Influence of ionic mobility on the phase transformation route in Y₃Al₅O₁₂ (YAG) stoichiometry

Ionic mobilities of Y³⁺ and Al³⁺ are controlled using either amorphous or well-crystallized precursors and enhanced by the addition of molten NaCl flux. Experimental results indicate that phase transformation prefers a relatively slow reaction route via YAM-YAP-YAG because of limited ionic mobility when either Y³⁺ or Al³⁺ is confined to a crystallized structure. The selection of intermediate phases can be either Y-rich (YAM) or Al-rich (YAP), depending on the competition of ionic mobility between Y³⁺ and Al³⁺ during phase transformation. The YAH-YAG route is selected only when the ionic mobility of Y³⁺ or Al³⁺ is high enough to induce a nucleation and growth reaction while a certain degree of stoichiometric inhomogeneity is presented; otherwise YAG is crystallized directly from amorphous precursors without passing through any intermediate phase.