NMR and computational study of Ba 8 Cu x Ge 46-x clathrate semiconductors Jing-Han Chen ⇑ , Ali Sirusi Arvij, Xiang Zheng, Sergio Y. Rodriguez, Joseph H. Ross Jr. Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA article info Article history: Received 12 September 2013 Received in revised form 31 December 2013 Accepted 6 January 2014 Available online 20 January 2014 Keywords: Clathrates NMR in condensed matter Band structure of semiconductors abstract Ba 8 Cu x Ge 46-x is a type-I clathrate material that forms as a semiconductor in a narrow composition range corresponding to the electron-balanced Zintl composition, with x = 5.3. We use NMR spectroscopy com- bined with ab initio electronic structure calculations to probe the electronic and structural behavior of these materials. Computational results based on a superstructure model for the atomic configuration of the alloy provide good agreement with the electric quadrupole-broadened NMR lineshapes. Modeling using the modified Becke–Johnson (TB-mBJ) exchange potential is also shown to agree well with exper- imental NMR Knight shifts. The results indicate that the Cu–Ge balance is the main factor determining the carrier density, within a narrow stability range near the ideal Zintl composition. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction In recent years, clathrates have gained significant attention due to their thermoelectric properties, and potential for device applica- tions [1–10]. Semiconducting clathrates consist of cages of silicon, germanium, or tin in a crystalline framework, with guest atoms located inside the cages. The framework can provide a high See- beck coefficient (S) and electrical conductivity (r) while acoustic phonons resonantly scattered from loosely bound guest atoms [11,12] lead to a very low thermal conductivity (j). These proper- ties contribute to high values of the thermoelectric figure of merit ZT ¼ S 2 r=jT . Besides thermal properties, clathrates doped with metal atoms are also interesting for a wide variety of electronic behavior, such as superconductivity and magnetism in Ba 8 Si 46 and related materials [6,13–15]. Ni- and Au-doped type-I Ba 8 Ge 46 have been found to have metal-to-insulator transitions with varying the concentrations of dopants [16,17]. In electron microprobe studies copper clathrates with the nom- inal composition Ba 8 Cu x Ge 46-x were found to have final composi- tions [18] in a range close to x ¼ 5 1 3 . This composition balances the electron count in the structural framework [18], with each Ba ion donating 2 electrons to the framework to make up for the electron deficit of Cu vs. Ge. In this way the four-bonded Cu–Ge network maintains four electrons per site needed to fill the valence band, fulfilling what is called the Zintl condition [19]. Johnsen et al. [20] examined a series of these materials, and found n-type behav- ior with samples having composition x = 6 exhibiting the lowest carrier density, contrary to the expected Zintl argument. The low- est-n materials also exhibited an unexpected highly resistance behavior, pointing to the possible presence of an impurity band or similar situation involving a large number of scattering centers. One mechanism for this may be the spontaneous occurrence of framework vacancies, which can also help to restore the electron-balance condition [18,20]. In order to better understand the structural configurations of these materials, and the conditions affecting the transport behavior, we used NMR spectroscopy and electronic structure cal- culations combined with electron microprobe and crystallographic studies to better understand the band-edge electronic behavior and the corresponding structural properties. Computational analysis included the modified Becke–Johnson (TB-mBJ) [21,22] exchange potential, which was recently shown to provide accurate modeling of semiconductor band-gaps. We find that in computing NMR shifts and relaxation behavior this model also provides good agreement with experiment. Comparing both the wide-line quad- rupole NMR spectra and the extracted paramagnetic contributions to the shifts and spin-lattice relaxation times, from computational modeling we find the system to be well-described by a full-frame- work model, without large numbers of vacancies. 2. Experiment 2.1. Synthesis and sample characterization For initial processing the elemental materials were mixed in stoichiometric quantities, with a small Ba excess included to account for its vapor pressure. Samples were arc melted in argon and then annealed in BN crucibles in evacuated ampoules at 950  C for 3 days, followed by 700  C for 4 days. Three Ba 8 Cu x Ge 46-x samples were made in this way, with starting compositions x = 4, 5.3, and 6. These samples will be designated Cu4, Cu5.3 and Cu6, respectively. Powder X-ray diffrac- tion (XRD) was performed using a Bruker D8 Advance spectrometer, using Cu Ka radiation. Electron microprobe measurements were carried out using wavelength dispersive spectrometry (WDS) methods on a Cameca SX50 equipped with four wavelength-dispersive X-ray spectrometers. 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2014.01.034 ⇑ Corresponding author. Tel.: +1 979 845 7823. E-mail address: jhchen@tamu.edu (J.-H. Chen). Journal of Alloys and Compounds 593 (2014) 261–266 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom