X-ray absorption near edge structure study of T '-type revived superconducting compounds, Nd(1.82-z)A(z)Ce(0.18)CuO(y) (NACCO), A = Ca, Sr or Ba, z <= 0.18

Effects of the substitution of divalent A (Ca, Sr or Ba) ion in Ce excess n on-superconducting Nd1.82Ce0.18CuO4.09 compound, leading to revival of supe rconductivity, are studied by measuring absorption edges of all the cations with synchrotron radiation. It is found that in these NACCO samples, Cu ex ists mainly in the 2+ state; the weak pre-edge feature near the Cu+ positio n is marginally affected due to A(2+) ions. Except for the case of Pa dopin g, a fall in the intensity of the white line at the Cu K-edge is observed. This is attributed to a decrease in lattice oxygen content (oxygen vacancy creation) leading to an increase in electron carriers. It is noted that whe n the effective charge on Cu is less than or equal to 1.88, the T'-type sam ples are superconducting.