Physical Properties of A_6ClFe_24Se_26(A=K, Rb) with Self-Similar Structure

We have successfully synthesized two novel compounds A_6ClFe_24Se_26 (A = K, Rb). The key structural units of them are FeSe octamers, consisting of edge-shared FeSe_4 tetrahedra. Two kinds of FeSe octamer layers with different connection configurations stack along the c axis, forming a three-dimensional (3D) TiAl_3-type structure. Interestingly, the 3D structural topology of these ocatmers in one unit cell is similar to the local atomic arrangement of themselves, i.e., self-similarity in structure. Physical property characterizations indicate that both the compounds exhibit insulating antiferromagnetism with Neel temperatures T_\rm N\sim 110 K and 75 K for K_6ClFe_24Se_26 and Rb_6ClFe_24Se_26.