MODULATIONS IN B8 TYPE INTERMETALLICS

Sven Lidin

Inorganic Chemistry, Stockholm University, 106 91 Stockholm, Sweden, sven@inorg.su.se

Keywords: Incommensurate structures, Superstructures

The B8 type intermetallics constitue a big group of compounds, and their study is as old as X-ray crystallography itself. The minority constituent is a main group element that forms an hexagonal close packing, while the majority component occupies the octahedral interstices and a part of the trigonal bipyramids. The structure shows a remarkable stoichiometric flexibility with compositions varying from 1:1 or even slightly less, as in Ni deficient NiBi, to 2:1 or slightly above as in the case of Cu₇In₃.

Compositions differing from a 1:1 stoichiometry normally gives rise to superstructure ordering, and about 30 different structure types have been reported. Prominently complex systems are Mn-Sn, Cu-Sn and Ni-Ge each with three known ordered B8 phases and Cu-In with five phases. In addititon to these ordered systems, it has been shown that many compounds show highly structured diffuse scattering at elevated temperatures, indicative of less well defined order.

We have studied the incommensurately modulated phases in a number of systems, and will present some results from the systems Cu-In, Ni-Bi and Mn-Sn. These structure determinations have some characteristics in common: the proper determination of unit cell parameters for is not straight forward because of the low intensities of satellite reflections and a further complication is the formation of pseudo merohedral twins that lead to apparent higher symmetries. The method that we have used to alleviate these problems is electron diffraction. Here the intensities of weak reflections is enhanced due to the strong interaction between the electron beam and the sample, and twinning problems may be resolved by selected area diffraction that allows the study of small, thin, regions of crystals that may be single crystalline, or at least show unbalance in the ratio between different individual twinning directions. An additional bonus is the two dimensional nature of the detection of diffracted intensity; it is easy to spot diffuse scattering, streaking and other signs of disorder, and by two dimensional pattern recognition, it becomes possible to identify substructures.

Mn8Sn5 is an incommensurately modulated phase whose parent structure is B8. It has the superspace group P:Cmcm:-1s0, but the basic lattice remains metrically hexagonal, and pseudo merohedral twinning leads to hexagonal diffraction symmetry. Selected area electron diffraction shows conclusiely that the modulation is one dimensional, and that the structure has a 4d orthorhombic superspace group rather than an hexagonal 5d one. Once the symmetry was determined, the superspace refinement was straight forward.

NiBi is again a derivative of the B8 type structure. In this case there are two independent modulations present; one commensurate and one incommensurate. Because of pseudo merohedry there are six modulation directions that appear simultaneously in bulk twins, but electron diffraction reveils that they only occur in certain combinations giving rise to a structure in the superspace group W:Pmcm:ss-1.

The phase diagram of Cu-In at he equimolar composition is rather complex. We have identified three distinct phases, and a continuum uniting two of these. One structure is partially solved and will be presented.