TEM measurement, data collection, structure determination, and refinement
This protocol is extracted from research article:
A nanocrystalline monoclinic CaCO3 precursor of metastable aragonite
Sci Adv, Dec 12, 2018; DOI: 10.1126/sciadv.aau6178

Bright-field TEM (BFTEM) images and two-dimensional SAED patterns were taken with a Philips CM20 electron microscope operating at 200 keV. BFTEM images and SAED patterns were recorded on imaging plates. Dark-field scanning TEM images and EDT data were taken with a Zeiss Libra 120 electron microscope operating at 120 kV and equipped with a LaB6 thermionic gun, a Nanomegas Spinningstar device for beam precession, and an ASI Medipix single-electron detector for electron diffraction recording. EDT data were acquired at steps of 1° with a beam precession semi-angle of 1° following the protocol described in (41). Data analysis, including three-dimensional diffraction volume reconstruction, cell parameter determination, and reflection intensity integration, were performed using ADT3D software (41) and in-home developed Matlab routines. Details of the EDT measurement were reported in tables S1 and S2. Energy-dispersive x-ray microanalysis was performed using a NORAN Voyager system and a Bruker EDX XFlash6T-60 detector attached to the Philips and ZEISS electron microscopes, respectively. For the matrix-based unit-cell transformation, the Crystal Cracker software (http://multianvil.asu.edu/Crystal_Cracker/CrystalCracker.html) was used.

Ab initio structure determination, assuming kinematical approximation with Ihkl ~ Fhkl2, was performed by direct methods implemented in the SIR2014 software (42). Least-squares structure refinement was performed by SHELXL (43). Data were reported in tables S1 to S5 and data files S1 to S5.

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