## 3 Results and Discussion
The present results demonstrate that La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) \"stuffed\" pyrochlore powder rehydrates rather easily during cooling and the water is then present in the form of not only physically bound water but structurally bound water and interstitial protons. Figure 4b displays TG curves of the La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) \"stuffed\" pyrochlore powder (curve 1) and pure La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) powder (curve 2). It can easily be seen that La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) loses surface water and hydroxyl ions up to 650 \({}^{\circ}\)C and above this temperature there is no weight loss, while La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) loses structurally bound water and protons in the 650-1000 \({}^{\circ}\)C temperature interval (\(\Delta\)M\({}_{1}\) = 0.08%). In this work, the sharpest weight loss (50-650 \({}^{\circ}\)C) for La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) (Figure 4b, curve 2) is associated with the release of H\({}_{2}\)O and CO\({}_{2}\). This can be partially assigned to the decomposition of La-containing carbonates and hydroxycarbonates formed due to CO\({}_{2}\) trapping from air during cooling and located at grain boundaries or in pores. The exoeffect on the DSC curve of La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) near 800 \({}^{\circ}\)C (Figure 4a, curve 1) can be caused by the structure relaxation after the removal of  
Figure 4: (a) DSC curves (1—first heating: 2—second heating) and TG curves (1a—first heating; 2a—second heating) of the powder sample La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) (left arrow—TG axis, right arrow—DSC axis); (b) TG curves: (1) first heating of La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) and (2) La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) powder sample. bulk CO\({}_{3}\)\({}^{2-}\) groups. It should be noted that after the second heating this effect disappeared (Figure 4a, curve 2). Four-Probe Proton Conductivity Measurements for the La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) Stuffed Pyrochlore in Dry and Wet Nitrogen and Air  
The four-probe conductivity measurement results for La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) are presented in Figure 5. In the measurements, we used a bar-shaped sample cut from the center of the ceramic pellet. It can be seen that the conductivity in both wet air and wet N\({}_{2}\) exceeds that in the dry atmospheres. The conductivity of La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) in wet air is \(\sim\)8 \(\times\) 10\({}^{-5}\) S/cm at 700 \({}^{\circ}\)C, in agreement with the two-probe measurement results (Figure 3). The total conductivity in wet nitrogen exceeds that in dry nitrogen, indicating that the La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) \"stuffed\" pyrochlore has proton conductivity. Clearly, the oxygen vacancies in position 48f of the La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) \"stuffed\" pyrochlore interact with water. A crucial role is played by the ability of the oxygen vacancies in position 48f, i.e., in the center of the \(Ln^{3+}\)2 Hf\({}^{4+}\)2 (\(Ln\) = La, Nd) tetrahedra in the pyrochlore structure, to be hydrated, which is due to the basicity of lanthanum and neodymium compounds. The lower basicity of the \(Ln_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{7-x/2}\) (\(Ln\) = Sm, Eu) \"stuffed\" pyrochlores makes them purely oxygen ion conductors [16]. La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) \"stuffed\" hafnate pyrochlores demonstrated proton contribution \(\sim\)1\(\times\) 10\({}^{-5}\) S/cm at 600 \({}^{\circ}\)C, whereas the conductivity of Ca-doped zirconate pyrochlores is higher by \(\sim\)1-1.5 orders of magnitude.