## 3 Results and Discussion
### In-Situ XRD Studies of the Electrode Materials
Merging the \(a\) and \(b\) parameters due to a phase transition from a _Bmab_ (_N_ _69_) structure to an/\(4/mmm\) (_N_ 139) structure took place at slightly different temperatures for the different samples and gas atmospheres. This transition was reversible upon cooling, but the temperature of the transition was lower in air and, conversely, was higher in He (Table 4). It should be noted that an/\(4/mmm\) sp.gr. is a common space group for a relaxed high-temperature state of the first order R-P phases [28]. This transition can be associated with the release of the structure strain at high temperatures, which, at lower temperatures, is compensated by the canting of the oxygen octahedrons and, hence, is characterized by orthorhombic distortions. Additionally, the strain can be compensated by the incorporation of the interstitial oxygen, the amount of which decreases with increasing copper content. Hence, the temperature of the transition varies depending on the amount of interstitial oxygen. A lesser amount of interstitial oxygen should increase the temperature of the transition, and that is what was observed in the experiments: in a helium atmosphere, after the release of the interstitial oxygen (for the NCNCO00-NCNCO02 samples), the temperature of the phase transition increased compared to those for the air-treated samples (Table 4). Interestingly, there was nostrong  
Figure 2: Concentration dependencies of the lattice parameters (**a**) and the unit cell volume (**b**) of the as-synthesized NCNCO00-NCNCO04 samples. (Error bars are not visible on the graph as they are of the size of the symbols for the data points). correlation between the temperatures of the phase transition and the copper content. Kinetic factors may have influenced the temperature of the phase transition in each particular experiment. Furthermore, it is problematic to deduce the exact moment of the phase transition from the XRD data (i.e., at what moment exactly the \(a\) and \(b\) parameters merge). Hence, the temperature of the transition for any particular case may be \(\pm 30\)\({}^{\circ}\)C from the temperatures stated in Table 4.  
Figure 4: Changes in the \(a\) and \(b\) lattice parameters upon heating/cooling in the He flow (**a**); the unit cell volume and the \(c\) parameter change (**b**) for the NCNCO02 sample. Parameters of the tetragonal structure are given in an unconventional \(F4\)/\(mmm\) sp.gr. for the direct comparison with those of a \(Bmab\) sp.gr. Figure 3: Changes in the \(a\) and \(b\) lattice parameters upon heating/cooling in the air flow (**a**); the unit cell volume and the \(c\) parameter change (**b**) for the NCNCO02 sample. Parameters of the tetragonal structure are given in an unconventional \(F4\)/\(mmm\) sp.gr. for the direct comparison with those of a \(Bmab\) sp.gr. The values of the volumetric thermal expansion coefficients are presented in Table 4. There were at least two distinct linear regions on the volume cell temperature dependencies in both air and helium atmospheres. These two regions corresponded to the orthorhombic and tetragonal phases of the samples and were seen as a slight slope increase when sp.gr. changed to _I4/mmm_ and returned to the original one upon cooling after the phase transition into _Bmab_ sp.gr. Hence, the VTEC values of the orthorhombic phase were lower than those of the tetragonal phase, which can be explained by a compensating change in the \(a\) and \(b\) lattice parameters upon changing the temperature for this syngony. Copper doping lowers VTEC at y(Cu) = 0.1; however, further doping (y(Cu) = 0.2) slightly increases it. Overall, the VTEC value in the range (8-10) \(\times\) 10\({}^{-6}\) K\({}^{-1}\) are close to the TEC values of the commonly used electrolytes for the intermediate-temperature SOFCs [20; 21; 22]. The phase transition does not cause a significant volume jump, and an increase in VTEC is not as high as those registered in the dilatometric studies in [49].