Quantitative determination of the crystal structure of Ni4Ti3 precipitates

The structure of Ni4Ti3, which occurs as a precipitate in Ni-rich NiTi, has already been determined of having the R-3 space group. This was done using basic electron diffraction by Tadaki et al. also providing an approximation of the atomic coordinates. In the present work a least squares refinement method based on quantitative electron diffraction is used to optimise the structure of the Ni4Ti3 unit cell. This method, in combination with Density Functional Theory (DFT) calculations, results in a structure for which the composition and R-3 space group of the Tadaki model is maintained but the atom positions are clearly shuffled. The R-factor (similar to the one used in X-ray diffraction refinement) drops from 12.5% for the original Tadaki structure to 8.2% for the newly refined one, and moreover the change of atomic coordinates explains in a better manner the shrinking of the Ni4Ti3 unit cell in reference to the NiTi matrix.