vacancies are ordered in lines parallel to , resulting tetrahedral coordination for a half of (AI,Fe) ions. Because of a sixth of oxygen sites in the
corresponding perovskite structure are vacant, this material could be a candidate for fast oxygen ion conductor. Goodenough et al. [21 have indeed observed a first-order transition to a fast oxide-ion conductor at 930 C for Ba2In 2 0 5 which adapts brownmillerite structure at ambient temperature. More recently Sr2ScAlO [3] and Ba2GdIn , .1 GaxO5 [41 have been reported as fast oxide-ion conductors. Shin et al. [5] have reported that Sr 2 Fe 20s with brownmillerite structure transforms to an oxygen disordered cubic perovskite structure above 700' C. However the details of the diffusion mechanism and its relation with structural transition of the compound have not been reported to date. The purpose of present study is to obtain the insight of the mechanism of the oxygen ion diffusion in Ba2In 2O5 . We employed molecular dynamics (MD) simulation technique for this purpose. The MD simulation has been used to study the diffusion behavior of the superionic conductors [6]. However no MD study has been reported for the compounds with brownmillerite structure. In order to examine the effect of composition on the transition behavior, we extended the simulation to A2 1n 20 5 (A=Ba, Sr, Ca; B=Al, Fe, In) systems as well. These MD simulation results were compared with the experimental results. EXPERIMENTAL 1. Molecular dynamic simulation A MD simulation program (MXDORTO and MXDTRICL) developed by Prof. K. Kawamura of Tokyo Institute of Technology was employed [7]. The MD simulation procedure 193 Mat. Res. Soc. Symp. Proc. Vol. 496 © 1998 Materials Research Society
was similar to our previous study of ACuO2 [8]. We used full ionic two-body potential with Born-Mayer-type short-range repulsion term shown below. (1) u.=cqq/r.+f0(b•+b)exp[(ai+ajrij)/(bi+ bj)] First term is for electrostatic potential between ions i and j separated by ri., having formal charges of qj and qj, respectively. Second term is for repulsive potential and fo is a constant Repulsion parameters (a,, bi) for ions used in this simulation were taken from Kunz and Armbruster [9]. Total number of atoms in a simulated cell was 324 (containing 9 unit cells; 3a, lb, 3c). Initial internal coordinate for each ion was taken from that of Ca2AlFeO5 [1], and was relaxed at 300 K. In order to study the structure at high temperature, the system was heated up to 5000 K with a heating rate of 0.5 K/step (total 104 steps). At temperatures near the transitions, constant temperature MD runs were also performed in order to obtain details of oxygen ion diffusion behavior. 2. Sample preparation and measurements The sample were prepared by the following procedure. The starting materials were high purity The mixed powders were pressed into cylindrical pellets at 2 BaCO 3,0InO3 and others. The specimens ton/cm2 and sintered at 1300' C to 1350' C for 10 to 24 hours in
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