Four and six-coordinated Al in a fluorite-type structure: A key to the stabilization of Sc2O3-stabilized ZrO2

Al is known as a unique element to enhance the stability of Sc₂O₃-stabilized ZrO₂ (ScSZ); however, understanding the Al state in the material is insufficient for the mechanism to be understood. In this study, the states and roles of Al in the ScSZ-based materials are elucidated by ²⁷Al NMR spectroscopy, DFT calculations, and detailed structural analysis concerning cubicity. The ²⁷Al NMR and DFT calculations reveal that Al substitutes Zr sites as 6-, 7- and 8-coordinated states in ScSZ even though the ionic radius of Al is much smaller than that of Zr. The formation of 6-coordinated Al with two oxygen vacancies in its vicinity indicates oxygen vacancies are preferentially located around the smaller cations. The local structure revealed by DFT calculations suggests that the coordination polyhedron of 7- and 8-coordinated Al is effectively 4-coordinated Al. The ²⁷Al NMR results also support this unique local structure. The results of this study show that manipulating the Al state is a key step in stabilizing Sc₂O₃-stabilized ZrO₂ and help to clarify the suppression mechanism of the degradation of conductivity.