Structural transformation of the oxygen and proton conductor Ba2In2O5 in humid air: An in-situ X-ray powder diffraction study

The oxygen and proton conductor Ba2In2O5 is known to undergo a structural t ransformation in humid atmosphere at about 300 degrees C. The resulting pha se Ba2In2O5 . H2O was investigated by in-situ high-temperature X-ray diffra ction. During the transformation, a transient structure existing near 320 d egrees C in a narrow temperature interval is formed. Ba2In2O5 . H2O is stab le from approximate to 300 degrees C down to room temperature and may be ch aracterized by S.G, P4/mmm, No. 123, tetragonal, with a = 4.1827(10) Angstr om, b = 4.1827(10) Angstrom and c = 8.9617(10) Angstrom. The atomic rearran gement during the transformation together with the change of the coordinati on of one half of the In atoms from tetrahedral to octahedral symmetry redu ces the unit cell volume of Ba2In2O5 . H2O to one quarter of the initial va lue. Linear thermal expansion coefficients of both phases show a strong ani sotropy. (C) 1999 Published by Elsevier Science B.V. All rights reserved.