Quasicrystal approximant Ho₁₃Zn_{58 + δ (δ ≈ 0.6)}: an incommensurate modulated tunnel structure

Abstract

In a systematic study of the binary quasicrystal approximants RE₁₃Zn_∼58 (RE = rare earth elements) system, single crystals of Ho₁₃Zn_58 + δ have been prepared. It is unique among family members of the RE₁₃Zn_∼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 Ho₁₃Zn_58 +δ structure can be described in the (3 + 1)-dimensional superspace group Pc2₁n(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 RE₁₃Zn_∼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.

Acknowledgement

Financial support from the Swedish Natural Science Research Council and the Foundation for Strategic Research are gratefully acknowledged.