## 3 Results and Discussion
As shown for A\({}_{2}\)(B\({}_{2-x}\)A\({}_{x}\))O\({}_{7-x/2}\) \"stuffed\" pyrochlores [2] using the Tm\({}_{2}\)(Ti\({}_{2-x}\)Tm\({}_{x}\))O\({}_{7-x/2}\) (\(x\) = 0-0.67) system as an example, at low degrees of Ti-for-Tm substitution (at \(x\) = 0-0.1), oxygen vacancies are formed predominantly in position 48f, opposed to the change in the vacancies formation in two positions (48f and 8b) in Y\({}_{2}\)(Zr\({}_{y}\)Ti\({}_{1-y}\))O\({}_{7}\)[36] and Y\({}_{2}\)Ti\({}_{2-x}\)Hf\({}_{x}\)O\({}_{7}\)[37] as a result of isovalent substitutions in the tetravalent metal sublattice. Note that in the case of the formation of anti-site pairs in nominally stoichiometric pyrochlores at high temperatures (e.g., in the oxygen- and proton-conducting neodymium hafnate Nd\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) synthesized at 1700 \({}^{\circ}\)C), oxygen vacancies are formed in position 48f [22]. It can be assumed that there a similar situation obtains for the Ln\({}_{2}\)(Hf\({}_{2-x}\)Ln\({}_{x}\))O\({}_{7-x/2}\) (\(Ln\) = La, Nd; \(x\) = 0.1) hafnate systems and the Tm\({}_{2}\)(Ti\({}_{2-x}\)Tm\({}_{x}\))O\({}_{7-x/2}\) (\(x\) = 0-0.1) titanate system. La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) \"stuffed\" pyrochlore was shown to contain oxygen vacancies only in position 48f (Table 1). Table 2 presents the La/Hf ratio on the outer and fracture surfaces of La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) and La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\). The La/Hf ratio on the fracture surfaces is similar to the intended one, whereas the outer surface is found to be enriched with lanthanum. Note that the nominally stoichiometric La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) ceramic is visually inhomogeneous and two-colored; its microstructure is presented in Figure 2a,b. Its surface is white, which correlates with the La\({}_{2}\)O\({}_{3}\) excess detected by EDS/EDX (energy dispersive X-ray spectroscopy X-ray analysis), and there is a brown layer in its interior. The micrograph of a fracture surface of the ceramic in Figure 2a demonstrates its inhomogeneity. Nevertheless, the La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) ceramic is dense: its relative density is 7.4 g/cm\({}^{3}\), i.e., 93.3% of its theoretical density. The La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) ceramic is light brown in colour and more homogeneous than the La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) ceramic prepared under the same conditions. The La/Hf ratio is near the expected one (Table 2). The microstructure of the La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) ceramic is shown in Figure 2c,d. Note the La excess is observed on the outer surface of the La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) compared to La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) \"stuffed\" pyrochlore (Table 2). The density of the La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) ceramic is 7.27 g/cm\({}^{3}\), which is 92.5% of its theoretical density, whereas its density determined by hydrostatic weighing is higher: 94.8%. The average La/Hf ratio value on the fracture surface exceeds the nominal stoichiometry within the standard error of the mean (SEM), but it is close to it and significantly less than this ratio on the outer surface (Table 2). In addition, visual information about the inhomogeneity of the color of the samples cannot serve as a basis for considering the samples as polyphasic; however, based on the EDX analysis data, we can conclude that