Quantitative determination of the crystal structure of Ni 4Ti 3 precipitates

Abstract

The structure of Ni 4Ti 3, 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 Ni 4Ti 3 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 Ni 4Ti 3 unit cell in reference to the NiTi matrix.