Electron spin resonance, dynamic Jahn-Teller effect, and electric transport mechanism in Na-doped type II silicon clathrates

Abstract

Silicon clathrates are Si-cage-based structures capable of hosting other atoms, such as sodium, in their Si-polyhedron cages. Electron spin resonance (ESR) can provide information on the electronic structure of localized or delocalized electrons in cage-like type II silicon clathrates of the form NaxSi136 (0<x≤ 24). The ESR lines observed at low temperatures are classified into four distinct groups ascribed to (1) an isolated single-Na-containing cage, (2) a pair of single-Na-containing cages or clusters composed of more than two single-Na-containing cages, (3) an electron localized around a silicon deficit with dangling bonds, and (4) a conduction electron. The spin-Hamiltonian parameters for the Na atom of group 1 can be explained by the combination of an off-center model and the dynamic Jahn-Teller effect for the single-Na-containing Si polyhedron. The Na content, x, dependence of the ESR signal of group 1 can be explained by the statistical formation probability of the isolated single-Na-containing cage in type II silicon clathrates. Deviation of the ESR signals of groups 1–3 from Curie's law as temperature increases from 110 to 500 K indicates the transition of bound electrons to delocalized states in type II silicon clathrates. The higher transition temperatures of approximately 170 K for the ESR signals of groups 1 and 2 and approximately 300 K for the ESR signals of group 3 correspond to large activation energies of approximately 0.12 eV and approximately 0.35 eV, respectively, from the electron-localized states to the conduction band.