## 3 Results and Discussion  
Study of the La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) and La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) Structure by the X-ray Diffraction Method  
The pyrochlore structure of La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) has two distinct cation sites--those of La, in the center of a distorted LaO\({}_{8}\) cube (scalenohedron), and Hf, in the center of a distorted HfO\({}_{6}\) octahedron (trigonal antiprism)--and three distinct anion sites: 48f (O1), 8a (O2), and 8b (O3). Each O1 oxygen (48f) is surrounded by two La\({}^{3+}\) and two Hf\({}^{4+}\) cations, each O2 oxygen (8a) is coordinated by four La\({}^{3+}\) cations, and each O3 oxygen, which is alwaysempty in ordered stoichiometric pyrochlores, is coordinated by four Hf\({}^{4+}\) cations. La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) is the most ordered pyrochlore phase in the \(Ln_{2}\)Hf\({}_{2}\)O\({}_{7}\) (\(Ln\) = La-Tb) series, without cation disordering. In the case of La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\), the Hf-to-La substitution leads to a decrease in the intensity of the 111, 311, 331, and 531 pyrochlore superstructure reflections compared with La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) (Figure 1a, scans 1 and 2, respectively). In addition, the cubic pyrochlore cell parameter increases from 10.7731(2) A for nominally stoichiometric La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) to 10.7833 (1) A for La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) because the ionic radius of lanthanum in the center position of the distorted octahedra exceeds that of hafnium (R La\({}^{3+}\)CN\({}_{6}\)= 1.032A; R Hf\({}^{4+}\)CN\({}_{6}\)= 0.71A). A detailed analysis of the phase composition and crystal structure of the obtained materials was carried out by refining diffraction patterns by the Rietveld method (by means of the TOPAS software). The results of such a refinement are summarized in Table 1 and in Figure 1b,c. According to the XRD data, the La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) sample is a single-phase pyrochlore with no detectable secondary phase. This result agrees well with [PERSON]'s data [35], according to which the homogeneity region of this solid solution extends to \(x\) = 0.16 at a temperature of 1600 \({}^{\circ}\)C.  
Similar results were previously obtained for Nd\({}_{2}\)(Hf\({}_{2-x}\)Nd\({}_{x}\))O\({}_{7-x/2}\) (\(x\) = 0, 0.1) synthesized at 1600 \({}^{\circ}\)C [22]. The incorporation of 5% La into the Hf sublattice leads to the  
Figure 1: **(a)** X-ray diffraction patterns of (1) La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) and (2) La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\); **(b)** Rietveld refinement of La\({}_{2}\)Hf\({}_{2}\)O\({}_{7}\) X-ray diffraction pattern: the experimental data (green dots), the calculated data (red line), and the difference between the experimental and calculated data (gray line); **(c)** Rietveld refinement of La\({}_{2}\)(Hf\({}_{1.9}\)La\({}_{0.1}\))O\({}_{6.95}\) X-ray diffraction pattern: the experimental data (blue line), the calculated data (red line), and the difference between the experimental and calculated data (gray line). formation of oxygen vacancies and a defect pyrochlore structure according to the following scheme:  
\[\text{La}_{2}\text{O}_{3}+\text{Hf}_{\text{Hf}}^{\text{x}}+1/2\text{O}_{\text{O}}^ {\text{x}}\rightarrow\text{La}^{\prime}_{\text{Hf}}+1/2\text{V}_{\text{O}}^{ \bullet\bullet}+1/2\text{La}_{2}\text{Hf}_{2}\text{O}_{7} \tag{2}\]  
According to [PERSON] et al. [2], only one pyrochlore phase exists in the Tm\({}_{2}\)(Ti\({}_{2-x}\)Tm\({}_{3}\))O\({}_{7-x/2}\) (\(x\) = 0-0.67) system at a low substitution degree, \(x\) = 0-0.1, unlike at higher degrees of substitution (\(x\) > 0.1) [2].