The　performance　of　rare-earth　iron　garnets　(ReIGs)　depends　on　their　lattice　integrity　and　heteroatomic　substitution　sites.　However,　the　lack　of　information　on　microstructural　evolution　during　the　liquid-phase　epitaxy　(LPE)　of　ReIGs　limits　quality　control　and　understanding　of　atomic　substitution.　Herein,　density　functional　theory　(DFT)　and　in　situ　Raman　spectroscopy　were　used　to　reveal　the　microstructure　geometry　and　self-assembly　mechanism　of　(TbBi)3FeO5O12-based　melts　[(Fe2O3-Tb4O7-Bi2O3),　1;　(Fe2O3-Tb4O7-Bi2O3-B2O3),　2].　Results　show　that　both　1　and　2　exhibit　anion　and　cation　cluster　characteristics　with　multi-level　size　distributions,　in　which　chain-like　ion　clusters　[Fe(iii)On-AO2-Fe(iii)O2-AOm][4-2(m+n)]-　[A　=　Tb3+　or　Bi3+;　n　(m)　=　2　and　3]　are　confirmed　to　be　the　growth　units　of　ReIGs　due　to　their　dominant　presence　in　the　melt.　Notably,　solidification　kinetics　show　that　the　growth　units　of　1　restructure　into　periodic　long　chains　[-AO2-Fe(iii)O2-]n2n-　during　cooling.　Electrostatic　potential　analysis　shows　that　the　growth　of　ReIGs　may　follow　the　electrostatic　bonding　self-assembly　of　[Fe(iii)On-AO2-Fe(iii)O2-AOm][4-2(m+n)]-　and　free　Fe3+　to　form　growth　units　with　lattice　structures,　which　are　superimposed　onto　the　growth　interface　to　achieve　the　growth　process.　In　addition,　trace　amounts　of　B2O3　enhance　the　freedom　and　stability　of　the　system　by　increasing　the　concentration　of　non-bridging　oxygen　and　interaction　between　the　clusters.　This　work　illustrates　the　self-assembly　mechanism　of　the　crystal　growth　of　ReIGs　at　the　atomic　scale,　providing　new　insights　into　the　optimization　of　single-crystal　performance　by　regulating　the　melt　structure.