In summary, the second phases found were LaSrGa3.00O7.00, in the undoped samples, or LaSr(Ga1−y,Niy)3.00O7.00 in the doped ones with x=0.10, independently of the sintering time; LaSr(Ga1−y′,Niy′)3.00O7.00 when x=0.20, that is only detected by BSE-EPMA and only in the samples treated for 48 h. In addition, (La1−z,Srz)4.00Ga2.00O9.00 is only detected by EPMA for the undoped sample sintered for 48 h and (La1−z′,Srz′)4.00(Ga1−u,Niu)2.00O9.00, with variable values for z′, is only detected by EPMA when x=0.10 and the sintering time was 48 h and x=0.20 for any sintering time, which means that those latter phases are only present in a small amount. Finally, the most significant difference in the composition between the two perovskites of each sample sintered for 48 h is for Ga. It can be related with a higher CORM-3 coefficient for that element. In particular, previous studies have shown cation or metal diffusion in LSGM and LaSrMn (LSM) perovskites at temperatures from 1300 °C. For instance, Dotelli et al.  studied the cation diffusion across LSGM/LSM electrode/electrolyte interfaces by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and EDS. LA-ICP-MS showed that Ga and Mg ions diffuse across the interface, and Ga diffuses even over very long distances after sintering of a two-layer system screen printed onto a LSM pellet at 1300 °C for 1 h. Furthermore, Lee et al.  demonstrated in an investigation of the interfacial stability between the anode (Ce1−xGdxO2−δ (GDC)+NiO) and electrolyte layer of LSGM-based SOFCs that the reaction zone became extended as the heating temperature increased and as the duration time increased, which strongly indicated that this type of interfacial reaction is controlled by a conventional thermally-activated diffusional process of corresponding species.