Phase transition of Ba2In2O5 in humid air studied by in-situ X-ray powder diffraction

The oxygen and proton conductor Ba(2)In(2)O(5) is known to undergo a phase transformation in humid atmosphere at about 300 degrees C. The resulting phase, Ba(2)In(2)O(5). H(2)O (beta-phase), was investigated by insitu high-temperature X-ray diffraction. During the transition, a transient structure existing near 320 degrees C in a narrow temperature range is formed. The beta-phase is stable from approximate to 300 degrees C down to room temperature and may be characterized by S.G. P4/mmm, No.123, tetragonal, with a = 4.1827(10) Angstrom and c = 8.9617(10) Angstrom. The atomic rearrangement during the transformation together with the change of the coordination of one half of the In atoms from tetrahedral to octahedral symmetry reduces the unit cell volume of the beta-phase to one quarter of the initial value. Linear thermal expansion coefficients of both phases show a strong anisotropy.