In situ vibrational spectroscopy was used to reveal the microstructure evolution mechanism of iron garnet single crystals growing from multi-component oxide melts

Abstract

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)O_n–AO₂–Fe(III)O₂–AO_m]^{[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)O_n–AO₂–Fe(III)O₂–AO_m]^{[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.