NiAs/Ni2In-type phase Ni1+xSn (0.35x0.45) with incommensurate occupationalordering of Ni

The previously reported, ordered low-temperature NiAs/Ni2In-type phase Ni1xSn comprises in the concentration range 0.35 < x < 0.53 in fact two separa te, ordered phases. Both phases are based on a NiAs-type arrangement, "NiSn ," with the additional Ni atoms distributed over the trigonal-bipyramidal i nterstitial 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 wi th unit cell parameters a(LT) approximate to 2a(HT), b(LT) approximate to 3 (1/2)a(HT), c(LT) approximate to c(HT), where the subscript I-IT refers to the corresponding, disordered hexagonal high-temperature phase. For lower N i contents (0.35 < x < 0.45) an incommensurate phase, that is termed "LT' p hase," occurs with a crystal structure closely related to the LT phase. Acc ording to X-ray powder diffraction and selected area electron diffraction a nalyses, the average unit cell exhibits orthorhombic Cmcm symmetry, has uni t cell parameters a(orth) approximate to a(HT), b(orth) approximate to 3(1/ 2)b(HT), c(orth) approximate to c(HT), and a modulation vector (q) over rig ht arrow = alpha(a) over right arrow*(orth) with alpha approximate to 0.493 for x = 0.44 and alpha approximate to 0.428 for x = 0.35, The LT phase may be regarded as a "lock-in" phase of the LT' phase with alpha = 1/2 A model for the long-range occupational ordering of Ni on the trigonal-bipyramidal interstitial sites has been suggested. (C) 2001 Academic Press.