The critical role of alkali cations in synthesizing Bi5FeTi3O15 nanocrystals

Recently, BFTO compounds attract much attention due to their potential as single-phase multiferroic materials. However, it is still challenging to synthesize pure phase BFTO nanocrystals due to their structural and compositional complexity. In this article, BFTO nanocrystals were successfully synthesized by adopting MOH (M = Li+, Na+ and K+) as mineralizers, and the critical role of M+ ion is expatiated in detail. Based on the anion coordination polyhedron growth unit model, growth unit/OH−/M+ core/shell capping layers would form during the syntheses process, and the outermost M+ layer can hinder the growth and formation of pure phase BFTO nanocrystals via the effective passivation beyond a certain critical concentration of M+, i.e., 0.5 M, 2.5 M and 0.5 M for LiOH, NaOH and KOH, respectively, proportional to 1/RM+ (the solvated cation radius) and \( {\text{K}}_{{\rm{D}}}^{{{\rm{MOH}}}} \) (the dissociation constant of MOH). The coefficient of \( {\text{R}}_{\text{Li + }} > {\text{R}}_{\text{Na + }} > {\text{R}}_{\text{K + }} \) and \( {\text{K}}_{\text{D}}^{\text{LiOH}} < {\text{K}}_{\text{D}}^{\text{NaOH}} < {\text{K}}_{\text{D}}^{\text{KOH}} \) results in the highest critical concentration of NaOH among all the MOH bases.