INTERSTITIAL TRANSITION METAL ORDERING IN WIDE RANGE NON-STOICHIOMETRIC M_2+xSn₂ (M=Co,Ni) SOLID SOLUTIONS. R.L. Withers and A-K.Larsson, Research School of Chemistry, Australian National University, Canberra, ACT, 0200 Australia

An electron diffraction study of "disordered" [[gamma]]-Co_2+xSn₂ (0.76<x<1.26) and [[gamma]]-Ni_2+xSn₂ (0.70<x<1.19) has revealed the presence of a spectacular, sharp and highly structured diffuse intensity distribution to which the low temperature ordered [[gamma]]' superstructures are clearly closely related. The underlying M₂Sn₂ average structure is of NiAs type (space group symmetry P6₃/mmc, a ~ 4.1 Å, c ~ 5.2 Å) with additional interstitial transition metal atoms occupying trigonal bipyramidal sites in the Sn sub-lattice. The ordering of these interstitial transition metal atoms and the associated structural relaxation gives rise to the observed diffuse distribution.

The basic shape of the diffuse distribution appears to be based upon intertwined, undulating, approximately cylindrical channels of diffuse intensity running along the c* directions of reciprocal space. The intensity observed when rotating around c*, however, depends strongly upon azimuthal angle. The general topology of the diffuse distribution does not change significantly with composition across the solid solution field, but the amplitude of the undulating channels does: this amplitude is shown to be directly proportional to composition. The strongest satellite reflections characteristic of the low temperature ordered [[gamma]]'-phases fall directly onto the diffuse distribution of the [[gamma]]-phase while the characteristic extinction conditions of the ordered [[gamma]]'-phases are mirrored in the diffuse distribution of the [[gamma]]-phase.

A modulation wave approach is used to deduce the implications of the observed diffuse distribution for local interstitial transition metal ordering. Minimization of macroscopic strain along close packed <110> directions is shown to be responsible for the characteristic absence of diffuse intensity in {h,-h,l} * reciprocal lattice planes.