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.
SAED patterns of taken along , , , and  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.
SAED patterns of taken by tilting the thin foil specimen from  to - and -type zone-axis orientations, showing the threefold symmetry of the  direction.
Group-subgroup relation for the parent space group of .
Possible vacancy positions on the planes of and in the superlattice unit cell with the space group of .
Experimental HRTEM images of taken along the  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)].
 projection of and sites; (a) and (b) are for models with Ge vacancies in sites and sites, respectively.
(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).
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 .
(a) Electrical resistivity, (b) Seebeck coefficient, and (c) thermal conductivity of plotted as a function of temperature.
SEM backscattered electron image of the specimen after the measurement, indicating the phase decomposition of into (gray) and Ge (black) phases.
Dimensionless figure of merit ZT of plotted as a function of temperature.
Schematic illustrations of density-of-state curves for (a) disordered and (b) ordered structures.
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 .
Article metrics loading...
Full text loading...