Atomic structure of the unique antiferromagnetic 2/1 quasicrystal approximant

The atomic structure of the recently discovered antiferromagnetic Ga50Pd35.5Tb14.5 2/1 approximant to quasicrystal with the space group of Pa-3(No. 205), a = 23.1449(0) angstrom was determined by means of a single crystal X-ray diffraction. The refined structure model revealed two main building units, namely, a Tsai-type rhombic triacontahedron (RTH) cluster with three concentric inner shells and an acute rhombohedron filling the gaps in between the RTH clusters. One of the interesting findings was a very low number of chemically mixed sites in the structure, which amount to only 7.40 % of the all the atomic sites within an RTH cluster. In particular, a disorder-free environment was noticed within a nearest neighbor of an isolated Tb3+ ion, which is presumably one of the main contributors in enhancing antiferromagnetic order in the present compound. The second significant finding was the observance of an orientationally ordered trigonal pyramid-like unit with a height of 4.2441(7) angstrom at the center of the RTH cluster, which has never been observed in Tsai-type compounds before. Such unit is noticed to bring structural distortion to outer shells, in particular, to the surrounding dodecahedron cage being another possible contributor of the antiferromagnetic order establishment in the present compound. The results, therefore, are suggestive of a possible link between chemical/positional order and the antiferromagnetic order establishment.