MS-13-P-5721 Refinement of lattice parameter of hematite by Nano-beam electron diffraction

The convergent-beam electron diffraction (CBED) has been used in the lattice parameter determination with good accuracy by using HOLZ reflection lines [1]–[4], and recently a new method to refine the lattice parameters by using the HOLZ reflections observed in the nano-beam electron diffraction pattern (NBDp) has been developed [5]. The HOLZ reflections observed in the NBDp were not used so far because of the displacement in HOLZ spot positions due to the distortion of the projection lens system of the TEM, but such displacements are nevertheless proportional to the distortion coefficients (Crad, Cspi, Cell), and can be taking into account in order to correct the spot positions observed in the NBDp [5]. These microscope parameters are obtained by fitting the spot positions between the experimental and simulated NBD patterns in order to obtain the minimum value of the residual sum of the chi-square [5]. This new method was applied here using a Si standard sample in order to determine the distortion coefficients, and the other required microscope parameters, and then applied in the refinement of the lattice parameters of a sample of hematite containing Al, using the distortion coefficients determined previously. The simulate NBDp were done by using the JEMS software [6] and a standard hematite crystal structure (R-3c, a=0.5035nm and c=1.3732nm) was used in the simulation of the diffraction pattern. The NBDp of both samples (Figures 1 and 2) were performed in the Center of Microscopy of the Universidade Federal de Minas Gerais, using a Tecnai G2-20 (FEI) TEM, with LaB6, operating at 200kV, with camera length of 300mm. The camera length was calibrated by comparison between the experimental and simulated NBDp and the experimental spot positions were measured using ImageJ software by fitting a circle mask in each spot and taking into account its center of mass in pixel units. The distortion coefficients Crad, Cspi, and Cell were estimated in 1E-17, 1E-18, and 1E-2 respectively. The a lattice parameter of hematite sample was measured taking into account the corrected ZOLZ spot positions and it was found a=0.5041nm. The c lattice parameter was kept constant.

References:

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