Atomic-Scale Insight into the Structure of Metastable γ-Ga2O3 Nanocrystals and their Thermally-Driven Transformation to β-Ga2O3

Metastable gamma-Ga2O3 nanocrystals have gained growing interest for a broad range of technological applications. However, a precise description of their atomic structure and changes thereof during thermally induced transformations that is required to understand and fully exploit their physical and chemical properties is still lacking. In this work, we investigate the structure of gamma-Ga2O3 nanocrystals (2.5 nm in diameter) obtained via a colloidal synthesis route and their evolution with thermal treatment. To this end, we have applied synchrotron X-ray atomic pair distribution function (PDF) analysis, complemented by Ga-71 solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), X-ray absorption spectroscopy (XAS), and electron microscopy. The local structure of the gamma-Ga2O3 nanocrystals deviates from the average cubic spinel-type structure, revealing a high degree of structural disorder. The average structure of the gamma-Ga2O3 nanocrystals is described as a defective spinel with gallium sites in tetrahedral and octahedral (GaIV and GaVI) coordination with oxygen atoms. The modeling of the local structure revealed a lowsymmetry distortion of the polyhedra, which are disorderly oriented. The surface structure of the gamma-Ga2O3 nanocrystals is different from their bulk, whereby GaVI sites at the outermost layers of the nanocrystals are found in a nonperiodical stacking arrangement with a higher occupancy than in the core, as revealed by high-angle annular dark field imaging scanning transmission electron microscopy (HAADF-STEM). The structural evolution of gamma-Ga2O3 nanocrystals upon thermal treatment in air was probed by in situ time-resolved PDF. A gradual transformation of the gamma-Ga2O3 nanocrystals toward the thermodynamically stable beta-Ga2O3 polymorph occurs at different structural domains at different temperatures. At ca. 300 degrees C, changes in the local structure showed an increased distortion of the polyhedral units and revealed the appearance of small beta-Ga2O3 domains (ca. <1 nm), while the bulk phase transformation took place between 550 and 750 degrees C and was associated with an increase in the coherence length of the beta-Ga2O3 phase.