AN ELECTRON-DIFFRACTION STUDY OF MODULATED NI1+XGE B8 TYPE PHASES

Structural modulations of NiAs-Ni-2 In type structures within the B8 p hase field of the Ni-Ge system have been investigated with electron di ffraction. The modulations of the average B8 substructure (caused by o rdering of interstitial Ni atoms in trigonal bipyramidal sites) can in reciprocal space be described by H=G+mq, where G are the Bragg reflec tions corresponding to the P6(3)/mmc average substructure, m is an int eger and q is a primary modulation-wave vector of the form [h,h,2 (h) over bar,1]. Ni5Ge3 is described by q=1/3 [11 (2) over bar 2]* which corresponds to a=11.682(6) Angstrom, b=6.737(3) Angstrom, c=6.264 (3) Angstrom, b=52.11(1)degrees and space group C2/c, Ni19Ge12 and Ni17Ge1 2 are both described by q=1/6 [22 (4) over bar 3]. They are each othe rs antistructure with respect to the interstitial sites and have hence identical primary modulation-wave vectors. Ni7Ge4 exhibits a complica ted diffuse distribution partly condensed out at q(1) approximate to 0 .18[22 (4) over bar 3] and q(2) approximate to 0.27[11 (2) over bar 2 ]. The diffuse scattering resembles the undulating channels along c* previously observed for Co1+x Sn and Ni1+x Sn but in Ni7Ge4 two indepe ndent sets, cutting through q(1) and q(2) respectively, are present. ( C) 1998 Elsevier Science S.A.