In fact, rs-Ni 1-x Zn x O solid solutions show distinct structural features. As previously found by EXAFS spectroscopy, the Ni-O and Zn-O interatomic distances do not depend on the composition and are equal to r NiO = 2.08 Å and r ZnO= 2.14 Å, while the distances between the metal cations are not fixed . X-ray diffraction methods are insensitive to this type of anomaly, and the latter can only be detected by local analysis methods. Consequently, if the increase in Ni-O and Zn-O distances during heating is linear, but each according to its own dependence, the overall change will already be non-linear. Moreover, this deviation from linearity should only occur for compositions with a high zinc oxide content, and may indicate that the solid solution concerned is nonequilibrium.