Electron diffraction study of interstitial transition metal ordering in T2+xSn2 (T=Co, Ni) B8-type solid solutions

Interstitial transition metal ordering in the ''disordered,'' wide-range nonstoichiometric Ni2+xSn2, 0.70 < x < 1.19, and Co2+xSn2, 0.76 < x < 1.26, solid solution fields has been investigated via electron and X-ray powder diffraction. The sublattice is of the NiAs type with additional interstitial transition metal ions occupying trigonal bipyramidal sites in the Sn sub-lattice. That the distribution of interstitial sites is far from random in these ''disordered'' phases is clear from the presence of a spectacular and highly structured diffuse intensity distribution which appears to take the form of undulating, approximately cylindrical channels of diffuse intensity running along the c* direction of reciprocal space. The strongest satellite reflections of a low-temperature superstructure phase in each system are shown to fall more or less directly onto the diffuse distribution of the ''disordered'' phase. The characteristic extinction conditions of the superstructure phase are mirrored in the characteristic shape of the diffuse distribution of the ''disordered'' phase. The composition dependence of the diffuse distribution is investigated. (C) 1996 Academic Press.