Synthesis, Crystal Structure and Reducibility of K_2NiF_4 Type Oxides Sm_(2-x)Sr_xCuO_4

摘要

K_2NiF_4 type compounds Sm_(2-x)Sr_x CuO_4 (0 ≤ x ≤ 1.2) were prepared. Rietveld refinement of powder diffraction data shows that the crystal system of these compounds belongs to T' type tetragonal structure of space group I4/mmm , and the addition of Sr causes a dramatic shift of the O(Ⅰ) ions along the c axis from Sm (Sr) toward Cu while scarcely affecting the Cu―O (Ⅱ) bond in basal CuO_4 plane. From XRD data, it can be seen that when 0.0 ≤ x ≤ 0.6, Sm_(2-x)Sr_xCuO_4 belongs to a single-phase K_2NiF_4 type structure, while at x ≥ 0.7, both the free SrO phase and the CuO phase exist in these compounds. From the results of XPS and Rietveld refinement, it can be seen that after replacing Sm~(3+) with Sr~(2+) of lower valence, the valence of Cu and Sm does not change apparently, and thus some defect must be formed at the oxygen ion positions and/or position A on A_2BO_4 to keep charge balance and to stabilize the structure.