Toward a unified treatment of AS=0 parity violation in low-energy nuclear processes

The atomic structure of the recently discovered antiferromagnetic (AFM) Ga50Pd35.5Tb14.5 2/1 approximant to quasicrystal with the space group of Pa3 over bar (No. 205), a = 23.1449(0) angstrom was determined by means of 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. 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 AFM order in the present compound. The second significant finding was the observance of an orientationally ordered trigonal pyramidlike unit with a height of 4.2441(7) angstrom at the center of the RTH cluster. Such a unit is noticed to bring structural distortion to outer shells, particularly to the surrounding dodecahedron cage being another possible contributor to the AFM order establishment in the present compound. The results, therefore, are suggestive of a possible link between chemical/positional order and the AFM order establishment.