Analysis of crystal structure and phase relationship of Ba_2-xLa_xIn₂O_5+δ by high temperature synchrotron X-ray diffraction and thermal analyses - Control of electrical conductivity and crystal structure by concentration of oxide ion vacancy

Abstract

Phase relationship of Ba_2-xLa_xIn₂O_5+δ has been investigated by using dilatometry, differential scanning calorimetry and synchrotron X-ray diffraction at high temperatures. Decrease of the temperature of the first order phase transition, from brownmillerite orthorhombic to tetragonal, was observed with an increase of La content in Ba_2-xLa_xIn₂O_5+δ for x = 0.0-0.2, whereas little variation was detected on temperature of the second order phase transition, from tetragonal to distorted cubic, in the specimens with x = 0.0-0.3. Ba_2-xLa_xIn₂O_5+δ with x larger than 0.4 showed no phase transition by temperature. Crystal structure of the specimens with 0.4 ≤ x ≤ 1.0, 1.2 ≤ x ≤ 1.7 and x = 2.0 were ideal cubic, tetragonal and orthorhombic distorted perovskite, respectively. Ba_1.3La_0.7In₂O_5+δ with ideal cubic perovskite structure showed higher oxide ion conductivity below 800°C than that of Ba₂In_2-xGa_xO₅ with distorted cubic structure, supporting that removal of the structural distortion should be effective to improve mobility of oxide ion, resulting in as high oxide ion conductivity as that of yttria stabilized zirconia.