Theoretical and Experimental Study on Thermoelectric Properties of Ba8TM x Ga y Ge46–x–y (TM = Zn, Cu, Ag) Type I Clathrates

Abstract

In the type I clathrates Ba 8 TM x Ga y Ge 46-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 bandstructure 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 Ba 8 TM x Ga y Ge 46-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.

Suppression of vacancies boosts thermoelectric performance in type-I clathrates

“…Due to their similar X-ray scattering factors, the occupations of Cu, Ge, and Ga in Ba 8 Cu 4.8 Ga 1 Ge 40.2 are hard to derive from X-ray diffraction. Thus a number of different techniques 11,14,[17][18][19]23,24,[37][38][39] have been applied to determine structural details and several basic rules have been established. For instance, TM elements and possible vacancies preferentially occupy the 6c site 17,21,23,24,36,37,[40][41][42][43] and avoid direct 13 element bonding.…”

“…Thus a number of different techniques 11,14,[17][18][19]23,24,[37][38][39] have been applied to determine structural details and several basic rules have been established. For instance, TM elements and possible vacancies preferentially occupy the 6c site 17,21,23,24,36,37,[40][41][42][43] and avoid direct 13 element bonding. 14,18,19,38,44,45 Structural details can also be extracted from thermal parameters obtained by refining diffraction data or NMR measurements, 37,46 or from changes of interatomic distances induced by composition variations as shown here.…”

“…If there are more (less) valence electrons than needed to complete the bonding, the system is an n-type (a p-type) semiconductor. Thus, the composition as well as details of the crystal structure (e.g., whether there are vacancies and how atoms distribute in the crystal structure [17][18][19] ) are critical for the thermoelectric properties of clathrates. 11,16,20 So far, the type-I clathrates most promising for thermoelectric applications are Ba-Ga-Ge(Si)-based compounds with a Ga content around 16 atoms per unit cell (u.c.).…”

“…11 Attempts were then performed to change the atomic environment by elemental substitution, for instance with Sn (Ref. 24) or Ga. 17,25 Interesting results including improved charge carrier mobility and enhanced TE properties have been observed. 24,25 In the present work, we study variations of the Ga content in the framework of Ba 8 (Cu,Ga,Ge,2) 46 clathrates, and their effects on the structural and TE properties.…”

Crystal Chemistry and Thermoelectric Properties of Type-I Clathrate Ba8Ni∼3.8SixGe42.2−x (x = 0, 10, 20, 42.2)

“…Both electronic properties and thermal conductivity need to be optimized. A “cross-substitution” could improve these properties by more flexibly changing the electronic band structure and introducing more scattering centers [8,26,37,38,39,40,41,42,43,44,45,46,47]. It brings in the meantime interest for the fundamental investigation on chemical properties such as structural variations and stabilities.…”