Crystal structure and thermoelectric properties of the type-I clathrate compound Ba8Ge43 with an ordered arrangement of Ge vacancies

Abstract

The crystal structure of the type-I clathrate compound Ba8Ge43 has been investigated by x-ray diffraction and transmission electron microscopy. The thermoelectric properties of Ba8Ge43 have also been investigated. The crystal structure of Ba8Ge43 is different from that reported for the usual type-I clathrate compounds with the space group of Pm3¯n but is a superlattice structure based on the usual type-I clathrate structure due to the ordering of Ge vacancies in half the 6c sites of the usual type-I clathrate structure. The crystal structure of Ba8Ge43 belongs to the space group of Ia3¯d and Ge vacancies exclusively occupy the 24c sites. The thermoelectric properties of Ba8Ge43 are not particularly good, as exemplified by the rather low ZT value of 0.057 because of the high value of electrical resistivity, which may arise from the existence of Ge vacancies.

Thermoelectric Properties of the Clathrate I Ba8Ge43□3

“…3a, b, respectively. The first essential outcome of these measurements is the observation of metallike electrical resistivity below 280 K. Resistivity values are two orders of magnitude lower than those previously reported, 5 corroborating the high quality of the samples. The thermopower exhibits a rather complex behavior with two sign changes: from n-type to p-type near 90 K (Fig.…”

“…The observed semiconducting-like behavior above room temperature is in good agreement with the previously reported result. 5 The origin of the observed transition, however, must await further structure investigations to determine whether or not an intimate link between possible crystal structure modifications and this electronic transition exists. …”

“…Experimentally, it was found that the compound shows semiconducting behavior. 5,6 The apparent discrepancy from the simple valence rules has been tentatively ascribed to an ordering of the framework vacancies resulting in modifications of the electronic band structure near the Fermi level. 5 On the other hand, investigations on the ternary system Ba 8Àx Eu x Ge 43 (x < 0.6) revealed metallic behavior for the whole homogeneity range.…”

“…5,6 The apparent discrepancy from the simple valence rules has been tentatively ascribed to an ordering of the framework vacancies resulting in modifications of the electronic band structure near the Fermi level. 5 On the other hand, investigations on the ternary system Ba 8Àx Eu x Ge 43 (x < 0.6) revealed metallic behavior for the whole homogeneity range. 7 Here we present detailed transport properties measurements of Ba 8 Ge 43 h 3 including electrical resistivity, thermopower, and thermal conductivity in the 5 K to 673 K temperature range.…”

“…The interaction between cage atoms and the host framework is a key property for understanding the thermoelectric properties of these compounds. So far, the highest thermoelectric figure of merit for a clathrate compound was reported for Ba 8 Ga 16 Ge 30 with ZT % 1.35 at 900 K. 2 In comparison, the binary archetype Ba 8 Ge 43 h 3 3,4 displays only very low ZT values, e.g., 0.06 at 693 K. 5 However, complete characterization of Ba 8 Ge 43 h 3 is important to identify the most promising clathrates among the variety of possible compositions.…”