Synthesis, Crystal Structure and Reducibility of K 2 NiF 4 Type Oxides Sm 2‐ x Sr x CuO 4

K 2 NiF 4 type compounds Sm 2‐ x Sr x CuO 4 (0 ≤ x ≤ 1.2) were prepared. Rietveld refinement of powder diffraction data snows that the crystal system of these compounds belongs to T type tetragonal structure of space group I 4/ 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 x CuO 4 belongs to a single‐phase K 2 NiF 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 2 BO 4 to keep charge balance and to stabilize the structure.