Dislocations are commonly present and important in metals but their effects have not been fully recognized in oxide ceramics. The large strain energy raised by the rigid ionic/covalent bonding in oxide ceramics leads to dislocations with low density (∼106 mm−2), thermodynamic instability and spatial inhomogeneity. In this paper, we report ultrahigh density (∼109 mm−2) of edge dislocations that are uniformly distributed in oxide ceramics with large compositional complexity. We demonstrate the dislocations are progressively and thermodynamically stabilized with increasing complexity of the composition, in which the entropy gain can compensate the strain energy of dislocations. We also find cracks are deflected and bridged with ∼70% enhancement of fracture toughness in the pyrochlore ceramics with multiple valence cations, due to the interaction with enlarged strain field around the immobile dislocations. This research provides a controllable approach to establish ultra-dense dislocations in oxide ceramics, which may open up another dimension to tune their properties.

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Yttria-stabilized zirconia (YSZ) has been used as a thermal barrier coating (TBC) material in gas turbines for several decades. Although continuous efforts have been made to develop novel TBC materials that can work at a higher temperature, no single material other than YSZ has all the desired attributes for the TBCs. In this paper, we report the in-situ synthesis of quasi-binary GdNbO4/Gd3NbO7 composites based on the simple Gd2O3-Nb2O5 binary phase diagram. The fracture toughness of these quasi-binary composites is remarkably enhanced compared with the value predicted by the rule of mixtures because the ferroelastic domain switching is more activated due to the residual stress in the quasi-binary composites, which triggers more crack defections due to the enlarged process zone. Additionally, the Gd3NbO7 phase provides a low thermal conductivity due to the substantial chemical inhomogeneity, which diffuses phonons. Gd3NbO7/GdNbO4 exhibits a balanced thermal conductivity of 1.6 W/(m·K) at 1073 K and a toughness value of 2.76 MPa·m0.5, and these values are among the best comprehensive properties that have been obtained for new TBC materials. The work demonstrates a feasible approach of designing a new TBC material with balanced properties and can be easily fabricated.