A NiAs/Ni2In-Type Phase Ni1+xSn (0.35

Abstract

The previously reported, ordered low-temperature NiAs/Ni2In-type phase Ni1+xSn comprises in the concentration range 0.35<x<0.53 in fact two separate, ordered phases. Both phases are based on a NiAs-type arrangement, “NiSn,” with the additional Ni atoms distributed over the trigonal–bipyramidal interstitial sites in an ordered way. Around the ideal composition Ni3Sn2 (0.45<x<0.53), the earlier reported commensurate structure occurs, termed the “LT phase,” in which the long-range ordering leads to Pbnm symmetry with unit cell parameters aLT≈2aHT, bLT≈31/2aHT, cLT≈cHT, where the subscript HT refers to the corresponding, disordered hexagonal high-temperature phase. For lower Ni contents (0.35<x<0.45) an incommensurate phase, that is termed “LT’ phase,” occurs with a crystal structure closely related to the LT phase. According to X-ray powder diffraction and selected area electron diffraction analyses, the average unit cell exhibits orthorhombic Cmcm symmetry, has unit cell parameters aorth≈aHT, borth≈31/2bHT, corth≈cHT, and a modulation vector q⃗=αa⃗*orth with α≈0.493 for x=0.44 and α≈0.428 for x=0.35. The LT phase may be regarded as a “lock-in” phase of the LT' phase with α=12. A model for the long-range occupational ordering of Ni on the trigonal–bipyramidal interstitial sites has been suggested.