ELECTRON-DIFFRACTION STUDY OF INTERSTITIAL TRANSITION-METAL ORDERING IN T2-TYPE SOLID-SOLUTIONS(XSN2 (T=CO, NI) B8)

Interstitial transition metal ordering in the ''disordered,'' wide-ran ge 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 addi tional interstitial transition metal ions occupying trigonal bipyramid al 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, approximate ly cylindrical channels of diffuse intensity running along the c dire ction of reciprocal space. The strongest satellite reflections of a lo w-temperature superstructure phase in each system are shown to fall mo re or less directly onto the diffuse distribution of the ''disordered' ' phase. The characteristic extinction conditions of the superstructur e phase are mirrored in the characteristic shape of the diffuse distri bution of the ''disordered'' phase. The composition dependence of the diffuse distribution is investigated. (C) 1996 Academic Press.