Inside Ceramics and Between MgO Grains: Solid-State Synthesis of Intergranular Semiconducting or Magnetic Spinels

Configurations of composite metal oxide nanoparticles are typically far off their thermodynamic equilibrium state. As such they represent a versatile but so far overlooked source material for the intergranular solid-state chemistry inside ceramics. Here, it is demonstrated how the admixture of Fe³⁺ and In³⁺ ions to MgO nanoparticles, as achieved by flame spray pyrolysis, can be used to engage ion exsolution, phase separation, and subsequent spinel formation inside the network of diamagnetic and insulating MgO grains. Extremely high uniformity in the distribution of intergranular ferrimagnetic MgFe₂O₄ films and grains with resulting magnetic coercivity values that depend on the nanoparticles' initial Fe³⁺ concentration is achieved. Moreover, percolating networks of semiconducting MgIn₂O₄ are derived from MgO nanoparticles with admixtures of 20 at% In³⁺ that gives rise to an enhancement of dc conductivity values by more than five orders of magnitude in comparison to the insulating MgO host. The here presented approach is general and applicable to the synthesis of a variety of functional spinel nanostructures embedded inside ceramic matrices. Nanoparticle loading with aliovalent impurity ions, the level of nanoparticle powder density after compaction, and sintering temperature are key parameters for this novel type of solid-state chemistry in between the host grains.