Low-temperature phase of the Zn–Sc approximant

Abstract

The low-temperature transition in the Zn–Sc 1/1 cubic approximant, reported by Tamura et al. [Phys. Rev. B 71 0902203 (2005)], has been confirmed by the following experimental evidence: (1) anomaly in temperature dependence of electrical resistivity at approximately 153 K, and (2) appearance of weak superlattice reflections both in electron and powder X-ray diffraction patterns. The low-temperature phase is monoclinic, belonging to the space group C2/c, with lattice parameters a = 1.947(5) nm, b = 1.379(3) nm, c = 1.955(5) nm and β = 89.931(3)°. The unit cell includes four unique Tsai-type clusters consisting of almost regular tetrahedron of Zn surrounded by triple shells. The first dodecahedral shell of Zn is highly distorted so as to avoid unrealistic short atomic distance to the Zn tetrahedron. Direct evidence of an orientational ordering of the clusters has been obtained by Reitveld analysis of the low-temperature phase. The occurrence of the transition is sensitive on alloy composition as well as heat treatment.