Shuying Piao

Complexity of hexagonal approximants in the RE13Zn~58 system (RE = Ce, Pr, Nd, Sm, Gd, Tb and Dy)

Abstract For the RE13Zn58quasi crystal approximants (RE = Ce, Pr, Nd, Sm, Gd, Tb, Dy), single crystals were prepared from the elements and their structures were refined from single-crystal X-ray diffraction data. They are derived from the Gd13Cd58type structure and crystallize in the space group P63/mmc or its subgroups. The crystal structures of Ce and Pr compounds are identical to that of the archetype, in the Nd compound a pair exchange disorder changes the composition and for Sm a further, partially occupied interstitial position lowers the symmetry to P63mc. For the compounds with Gd, Tb and Dy, the occupancy may order, lowering the symmetry further. For the case of Tb13Zn59 high quality single crystals allowed us to refine the fully ordered orthorhombic super structure.

Quasicrystal approximant Ho13Zn58 + δ (δ ≈ 0.6): an incommensurate modulated tunnel structure

In a systematic study of the binary quasicrystal approximants RE 13Zn∼58 (RE = rare earth elements) system, single crystals of Ho13Zn58 + δ have been prepared. It is unique among family members of the RE 13Zn∼58 system in that it shows a modulated superstructure ordering. Satellite reflections observed in single crystal X-ray diffraction pattern indicate the presence of a one-dimensional lattice distortion. The modulated structure has been solved and refined from X-ray data using the superspace group approach. The Ho13Zn58 +δ structure can be described in the (3 + 1)-dimensional superspace group Pc21 n(0β0)s0s with lattice parameters a = 24.6481(7) Å, b = 14.2484(4) Å, c = 14.0153(3) Å and a modulation vector q = (0, 0.741, 0). The crystal structure is closely related to those of other RE 13Zn∼58 compounds, comprising hexagon-shaped tunnels made up of rare earth-centred Zn polyhedra. All atoms in the structure are affected by a displacive modulation, but one set of Zn atoms are very strongly affected and display an almost composite behaviour with regard to the rest of the structure.

News from the world of modulated intermetallics

The number of modulated structures with more than one modulation vector is small, and such structures often pose special problems in solving and refining. This talk will be concentrated on three such cases; The 3+4 dimensionally modulated cubic structure of digenite, the 3+2 dimensionally modulated structure of Cu3In2, the 3+2 dimensionally modulated structure of AuZn3 and the 3+2 dimensionally modulated structure of Se(Sn4)2K10. The three former cases are interesting because they are relatively weakly ordered structures where modelling is straight-forward, but the model itself is less than obvious to understand while the latter case appears highly ordered, but presents formal modelling difficulties. From these and previously known multi dimensional cases it would appear that higher order modulations are very prone to disorder. The image shows the hk0 layer from Se(Sn4)2K10.