Daniel S. Gianola

NaSICON (Na Super Ionic Conductor) ceramics with formula Na _{1+ x} Zr ₂ Si _x P _{3− x} O ₁₂ (NZSP) are leading solid electrolyte candidates for sodium‐based energy storage. However, the mechanical properties governing membrane reliability, namely elastic modulus, hardness, fracture toughness, and flexural strength, have not been systematically characterized across the full composition range, owing to confounding effects from porosity, glassy grain‐boundary phases, and ZrO ₂ transformation‐induced and grain‐boundary microcracking in conventionally sintered materials. Here, we report the first comprehensive mechanical characterization of high‐purity (total secondary‐phase content < 10%), fully dense NZSP across x = 2.0–2.8, prepared by rapid induction hot‐pressing of powders prepared by solution‐assisted solid‐state reaction. Hot‐pressed samples achieve relative densities exceeding 99%, nanosized grains (150–200 nm), nanosized ZrO ₂ particles, and glass‐free grain boundaries across most compositions. Elastic modulus increases from 109 GPa ( x = 2.0) to 124 GPa ( x = 2.8), attributed to progressive replacement of compliant phosphate units by stiffer silicate groups and the corresponding increase in the stiffer rhombohedral phase. Hardness follows the same trend, rising from 7.20 to 9.61 GPa. Fracture toughness falls between 1.0 and 1.5 MPa·m ^1/2 . Biaxial flexure strength reaches a maximum of 195 MPa at x = 2.4. The critical grain size for thermal expansion anisotropy‐driven cracking falls from 9 µm ( x = 2) to 0.5 µm ( x = 3), for both phases above the hot‐pressed grain size at every composition.

Defects are fundamental to the behavior and performance of structural materials, yet their treatment in alloy design has often been decoupled from thermodynamic considerations of phase stability. The emerging concept of "defect phases" — chemically and structurally distinct configurations at lattice defects — offers a unified framework that integrates defect chemistry, thermodynamic stability, and mechanical behavior. While grain-boundary (two-dimensional) defect phases have gained recent attention, this article expands the scope to include defect phases across all dimensionalities, with a particular emphasis on dislocations (one-dimensional) as mobile carriers of plastic deformation and sites of complex phase behavior. We discuss how point, line, and planar defects can host distinct defect phases, how these phases compete for solute atoms, and how their stability can be mapped using defect phase diagrams constructed in chemical potential space. Through selected case studies in metallic solid solutions and ordered intermetallics, including Laves, B2, and µ-phases, we illustrate how dislocation-based defect phases can influence plasticity, strengthen alloys, or even drive local transformations that modify mechanical properties. By bridging defect physics with materials thermodynamics, we advocate for a defect phase-informed design paradigm that connects atomic-scale phenomena to bulk processing and performance. Graphical abstract