It is seen that the edge position of all the samples was quite similar to that of a mix between Ni metal at approximately 8333 eV and the NiO standard at approximately 8345 eV. In addition, the features of all the samples were very similar in terms of shape and position to those of a mix between Ni metal and the NiO standard. For more details about the edge position and features of all the samples, the first derivative absorption was considered, as shown in Figs. 11(a) and 11(b), respectively. From the first derivative absorption result, it can be seen that the features of all the samples were quite similar to those of the NiO standard mixed with some Ni metal. In Fig. 11(a) at 630 °C and in Fig. 11(b) at x = 0.02, only the feature of the NiO standard is shown. The results indicate that most of the Ni ions in our samples are in the Ni2+ valence state mixed with some Ni metal.