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Crystal structure and thermoelectric properties of the type-I clathrate compound with an ordered arrangement of Ge vacancies
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Image of FIG. 1.
FIG. 1.

Crystal structure of type-I clathrate compounds. A dodecahedron centered at the body center of the unit cell and one of its neighboring tetrakaidekahedra are indicated in the figure. The atoms in the sites are indicated with shaded circles.

Image of FIG. 2.
FIG. 2.

SAED patterns of taken along [001], [101], [111], and [112] directions. The upper row [(a)–(d)] corresponds to experimental patterns, while the lower row [(e)–(h)] corresponds to calculated ones on the assumption that Ge vacancies in sites are randomly distributed so as to maintain the space group of . Reflections indicated by arrows and double arrows in (a)–(d) are, respectively, superlattice reflections and forbidden reflections that appear as “double” reflections.

Image of FIG. 3.
FIG. 3.

SAED patterns of taken by tilting the thin foil specimen from [111] to - and -type zone-axis orientations, showing the threefold symmetry of the [111] direction.

Image of FIG. 4.
FIG. 4.

Group-subgroup relation for the parent space group of .

Image of FIG. 5.
FIG. 5.

Possible vacancy positions on the planes of and in the superlattice unit cell with the space group of .

Image of FIG. 6.
FIG. 6.

Experimental HRTEM images of taken along the [111] direction with five different defocus values [(a)–(e)]. Corresponding HRTEM images calculated for the structure models with Ge vacancies in sites [(f)–(j)] and in sites [(k)–(o)].

Image of FIG. 7.
FIG. 7.

[111] projection of and sites; (a) and (b) are for models with Ge vacancies in sites and sites, respectively.

Image of FIG. 8.
FIG. 8.

(a) Four different subunit cells with different distributions of Ge vacancies in sites of the usual type-I clathrate compound. (b) Unit cell of corresponding to a superlattice formed by stacking eight subunit cells shown in (a).

Image of FIG. 9.
FIG. 9.

APB’s observed in quenched from 790 °C: (a) dark-field image formed with a 112-type superlattice reflection and (b) HRTEM image taken along [111].

Image of FIG. 10.
FIG. 10.

(a) Electrical resistivity, (b) Seebeck coefficient, and (c) thermal conductivity of plotted as a function of temperature.

Image of FIG. 11.
FIG. 11.

SEM backscattered electron image of the specimen after the measurement, indicating the phase decomposition of into (gray) and Ge (black) phases.

Image of FIG. 12.
FIG. 12.

Dimensionless figure of merit ZT of plotted as a function of temperature.

Image of FIG. 13.
FIG. 13.

Schematic illustrations of density-of-state curves for (a) disordered and (b) ordered structures.


Generic image for table
Table I.

Indices of observed reflections in the SAED patterns [Figs. 2(a)–2(d)] and those of possible reflections for the five structure models with the space group of , , , , and .


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Crystal structure and thermoelectric properties of the type-I clathrate compound Ba8Ge43 with an ordered arrangement of Ge vacancies