Core-level photoemission spectra of Mo0.3Cu0.7Sr2ErCu2Oy, a superconducting perovskite derivative. Unconventional structure–property relationships

Abstract

Detailed studies of the electronic states for Mo_0.3Cu_0.7Sr₂ErCu₂O_y samples with different oxygen contents are presented here. The influence of oxygenation on the electronic states for the Mo_0.3Cu_0.7Sr₂ErCu₂O_y system from the semiconducting to the superconducting state has been investigated by means of X-ray photoelectron spectroscopy (XPS). The XPS studies show that Mo is in a mixed Mo^V and Mo^VI oxidation state and Mo^V is predominant over the Mo^VI in the as-prepared (AP) sample. Yet annealing under an oxygen atmosphere enhances the Mo^VI state. At the same time, a reduction in the copper species is observed. In the Cu 2p spectra, a larger energy separation between the satellite and main peaks (E_S–E_M) and a lower intensity ratio (I_S/I_M) are found to correlate with higher values of the superconducting transition temperature (T_C). Analysis of these spectra within the Configuration Interaction (CI) model suggests that higher values of T_C are related to lower values of the O 2p–Cu 3d charge transfer energy. The change in the Sr 3d and O 1s core level spectra correlates with the oxygen insertion in the (Mo/Cu)O_1+δ chain site, after oxygenation. The hole concentration (P_h) in the copper plane has been obtained using the room temperature thermoelectric power (TEP) value; this shows an increasing tendency with increasing T_C, after oxygenation. From these experimental results, one observes that T_Cincreases with decreasing charge transfer energy. This is, indeed, opposite to the accepted views and occurs in parallel with the shortening of the apical copper–oxygen distance (Cu2–O2) and the increasing of the CuO₂ plane buckling angle.