Tuning Complexity by Lithiation: A Family of Intergrowth Structures Using Condensed hypho-Icosahedra in the Li-Doped Ca-Zn System.

Cluster chemistry of intermetallics with valence electron counts (VECs) in the range of 2.0-3.0 is intriguing. Lithiation of polar intermetallics in this VEC region is found to be an effective chemical route to produce new complex structures with different stability mechanisms. In this work, two new complex intermetallic structures have been discovered in the Ca-Li-Zn system: Ca12LixZn59-x and Ca15LixZn75-x. Ca12LixZn59-x, x ≈ 5.65(3)-14.95(3), forms in the trigonal space group R3̅m, with a = 9.074(1)-9.1699(2) Å, c = 53.353(1)-53.602(1) Å, and Z = 3. In comparison, Ca15LixZn75-x, x ≈ 19.07(2), crystallizes in the space group P63/mmc, with a ≈ 9.183(1) Å, c ≈ 45.191(5) Å), and Z = 2. Both structures are members of a large intergrowth family featuring slabs of dimers (D) and trimers (T) stacking along [001], with the sequences DTDDTDDTD for Ca12LixZn59-x and TDDDTDDD for Ca15LixZn75-x. Each dimer consists of two face-sharing Zn-centered hypho-icosahedra, and each trimer comprises a Li-centered icosahedron sandwiched by two hypho-icosahedra. This intergrowth family includes several known intermetallic structure types involving very electropositive metals, e.g., SrMg5.2, Ba2Li4.21Al4.79, and Sr9Li17.5Al25.5. Because of cluster defects and condensation, both Ca12LixZn59-x and Ca15LixZn75-x are electronically akin to close-packed metals, and their structural stabilities can be interpreted by a Hume-Rothery mechanism rather than the Zintl-Klemm concept.