Discovery of a vacancy-modified T'-hexaferrite phase in hydrothermally synthesized barium ferrite nanoplatelets

Hydrothermal synthesis of Co-substituted barium hexaferrite nanoplatelets (NPLs) revealed the formation of a previously unreported hexaferrite-related phase at elevated Co concentrations. At low Co contents (BaFe_12-xCo_xO₁₉, x ≤ 0.09), Co was incorporated into M-type barium hexaferrite NPLs. Atomic-resolution STEM imaging revealed depletion of trigonal Fe(2b) lattice sites in the as-synthesized particles, resulting in strongly reduced magnetic properties. Annealing at 800 °C restored the M-type structure and recovered the magnetic performance. At higher Co concentrations, a new platelet-shaped hexaferrite phase formed together with minor amounts of cobalt ferrite. X-ray diffraction and atomic-resolution HAADF-STEM imaging showed that this phase is structurally related to Ba₂Co₉O₁₄, which consists of T'-type structural blocks, but differs markedly from the reported structure through depletion of the Co2 lattice sites located in the middle of the T' block. The new T'-hexaferrite exhibits a composition close to Ba₂Fe_9-yCo_yO₁₄ (y ≈ 2-4), displays hard-magnetic behaviour, and remains stable up to approximately 900 °C. Density functional theory calculations combined with atomistic thermodynamics identified a Co2-site-depleted structural model that provides the best agreement with both the experimental STEM observations and the thermodynamic stability conditions relevant to hydrothermal synthesis. Solid-state synthesis at 800 °C further supports the conclusion that the T'-hexaferrite is a thermodynamically stable phase rather than a transient hydrothermal product. These findings demonstrate that transition-metal substitution can stabilize previously inaccessible vacancy-modified hexaferrite structures and provide new insight into the interplay between defect chemistry, phase stability, and crystal-structure evolution in complex oxide nanoplatelets.