From V8Ga36.9Zn4.1 and Cr8Ga29.8Zn11.2 to Mn8Ga27.4Zn13.6: A Remarkable Onset of Zn-Cluster Formation in an Intermetallic Framework

Abstract

The series of isotypic compounds V8Ga41 --> V8Ga36.9Zn4.1 --> Cr8Ga29.5Zn11.2 --> Mn8Ga27.4Zn13.6 with the V8Ga41 structure type (space group R3, Z = 3) was prepared and structurally characterised by X-ray diffraction experiments (V8Ga41: a 13.9351(5), 14.8828(12); V8Ga36.9Zn4.1: a = 13.9244(7), c = 14.8660(9): Cr8Ga29.8Zn11.2: 13.7153(5), c = 14.6872(9); Mn8Ga27.4Zn13.6: a = 13.6033(6), c = 14.6058(16)). The site occupancies of the ternary compounds were refined from neutron powder-diffraction data and exposed a startling segregation of Zn and Ga, which finally resulted in the formation of separated Zn13 cluster entities-corresponding to almost ideal centred cuboctahedra or small pieces of fcc metal-in the Mn compound, which has the highest Zn content in the series. The homogeneity ranges of the underlying phases T8Ga41 xZnx were determined to be 0 < x < 4.1(3), 8.7(3) < x < 11.2(3) and 13.6(4) < x < 16.5(3) for T = V, Cr and Mn, respectively. The different ranges of composition of the phases reflect the requirement of an optimum electron concentration for a stable V8Ga41-type structure, which is in the narrow range between 159 and 165 electrons per formula unit. First-principles electronic-structure calculations could explain this fact by the occurrence of a pseudo gap in the density of states at which the Fermi level is put for this particular electron concentration. Furthermore the nature of the Zn/Ga segregation was revealed: T-Zn interactions were found to be considerably weaker than those for T-Ga. This places the Zn atoms as far as possible from the T atoms, thus leading to the formation of cuboctahedral Zn13 entities.

Lithiation-Induced Zinc Clustering of Zn3, Zn12, and Zn18 Units in Zintl-Like Ca∼30Li3+xZn60–x (x = 0.44–1.38)

“…However, the packing of these cationic polyhedra does not form any known type of clathrate structure. 51 These three different polyhedra encapsulate, respectively, lithium-centered Zn 12 icosahedral clusters (shaded in turquoise), arachno-M 18 (M = Zn/Li) tubular clusters (rose), and Zn 3 triangular clusters (green). The three types of anionic clusters are interconnected via two-center, two-electron (2c-2e) intercluster bonding to form a three-dimensional extended network.…”

“…The centers of two hexagonal faces of the M 18 tubular cluster are occupied by Ca atom(s), either ordered or disordered. Also, each M 18 cluster is enclosed by a Ca 30 polyhedron, which exhibits 12 triangles, 12 pentagons, and 2 hexagonal faces (thus denoted as 3 12 5 12 6 2 ). Each of the 12 pentagonal faces of Ca 30 is intersected orthogonally by a M3− Zn4 exobond of the M 18 tubular cluster (Figure 3b).…”

“…Table 3 gives the electron counting scheme for the title phase using electron counting approaches of Wade−Mingos. 6,7 The results of extended Huckel MO calculations for the isolated Li@Zn 12 cluster ( Figure S1 in the SI). This value also agrees with Wade's rules (2n + 6) for the number of skeletal electrons of an arachno polyhedron.…”

“…11 Therefore, effective s− p orbital mixing for group 12 elements can only be accomplished by forming large clusters so that s and p orbitals become more dispersive. Two possible routes to achieving such large clusters of group 12 elements are (1) to combine with elements on the Zintl border, i.e., Ga, In, and Tl, which tend to form clusters, as seen in the ternary phases M 8 (Ga,Zn) 41 (M = V, Cr, Mn) 12 and (2) to target compounds with high percentages of group 12 elements, such as Ce 13 Zn 58 , La 13 Hg 58 , ScZn 6 , CaCd 6 , Dy 2 Zn 17 , BaCd 11 , NaZn 13 , and KCd 13 . 13 In contrast, binaries with relatively lower percentages of group 12 elements generally exhibit network structures, e.g., CaCd 2 , Ca 5 Zn 3 , and Ba 2 Hg.…”

“…To establish valence-electron-counting rules of the title phase, molecular orbitals (MOs) of isolated Li@Zn 12 Li 5 Zn 58 " reaction (X-ray pure phase) was mechanically polished into a cuboid for electrical resistivity measurement by the four-probe method. Resistivity data were collected using a Quantum Design Physical Measurement System (PPMS) within the temperature range of 2−300 K. For comparison, resistivity data of a high-quality single crystal of CaZn 2 in a cuboid shape were also collected.…”