ABSTRACT
The layered compound Sn2.8(4) Bi20.2(4) Se27 exhibits an extraordinarily long-periodic 150R stacking sequence. The crystal structure contains three different building blocks, which form upon the addition of Sn to a Bi-rich bismuth selenide. Sn-doped Bi2 double ("2") layers similar to those in elemental bismuth, Sn0.3 Bi1.7 Se3 quintuple ("5") layers and Sn0.4 Bi2.6 Se4 septuple ("7") layers are arranged in a 7525757525|7525757525|7525757525 sequence, which corresponds to a structure with a=4.1819(4) and c=282.64(6)â Å in space group R 3 â¾ m. The structure of a microcrystal was determined using microfocused synchrotron radiation and refined as a formally commensurately modulated structure in (3+1)D superspace (superspace group R 3 â¾ m(00γ)00), with a trivial basic structure that contains just one atom. The stacking sequence as well as the cation distribution are confirmed by aberration-corrected scanning transmission electron microscopy (STEM) in combination with chemical mapping by X-ray spectroscopy with atomic resolution. Stacking faults are not typical but have been observed occasionally.
ABSTRACT
Pseudobinary phases (SnSe) xBi2Se3 exhibit a very diverse structural chemistry characterized by different building blocks, all of which are cutouts of the NaCl type. For SnSe contents between x = 5 and x = 0.5, several new phases were discovered. Next to, for example, Sn4Bi2Se7 ( x = 4) in the NaCl structure type and SnBi4Se7 ( x = 0.5) in the layered defect GeSb2Te4 structure type, there are at least four compounds (0.8 ≤ x ≤ 3) with lillianite-like structures built up from distorted NaCl-type slabs (L4,4-type Sn2.22Bi2.52Se6, L4,5-type Sn9.52Bi10.96Se26, L4,7-type Sn11.49Bi12.39Se30, and L7,7-type Sn3.6Bi3.6Se9). For two of them (L4,7 and L7,7), the cation distributions were determined by resonant X-ray scattering, which also confirmed the presence of significant amounts of cation vacancies. Thermoelectric figures of merit ZT range from 0.04 for Sn4Bi2Se7 to 0.2 for layered SnBi4Se7; this is similar to that of the related compounds SnBi2Te4 or PbBi2Te4. Compounds of the lillianite series exhibit rather low thermal conductivities (â¼0.75 W/mK for maximal ZT). More than other "sulfosalts", compounds in the pseudobinary system SnSe-Bi2Se3 adapt to changes in the cation-anion ratio by copying structure types of compounds containing lighter or heavier homologues of Sn, Bi, or Se and can incorporate significant amounts of vacancies. Thus, (SnSe) xBi2Se3 is a multipurpose model system with vast possibilities for substitutional and structural modification aiming at the optimization of thermoelectric or other properties.
ABSTRACT
Heterostructures that consist of a germanium antimony telluride matrix and cobalt germanide precipitates can be obtained by straightforward solid-state synthesis including simple annealing and quenching procedures. The microscale precipitates are homogeneously distributed in a matrix with pronounced "herringbone-like" nanostructure associated with very low thermal conductivities. In comparison to the corresponding pure tellurides, the figure of merit (ZT) values of heterostructured materials are remarkably higher. This is mostly due to an increase of the Seebeck coefficient with only little impact on the electrical conductivity. In addition, the phononic part of the thermal conductivity is significantly reduced in some of the materials. As a result, ZT values of ca. 1.9 at 450 °C are achieved. Temperature-dependent changes of the thermoelectric properties are well-understood and correlate with complex phase transitions of the telluride matrix. However, the high ZT values are retained in multiple measurement cycles.
