Nonstoichiometry and chemical purity effects in thermoelectric Ba8Ga16Ge30 clathrate

Abstract

Zone melting purification experiments have been carried out on the clathrate, Ba8Ga16Ge30. The impurities present have been identified and their approximate concentrations measured. Trace impurities were determined to be approximately 240 parts per million (ppm) in the most impure sample to 17 ppm in the most pure sample. The temperature-dependent Seebeck coefficient, thermal conductivity, and electrical conductivity are reported as a function of sample purity as well as the room-temperature Hall coefficient. Microprobe analysis suggests that the samples are nonstoichiometric with excess Ge relative to Ga, and there are indications of the presence of defects. Single-crystal x-ray investigations as well as synchrotron powder diffraction measurements support the presence of defects, but the x-ray data cannot accurately determine the relative amounts of Ga and Ge. Band-structure calculations in the generalized gradient approximation show that the measured Hall and Seebeck coefficients are consistent with a defect lattice of approximate stoichiometry Ba8Ga14Ge31. Although the figure of merit (ZT) is found to be the highest for the purest sample, the dominant contribution to transport is conjectured to arise from deviations from the ideal stoichiometry and not impurities.

Thermal Stability and Phase Purity in Polycrystalline Ba8Ga x Ge46−x

“…The hysteresis above 1000 K is unaffected by the presence of Ge impurity. 9 The change in carrier concentration and constant mobility suggests that mainly the Ga/Ge ratio is affected. This might be attributed to: (1) oxidation during measurements and subsequent loss of BaO, which may lead to a changing Ga/Ge ratio; (2) hightemperature precipitation reactions; and/or (3) annealing effects during the thermal cycling.…”

“…This might be attributed to: (1) oxidation during measurements and subsequent loss of BaO, which may lead to a changing Ga/Ge ratio; (2) hightemperature precipitation reactions; and/or (3) annealing effects during the thermal cycling. For example, vacancies are known to exist on the cage sites 9 and their equilibrium concentration should depend on temperature. We do note that heating and cooling rates during measurements were faster and at lower temperature (200 K/h with T max = 1050 K and dwell time = 30 min) compared with the hot-pressing (1100 K with dwell time = 2 h).…”

“…In a similar study, Bryan et al 9 found that high purity in Ba 8 Ga 16 Ge 30 clathrates results in high ZT and emphasized that carrier concentration control is the most important factor that affects the transport properties. As carrier concentration control and phase stability are two of the primary concerns for utilizing Ba 8 Ga 16 Ge 30 , we have characterized the stability temperature for material both with and without Ge impurity.…”

“…11,12 According to previous studies on type I Ba 8 Ga 16 Ge 30 clathrates, heat treatment of some sort is necessary to remove the Ge impurity. 9,10,13,14 Generally, a small amount of Ge impurity ($1 wt.%) produced during the synthesis is unlikely to affect electron transport significantly. However, Ge loss may alter the electron concentration (via Ga:Ge ratio) in the matrix.…”

Structure and properties of Ba8Ga16Ge30 clathrates by a novel synthesis method using CO gas reductive atmosphere

“…In almost all of the researches on the clathrate materials, the compounds were prepared either by conventional solidification of melted bulk quantities, or by an arc-melting technique [5][6][7]. In either case, the starting materials used to be high purity elements of barium, gallium and germanium.…”