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5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Occupational disorder as the origin of flattening of the acoustic phonon branches in the clathrate Ba 8 ⁢Ga 16 ⁢Ge 30

In the search for high-performance thermoelectrics, materials such as clathrates have drawn attention due to having both glasslike low phonon thermal conductivity and crystal-like high electrical conductivity. Ba 8 ⁢Ga 16 ⁢Ge 30 (BGG) has a loosely bound guest Ba atom trapped inside rigid Ga-Ge cage structures. Avoided crossings between acoustic phonons and the flat guest atom branches have been proposed to be the source of the low lattice thermal conductivity of BGG. Ga-Ge site disorder with Ga and Ge exchanging places in different unit cells has also been reported. We used time-of-flight neutron scattering to measure the complete phonon spectrum in a large single crystal of BGG and compared these results with predictions of density functional theory to elucidate the effect of the disorder on heat-carrying phonons. Experimental results agreed much better with the calculation assuming the disorder than with the calculation assuming the ordered configuration. Although the atomic masses of Ga and Ge are nearly identical, we found that disorder strongly reduces phonon group velocities, which significantly reduces thermal conductivity. Finally, our work points to a path towards optimizing thermoelectrics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Materials Data on Ga3Ge by Materials Project

Ga3Ge is Uranium Silicide structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Ga is bonded to eight equivalent Ga and four equivalent Ge atoms to form GaGa8Ge4 cuboctahedra that share corners with twelve equivalent GaGa8Ge4 cuboctahedra, edges with eight equivalent GeGa12 cuboctahedra, edges with sixteen equivalent GaGa8Ge4 cuboctahedra, faces with four equivalent GeGa12 cuboctahedra, and faces with fourteen equivalent GaGa8Ge4 cuboctahedra. All Ga–Ga bond lengths are 2.99 Å. All Ga–Ge bond lengths are 2.99 Å. Ge is bonded to twelve equivalent Ga atoms to form GeGa12 cuboctahedra that share corners with twelve equivalent GeGa12 cuboctahedra, edges with twenty-four equivalent GaGa8Ge4 cuboctahedra, faces with six equivalent GeGa12 cuboctahedra, and faces with twelve equivalent GaGa8Ge4 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on GaGe3 by Materials Project

GaGe3 is Uranium Silicide structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Ga is bonded to twelve equivalent Ge atoms to form GaGe12 cuboctahedra that share corners with twelve equivalent GaGe12 cuboctahedra, edges with twenty-four equivalent GeGa4Ge8 cuboctahedra, faces with six equivalent GaGe12 cuboctahedra, and faces with twelve equivalent GeGa4Ge8 cuboctahedra. All Ga–Ge bond lengths are 3.03 Å. Ge is bonded to four equivalent Ga and eight equivalent Ge atoms to form distorted GeGa4Ge8 cuboctahedra that share corners with twelve equivalent GeGa4Ge8 cuboctahedra, edges with eight equivalent GaGe12 cuboctahedra, edges with sixteen equivalent GeGa4Ge8 cuboctahedra, faces with four equivalent GaGe12 cuboctahedra, and faces with fourteen equivalent GeGa4Ge8 cuboctahedra. All Ge–Ge bond lengths are 3.03 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ga3Ge by Materials Project

Ga3Ge crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. there are two inequivalent Ga sites. In the first Ga site, Ga is bonded to eight Ga and four equivalent Ge atoms to form distorted GaGa8Ge4 cuboctahedra that share corners with four equivalent GeGa12 cuboctahedra, corners with fourteen GaGa8Ge4 cuboctahedra, edges with six equivalent GeGa12 cuboctahedra, edges with twelve GaGa8Ge4 cuboctahedra, faces with four equivalent GeGa12 cuboctahedra, and faces with sixteen GaGa8Ge4 cuboctahedra. There are a spread of Ga–Ga bond distances ranging from 2.96–3.09 Å. There are two shorter (2.96 Å) and two longer (3.09 Å) Ga–Ge bond lengths. In the second Ga site, Ga is bonded to eight equivalent Ga and four equivalent Ge atoms to form distorted GaGa8Ge4 cuboctahedra that share corners with four equivalent GeGa12 cuboctahedra, corners with fourteen GaGa8Ge4 cuboctahedra, edges with six equivalent GeGa12 cuboctahedra, edges with twelve equivalent GaGa8Ge4 cuboctahedra, faces with four equivalent GeGa12 cuboctahedra, and faces with sixteen GaGa8Ge4 cuboctahedra. There are two shorter (2.96 Å) and two longer (3.07 Å) Ga–Ge bond lengths. Ge is bonded to twelve Ga atoms to form GeGa12 cuboctahedra that share corners with six equivalent GeGa12 cuboctahedra, corners with twelve GaGa8Ge4 cuboctahedra, edges with eighteen GaGa8Ge4 cuboctahedra, faces with eight equivalent GeGa12 cuboctahedra, and faces with twelve GaGa8Ge4 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on GaGe3 by Materials Project

GaGe3 crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Ga is bonded to twelve Ge atoms to form GaGe12 cuboctahedra that share corners with six equivalent GaGe12 cuboctahedra, corners with twelve equivalent GeGa4Ge8 cuboctahedra, edges with eighteen GeGa4Ge8 cuboctahedra, faces with eight equivalent GaGe12 cuboctahedra, and faces with twelve GeGa4Ge8 cuboctahedra. There are six shorter (2.99 Å) and six longer (3.07 Å) Ga–Ge bond lengths. There are two inequivalent Ge sites. In the first Ge site, Ge is bonded to four equivalent Ga and eight Ge atoms to form distorted GeGa4Ge8 cuboctahedra that share corners with four equivalent GaGe12 cuboctahedra, corners with fourteen GeGa4Ge8 cuboctahedra, edges with six equivalent GaGe12 cuboctahedra, edges with twelve GeGa4Ge8 cuboctahedra, faces with four equivalent GaGe12 cuboctahedra, and faces with sixteen GeGa4Ge8 cuboctahedra. There are a spread of Ge–Ge bond distances ranging from 2.94–3.05 Å. In the second Ge site, Ge is bonded to four equivalent Ga and eight equivalent Ge atoms to form distorted GeGa4Ge8 cuboctahedra that share corners with four equivalent GaGe12 cuboctahedra, corners with fourteen GeGa4Ge8 cuboctahedra, edges with six equivalent GaGe12 cuboctahedra, edges with twelve equivalent GeGa4Ge8 cuboctahedra, faces with four equivalent GaGe12 cuboctahedra, and faces with sixteen GeGa4Ge8 cuboctahedra.

36 MATERIALS SCIENCE↗