Hydride Synthesis and Thermoelectric Properties of Type-I Clathrate K₈E₈Ge₃₈ (E = Al, Ga, In)

Abstract

Type I clathrates of the composition K8E8Ge38 (E = Al, Ga, In) were prepared via the reaction of KH with E and Ge and thermoelectric properties measured in order to compare to K8Al8Si38, a promising thermoelectric material. The structures were confirmed with Rietveld refinement of powder diffraction patterns obtaining lattice parameters of 10.7729(2) Å, 10.7469(5) Å, and 10.9975(6) Å for E = Al, Ga, and In, respectively. Samples of K8E8Ge38 with E = Al and Ga were consolidated via spark plasma sintering for property measurements and determined to be 94.2% and 81.4% dense, respectively. K8In8Ge38 showed significant decomposition after sintering with both elemental In and Ge present in the powder diffraction pattern. The thermoelectric properties of K8E8Ge38 (E = Al, Ga) from 300–10 K were measured on sintered pellets. K8Al8Ge38 was found to have a Seebeck coefficient, electrical resistivity, and thermal conductivity of −35.8 μV/K, 2.56 mΩ·cm, 1.37 W/m·K at 300 K, respectively. K8Ga8Ge38 was found to be a compensated semiconductor with a Seebeck coefficient, electrical resistivity, and thermal conductivity of 4.19 μV/K, 1080 mΩ·cm, and 1.05 W/m·K at 300 K, respectively. The resistivity of K8Al8Ge38 is 46 times lower than K8Al8Si38 which has a Seebeck coefficient of −90.0 μV/K and thermal conductivity of 1.77 W/mK at 300 K, suggesting that a solid solution of K8Al8Si38‑xGe x has potential for optimal thermoelectric performance.

Computationally Driven Discovery of a Family of Layered LiNiB Polymorphs

“…[10] Boron atoms in both RT-a nd HT-LiNiB form pairs,w hich are further surrounded by 6Nia toms (Figures 3a nd 4). Consistent with the proposed structural models of RT-LiNiB,o nly as ingle 11 BNMR signal is observed. Thes pectrum of HT-LiNiB shows two 11 BNMR signals,b oth with shifts similar to that of RT-LiNiB.T he differences in topology of the [NiB] layers in RT-LiNiB and HT-LiNiB as well as the difference in boron coordination by Li most likely explain the variation in 11 Bshifts observed in the solid-state NMR spectra.…”

“…Consistent with the proposed structural models of RT-LiNiB,o nly as ingle 11 BNMR signal is observed. Thes pectrum of HT-LiNiB shows two 11 BNMR signals,b oth with shifts similar to that of RT-LiNiB.T he differences in topology of the [NiB] layers in RT-LiNiB and HT-LiNiB as well as the difference in boron coordination by Li most likely explain the variation in 11 Bshifts observed in the solid-state NMR spectra. However, two 11 BNMR signals are observed for HT-LiNiB,w hereas only one is expected based upon the crystal structure.T he additional 11 BNMR signal could arise from boride units that are near to stacking faults, which STEM images indicate are much more numerous in HT-LiNiB than RT-LiNiB.…”

“…Thes olid-state 11 BNMR spectra for RT-LiNiB and HT-LiNiB are consistent with their metallic behavior, and the 11 B chemical-shift range of 150-220 ppm is similar to the chemical shift of 170 ppm observed for LiNi 3 B 1.8 as well as for other metallic borides. [10] Boron atoms in both RT-a nd HT-LiNiB form pairs,w hich are further surrounded by 6Nia toms (Figures 3a nd 4).…”

“…Previously,weutilized alkali-metal hydrides AH(A = Li, Na, K) as mixable salt-like precursors for the fast experimental exploration of ternary systems containing alkali metals. [10,11] Herein, we report the computationally driven experimental discovery of two polymorphs of the novel transitionmetal boride LiNiB with au nique layered crystal structure. [10,11] Herein, we report the computationally driven experimental discovery of two polymorphs of the novel transitionmetal boride LiNiB with au nique layered crystal structure.…”

“…[10, 11a,b] Thehydride precursors remedy mixing issues,thus leading to faster reactions. [10,11] Herein, we report the computationally driven experimental discovery of two polymorphs of the novel transitionmetal boride LiNiB with au nique layered crystal structure. By using an adaptive genetic algorithm (AGA) for ab initio prediction of crystal structure [2,12] and afacile synthetic route from al ithium hydride precursor,t wo polymorphs of the novel layered LiNiB have been identified.…”