SmTaO4, Sm3TaO7 and SmTa3O9 ceramics were prepared via a high temperature solid-state reaction. The results show that the phase structures of the SmTaO4 ceramics are a monoclinic phase (space group: I2 (5)), the Sm3TaO7 ceramics are an ordered orthorhombic phase (space group: Ccmm), and the SmTa3O9 ceramics are an orthorhombic phase (space group: Cmmm). Macromolecular and crystal structures of Sm3TaO7 and SmTa3O9 are more complex than those of SmTaO4, which can enhance phonon scattering and reduce thermal conductivity. Compared with those of SmTaO4 and SmTa3O9, Sm3TaO7 has the smallest grain size, indicating that a small grain size has an effect on strength and ensures material stability. Because of a distorted TaO6 octahedron and a high concentration of cation vacancy in the lattice of SmTa3O9, its thermal conductivity increases with increasing temperature. Compared with that of 7–8 YSZ, Sm3TaO7 has a low thermal conductivity (1.3–1.7 W·m−1·K−1 at 100 °C–900 °C), and its minimum thermal conductivity is nearly 30% lower than that of YSZ. The Sm3TaO7 ceramics have the highest TECs at approximately 9.82 × 10−6 K−1 at 1200 °C, which is close to that of 7–8 YSZ (approximately 10 × 10−6). This indicates that Sm3TaO7 has the potential to be employed as a thermal barrier coating.
HfO 2 –Sm 3 TaO 7 ceramics are prepared through a solid-state reaction method. The X-ray diffraction and structural refinement show that the phase structures of HfO 2 –Sm 3 TaO 7 ceramics are an ordered orthorhombic phase and the space groups are belonging to Ccmm . The degree of the structural disorder increases with increasing HfO 2 content. The solid solution mechanism reveals that Hf 4+ exists in the form of interstitial ions that cause crystal expansion when the doping content is less than 4 mol%. When the doping concentration of HfO 2 ≥ 4 mol%, the Hf 4+ ions can substitute an equal number of Sm 3+ and Ta 5+ ions. The phase transition of Sm 3 TaO 7 ceramics is removed with increasing HfO 2 content, and the 8 mol% HfO 2 –Sm 3 TaO 7 ceramics have a high thermal expansion coefficient of 10.2 × 10 −6 K −1 at 1200 °C. The 2 mol% HfO 2 –Sm 3 TaO 7 ceramics have the lowest thermal conductivity (1.03 W/m K at 900 °C), which is lower than previous research of the 7–8 YSZ. The outstanding thermophysical properties of HfO 2 –Sm 3 TaO 7 ceramics indicate that they are potential thermal-barrier coating materials.
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