Preparation, crystal structure, and reducibility of K2NiF4 type oxides Eu2-xSrxNiO4+δ

K2NiF4 type compounds Eu2-xSrxNiO4+delta (0.5 less than or equal to x less than or equal to 1.2) have been prepared by a citric acid complex decomposition method. Rietveld refinement of the powder X-ray diffraction data showed that there was a crystal system transformation from orthorhombic bic Fmmm to tetragonal I4/mmm at around x = 0.6-0.7. The Sr substitution caused a drastic shift of the O-II ions along the c-axis from Eu(Sr) toward Ni while scarcely affecting the NiO4 network in the basal plane. The Jahn-Teller distortion of the NiO6 octahedron decreased while the percentage of Ni3+ increased with increasing x. The variation of the higher peak temperature on the TPR profiles with x are opposite to the variation of the cell parameters c. (C) 1998 Academic Press.