Stabilization mechanism of Si_(12) cage clusters by encapsulation of a transition-metal atom: A density-functional theory study

摘要

We systematically studied the geometrical and electronic structures of transition-metal (M)-encapsulating Si_(12) cage clusters, MSi_(12) (M=Hf, Ta, W, Re, Os, Ir, Pt, and Au), mainly focusing on their outstanding stability, using calculations based on density-functional theory. We found that the MSi_(12) clusters except HfSi_(12) belong to either of two distinct structural classes, the D_(6h)-symmetric hexagonal prism (HP; for M=Ta, W, Re, and Os; total number of valence electrons per cluster, N_ν, ranging from 53 to 56) and less-symmetric four pentagonal face (FPF; M=Re, Os, Ir, Pt, and Au; N_ν ranging from 55 to 59) structures. The HP structure is particularly stabilized at N_ν=54, which is understood in terms of the electronic shell closure of the M atoms due to the 18-electron rule, and the geometrical symmetry is maintained for N_ν=53, 55, and 56 by the covalent bonding between the M atom and the Si cage accompanied by the cage-to-M charge transfer. The FPF structure is lowest in energy for N_ν=56 and is maintained by the same covalent-bond/charge-transfer mechanism for other values of N_ν. We propose that all these results originate from the electronic "rigidness" of the HP and FPF Si cages against the variation of N_ν, which is the leading factor governing the stability of MSi_(12).