Diffusion-induced amorphization in yttrium-iron garnet imbedded with ZrO2 particles

Yttria-iron garnet (YIG)/ZrO2 (5 mol.%) composites, fired at 1450°C in oxygen atmosphere for 8, 16 and 96 h, were studied by transmission electron microscope with regard to a possible diffusion-induced amorphization (DIA) process within the YIG lattice. Lattice image shows an amorphous zoning of YIG around the intragranular ZrO2 particles where possible grain-boundary wetting effect can be avoided, and next to the YIG/ZrO2 interface where the gradient of inward Zr flux or outward Fe flux is presumed to be large. As a result of such a solid-state amorphization, the lattice planes are still parallel-aligned in the opposite sides of the amorphous zoning. Structural and chemical considerations further suggest that amorphization proceeds by cellular disordering of Zr4+, Fe3+ and Fe2+ (charge compensating defect due to Zr4+ substitution for Fe3+) in the array of octahedral sites and facilitated indirectly by dislocations acting as source and sink of vacancies. An ordering process, subsequent to DIA, caused the formation of non-dendritic α-Fe2O3 islands in the relatively high Fe/Zr ratio region of the amorphized zoning.