Ternary Ag - In - (Eu, Ce) 1/1 approximants are synthesized and their structures are studied by transmission electron microscopy (TEM). For both the approximants, superlattice spots are clearly observed at room temperature, and the superstructures of the Ag - In - (Eu, Ce) approximants are found to be similar to those of Cd25Eu4 and Cd37Ce 6, respectively. The Ag - In - Eu 1/1 approxi-mant has a double unit cell with space group Fd3̄ while the Ag - In - Ce 1/1 approximant has a normal unit cell with space group Pn3̄, both at room temperature. The results suggest that the tetrahedron at the center of the icosa-hedral cluster is ordered in the Ag - In - (Eu, Ce) approximants in the same manner as the corresponding binary approximants Cd6(Eu, Ce). Furthermore, TEM observations at high temperatures reveal that the Ag - In - (Eu, Ce) approximants undergo a phase transition to a bcc lattice at an elevated temperature. The transitions are understood as a consequence of disordering of the tetrahedron located at the center of the icosahedral cluster.
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