First-Principles Study of Electronic Structure of Type I Hybrid Carbon–Silicon Clathrates KWAI S. CHAN 1,3,4 and XIHONG PENG 2 1.—Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA. 2.—College of Letters and Sciences, Arizona State University, Mesa, AZ 85212, USA. 3.—e-mail: kwai. chan@swri.org. 4.—e-mail: kchan@swri.org A new class of type I hybrid carbon–silicon clathrates has been designed using computational methods by substituting some of the Si atoms in the silicon clathrate framework with carbon atoms. In this work, the electronic structure of hybrid carbon–silicon clathrates with and without alkaline or alkaline- earth metal guest atoms has been computed within the density functional theory framework. The theoretical calculations indicate that a small number of carbon substitutions in the Si 46 framework slightly reduces the density of states (DOS) near the band edge and narrows the bandgap of carbon–silicon clathrates. Weak hybridization of the conduction band occurs when alkaline metal (Li, Na, K) atoms are inserted into the structure, while strong hybridization of the conduction band occurs when alkaline-earth metal (Mg, Ca, Ba) atoms are inserted into the hybrid structure. Empty C y Si 46y clath- rates within the composition range of 2 £ y £ 15 can be tuned to exhibit indirect bandgaps of 1.5 eV or less, and may be considered as potential elec- tronic materials. Key words: Carbon–silicon clathrates, electronic structure, bandgap, first- principles computations INTRODUCTION Silicon clathrates are a class of intermetallic compounds comprising a Si framework containing intracrystalline cavities into which guest atoms, typically alkaline or alkaline-earth elements, can be intercalated. Based on the arrangement of the cage structure, 1–5 silicon clathrates can be categorized into several types, but most are type I or type II. A new Si clathrate, Si 24 , and several new sodium– silicon clathrate compounds have also been reported recently. 6,7 Empty type I clathrates (without guest atoms) have a simple cubic structure that belongs to space group Pm  3n (no. 223) with 46 atoms per unit cell, while empty type II clathrates have a face- centered cubic (fcc) structure that belongs to space group Fd  3m (no. 227) with 136 atoms per unit cell. The salient characteristic features of the silicon clathrate framework structure are (1) intercalation of guest atoms inside the cage structure, and (2) substitution of Si framework atoms by elements such as Al, Ga, Ag, Cu, Ni, and, among others, Au. 1,8 As many as 8 guest atoms may be inserted into the cages of type I clathrates, and 24 guest atoms may be present in the cages of type II clathrates. 1–8 These characteristics provide a range of avenues for tailoring properties through modifications of chem- istry and electronic structure. As a result, these clathrate compounds have received considerable attention due to their unique structure and their potential as magnetic, 1 electronic, 5 thermoelec- tric, 8–12 superconducting, 1,13–15 energy storage, 16–20 and hard materials. 1,21 Their properties are derived from their cage structure and the interactions between the framework and the guest atoms resid- ing within the cage cavities. 1–5 Silicon clathrates can be considered analogous to carbon clathrate materials and are composed of face-sharing Si 20 , Si 24 , and Si 28 cages linked through sp 3 covalent bonds. 2 (Received January 12, 2016; accepted April 30, 2016; published online May 23, 2016) Journal of ELECTRONIC MATERIALS, Vol. 45, No. 8, 2016 DOI: 10.1007/s11664-016-4621-3 Ó 2016 The Minerals, Metals & Materials Society 4246