A new polymorph of strontium hexaferrite stabilized at the nanoscale

During wet-chemical synthesis, metastable polymorphs frequently nucleate before the stable phase due to a lower nucleation barrier. These metastable polymorphs stabilized at the nanoscale represent some of the technologically most important nanomaterials, e.g. , photocatalytic anatase and magnetic maghemite nanoparticles. However, such nanomaterials are not restricted to simple oxides. Here, we present a new polymorph of hexagonal strontium ferrite, i.e. , hexaferrite, stabilized in the form of small discoid nanoplatelets. Under hydrothermal conditions the strontium ferrite forms as a result of reactions between Sr and Fe hydroxides in the presence of a high concentration of hydroxyl ions at temperatures below 80 °C. Atomic-resolution scanning-transmission electron microscopy showed that the primary discoid nanoplatelets (2-5 nm thick and a few tens of nm wide) exhibit an incredibly complex crystalline structure with a very large hexagonal unit cell ( a = 56.6 Å, c = 18.0 Å) and are weakly ferrimagnetic. With exaggerated growth above 160 °C, the discoid nanoplatelets having the new structure recrystallize to larger hexagonal nanoplatelets with an equilibrium magnetoplumbite structure. The discovery of a new hexaferrite polymorph demonstrates the immense potential of the stabilization of new metastable polymorphs of complex functional materials for the discovery of new nanomaterials. During hydrothermal synthesis the magnetoplumbite strontium-ferrite nanoplatelets form via the growth of primary discoid nanoplatelets with a new, incredibly complex hexagonal structure.