In nature it is common that a new mineral grows between two minerals due to the inter-diffusion of elements. Understanding its growth mechanism is critical for reconstructing conditions and rates of mineral formation. The growth of the new phase is controlled by the coupling of interface reaction and long-range diffusion. To understand the interface reaction, it is essential to figure out the atomic structure of the interfaces.
In this research, spinel (MgAl2O4, Spl) has been grown between periclase (MgO, Per) and corundum (Al2O3, Crn) via pulsed laser deposition  and uniaxial stress methods  for studying the early and late growth stages, respectively. Electron Backscatter Diffraction (EBSD) has been used to study the interfacial orientation relationship on both Per/Spl and Spl/Crn reaction interfaces. The EBSD mapping (Fig.1) shows that the Spl layer splits into two different sections: a thinner part in topotaxial orientation relationship with Per, and a thicker part topotaxial with Crn. Then Focused Ion Beam (FIB) was used to lift out the interface areas with representative orientation relationships, and the atomic structure studied by aberration-corrected Scanning Transmission Electron Microscopy (STEM).
The atomic resolution images of the Spl/Crn interface show that the interface is located where the (001) lattice plane of Crn coincides the (111) lattice plane of Spl, which are both occupied by Al atoms exclusively. In another side, the Per/Spl interface shows a periodic configuration (Fig.2a), consisting of curved segments (convex towards Per) . The image in Fig.2b reveals regularly spaced misfit dislocations at the positions of “cusps” (see the 2D model in Fig.2c), occurring every ~4.5 nm. A similar configuration is observed at another interface area equivalent with a 90° rotation of the structure in Fig.2b. These results unveil the 3D configuration of the interface, which has a grid of convex protrusions of spinel into periclase with misfit dislocation at each minimum (Fig2d). The structure reveals the mechanism of the interface migration: the climb of the misfit dislocations is the rate-limiting factor and therefore leads to this scalloped geometry. Furthermore, the extra atoms required for dislocation climb leave behind vacancies that eventually form pores at the interface, which provides additional resistance to interface motion and leads to doming of the interface on the scale of individual grains. These results also show that a fundamental understanding of the interface reaction and migration on the atomic scale is the key for understanding the interface migration on the larger scale.
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 This research was funded by the EU’s Horizon 2020 Marie Curie grants No. 656378–Interfacial Reactions (CL) and the Austrian Science Fund (FWF): I1704-N19 in the framework of FOR741-DACH (GH).
To cite this abstract:Chen Li, Thomas Griffiths, Timothy J. Pennycook, Clemens Mangler, Lutz C. Götze , Petr Jeřábek , Jannik Meyer, Gerlinde Habler, Abart Rainer; Interface migration mechanism on Corundum/Spinel/Periclase: atomic study via aberration-corrected STEM. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/interface-migration-mechanism-on-corundumspinelpericlase-atomic-study-via-aberration-corrected-stem/. Accessed: February 23, 2019
EMC Abstracts - https://emc-proceedings.com/abstract/interface-migration-mechanism-on-corundumspinelpericlase-atomic-study-via-aberration-corrected-stem/