  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.
