Stacking disorder in metastable NiSn4

The atomic structure of NiSn4 intermetallic forming at ambient temperature in different types of Sn/Ni diffusion couples was investigated by powder X-ray diffraction. The recorded diffraction patterns show narrow Bragg reflections as well as characteristic broad diffraction bands indicative of stacking disorder in the crystal structure of NiSn4. The crystal structure consists of NiSn4 layers with square-antiprismatic coordination of Ni by Sn. The stacking of these layers is irregular (faulted) by being an intermediate between the pseudotetragonal PtSn4-(space group Aeaa) and the tetragonal beta-IrSn4-type (space group I4(1)/acd) ideal structures. Thereby, the shape of the diffraction bands reveals that the stacking more closely corresponds to beta-IrSn4-than to the PtSn4-type. The obtained lattice parameters are: a(beta-IrSn4) = a(PtSn4) = b(PtSn4) = (6.248 +/- 0.001) angstrom and 2c(PtSn4) = c(beta-IrSn4) = (23.001 +/- 0.004) angstrom. Pronounced stacking corresponding to a PtPb4-type structure discussed in the literature is not present in NiSn4 on the basis of the diffraction data. The latter finding is compatible with density functional theory (DFT) based first-principles calculations, indicating a higher energy for PtPb4-type NiSn4 as compared to PtSn4- and beta-IrSn4-type NiSn4. (C) 2016 Elsevier Ltd. All rights reserved.