Cu2+O- hole pairs and O- spin bags in La2-x Mx CuO4 oxide mixtures

La2-xCexCuO4 (0 ≤ x ≤ 0.2) shows the usual Cu2+ EPR line. However, after use as catalyst for NO reduction by CO, the x = 0.2 sample shows also an unusual EPR feature, with apparent g values ranging between 5 and 2.5 depending on temperature. Ce4+, substituting for L3+, could transform into Ce3+ by removing one electron from the Cu-O2 plane. However this, by itself, would not account for the unusual behavior of the x = 0.2 used sample, as a very small amount of Ce3+ substitutes for La3+ ions, nearly identical in all of the examined samples. We propose that undissolved CeO2 partially transforms into Ce2O3 and acts as a sink of O2-. The last ions would transform into O- by reacting with the O atoms brought about by the NO reduction. When CeO2 is enough concentrated, as for x = 0.2, O- "spin bags" would form and interact with Cu2+ ions system.