Abstract
In the type I clathrates Ba8TM x Ga y Ge46–x–y (TM = group 10 to 12 elements) where some of the Ge framework atoms are substituted by Zn, Cu or Ag, the transition-metal elements prefer to occupy the 6c site. Preliminary band-structure calculations showed that this substitution implies modification of the electronic bands in the vicinity of the energy gap. By appropriate tailoring of the band structure, improved thermoelectric properties can be obtained. More detailed full-potential linearized augmented plane wave (FP-LAPW) method calculations within density functional theory (DFT) were performed using the WIEN2k package for compositions where the transition element TM fully occupies the 6c site. Additional analysis of the properties of the electron density topology within Bader’s atoms-in-molecules approach was carried out to study the chemical bonding in intermetallic clathrates. To verify the theoretical predictions, polycrystalline samples of the type I clathrates Ba8TM x Ga y Ge46–x–y (TM = Zn, Cu, Ag) modified by transition-metal element substitution for Ge were obtained. The samples were characterized using powder x-ray diffraction analysis, scanning electron microscopy, and energy-dispersive x-ray spectroscopy. The electrical conductivity, Seebeck coefficient, and thermal conductivity were measured in the temperature range from 320 K to 720 K. Several models were used to fit the experimental results for the electronic transport properties and to estimate the energy gap. Vacancies at the Ge site were considered responsible for deviations from the desired properties, and appropriate defect equations correlating the vacancies and TM concentration are presented. Finally, the results of DFT calculations are compared with the experiments, showing good agreement with theoretically predicted cell parameters and general observations of the transport properties.
Highlights
In the quest for better thermoelectric materials, the attention of researchers has focused on so-called (Received January 8, 2016; accepted May 13, 2016; published online June 21, 2016)cage compounds.[1,2,3] The particular feature of these compounds is the presence of large structural voids in their structure, which can be filled with ‘‘guest’’ atoms
We investigated the band structure of Ba–Ge type I clathrates with selected compositions modified by substitution of Zn, Cu, Ag, and Ga, as well as the impact of such substitution on the bond properties, band dispersion, and relative positions of bands, and we compare with electronic transport measurements
The band structure of Ba8Ga6Ge40 calculated in our previous study, where full occupation of 6c position by Ga was assumed, is very similar to those calculated for Ba8TM6Ge40
Summary
In the quest for better thermoelectric materials, the attention of researchers has focused on so-called (Received January 8, 2016; accepted May 13, 2016; published online June 21, 2016). Many theories have been used to explain this behavior, the most popular and illustrative being ‘‘rattling’’ of guest atoms, where loosely bonded atoms are regarded as Einstein oscillators acting as phonon scattering centers.[1] more recent inelastic neutron scattering experiments and Raman spectroscopy measurements point to different mechanisms for the reduced thermal conductivity of type I clathrates and refute the scenario of guest atoms as independent oscillators.[4] A comprehensive overview of the current state of knowledge on glass-like thermal conductivity in clathrates is given by Takanabe et al.[5] Due to their particular transport properties, germanium clathrates with type I structure are a group of promising thermoelectric materials Their structure can be described as a combination of two types of germanium polyhedra, namely two small dodecahedra and six larger tetrakaidecahedra, per unit cell (Fig. 1). Additional analysis of the electron density topology properties within Bader’s atoms-in-molecules approach was carried out for Ba8Ga6Ge40, Ba8Zn6Ge40, Ba8Cu3Zn3Ge40, Ba8Cu6Ge40, and Ba8Ag6Ge40
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