## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Composition** & **Analysing Area** & \begin{tabular}{c} **Nominal** \\ **Stoichiometry La/Hf** \\ \end{tabular} & **La/Hf Mean** & \begin{tabular}{c} **Standard Error of the** \\ **Mean** \\ \end{tabular} \\ \cline{2-5} La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) & \
\begin{tabular}{c} Outer surface \\ \end{tabular} & \(1.11\) & \(1.34\) & \(0.04\) \\ \cline{2-5}  & Fracture surface & \(1.24\) & \(0.05\) \\ \hline \multirow{3}{*}{La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\)} &
\begin{tabular}{c} Outer surface \\ Fracture surface \\ \end{tabular} & \(1.00\) & \(1.57\) & \(0.13\) \\ \cline{2-5}  & Fracture surface & \(1.05\) & \(0.04\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Average La/Hf ratio on the outer and fracture surfaces of ceramics: SEM/EDX ten point analysis of La\({}_{2}\)(Hf\({}_{2-x}\)La\({}_{x}\))O\({}_{7-x/2}\) (\(x\) = 0, 0.1). there is a gradient in the La/Hf ratio. This inhomogeneity is apparently associated with significant inertia of the system, as a result of which the diffusion of atoms proceeds slowly. Oxygen Ion and Proton Conductivity of La\({}_{2}\)(Hf\({}_{2-x}\)La\({}_{x}\))O\({}_{7-x/2}\) (\(x\) = 0; 0.1) in Dry and Wet Air  
Electrochemical impedance spectroscopy data for the La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) ceramic demonstrate that its impedance response corresponds to dielectric behaviour. No significant conductivity was detected in the range 300-900 \({}^{\circ}\)C with our measurement equipment. Therefore, La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) is a dielectric, unlike Nd\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\), studied previously [22], whose conductivity in wet air is \(\sim\)1.25 \(\times\) 10\({}^{-6}\) S/cm at 700 \({}^{\circ}\)C, approaching the proton conductivity of La\({}_{2}\)Zr\({}_{2}\)O\({}_{7}\)[38]. The negligible conductivity of La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) is attributable to the high covalence of the La-Hf bonds and the perfect structure of this pyrochlore phase. Figure 3 shows Arrhenius plots for a number of \(Ln_{2}\)(Hf\({}_{1.9}\)\(Ln_{0.1}\))O\({}_{6.95}\) ((1) \(Ln\) = La; (2) Nd [22]; (3) Sm [16]; (4) Eu) [16] \"stuffed\" pyrochlores in dry and wet air. La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) has considerable proton conductivity in the range 400-750 \({}^{\circ}\)C. The impedance spectra of La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) at 530 and 700 \({}^{\circ}\)C in dry and wet air are presented in Figure S1. Apparent activation energy (E\({}_{\text{a}}\)) of total conductivity temperature dependences of \(Ln_{2}\)(Hf\({}_{1.9}\)\(Ln_{0.1}\))O\({}_{6.95}\) (Ln = La, Nd, Sm, Eu) \"stuffed\" hafnate pyrochlores in dry and wet air are presented in Table 3. 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 (Figure 3), which differs very little from the conductivity of Nd\({}_{2}\)(Hf\({}_{1.9}\)Nd\({}_{0.1}\))O\({}_{6.95}\) under the same conditions [22]. The conductivity of \(Ln_{2}\)(Hf\({}_{1.9}\)\(Ln_{0.1}\))O\({}_{6.95}\) (\(Ln\) = Sm, Eu) is not higher in wet air [16], which indicates that Sm- and Eu-based \"stuffed\" hafnate pyrochlores are purely oxygen ion conductors (Figure 3). The \(Ln_{2}\)(Hf\({}_{1.9}\)\(Ln_{0.1}\))O\({}_{6.95}\) (\(Ln\) = La, Nd) \"stuffed\" pyrochlores, in which the major defects are oxygen vacancies in position 48f, have proton conductivity due to interaction of the oxygen vacancies in position 48f with water according to the Equation (3):  
\[\text{H}_{2}\text{O}+\text{V}_{\text{O}}^{\bullet\bullet}+\text{O}_{\text{O}}^ {\text{x}}\to 2\text{OH}_{\text{O}}^{\bullet} \tag{3}\]  
Figure 2: Microstructure of (**a**,**c**) a fracture surface and (**b**,**d**) the outer surface of the (**a**,**b**) La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) and (**c**,**d**) La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) ceramics.