3. Narrow-gap semiconductors with complex framework structures

The general goal of this project line is the synthesis and characterisation of new s-p bonded narrow-gap semiconductors with possibly interesting thermoelectric properties. Very careful crystallographic studies have to be performed in order to unambiguously determine structure and composition of the obtained materials. This is essential for the proper interpretation of the different physical properties, which define thermoelectric efficiency.

Our current efforts have been centred round the peculiar compounds in the binary system Zn-Sb. In 1997 b -Zn 4Sb 3 was recognised as an outstanding thermoelectric material mainly due to its extraordinarily low thermal conductivity, which is similar to that of glasses [ T. Caillat, J.-P. Fleurial, A. Borshchevsky, J. Phys. Chem. Solids1997, 58, 1119 ]. The glass-like behaviour of b -Zn 4Sb 3 has been recently explained by a high degree of disorder in the Zn substructure [ G. J. Snyder, M. Christensen, E. Nishibori , T. Caillat, B. B. Iversen,, Nat. Mater.2004, 3, 458]. Lately, we could characterise the complex structure of the low-temperature phase a -Zn 4Sb 3 and by that unambiguously identify the composition of the disordered b -phase. Currently we are working on the controlled doping of zinc antimonides and the synthesis of derivatives. Zinc antimonides and its derivatives can be considered as “electron-poor framework semiconductors” and are distinguished by complex crystal structures as a consequence of multi-centred bonding patterns. This is in contrast to more electron-rich III-V narrow-gap semiconductors (e.g., GaAs, InSb) with simple tetrahedral frameworks (“electron-precise” semiconductors). Structural complexity is considered to be an important ingredient for a promising thermoelectrics. Our preliminary study indicate that this complexity can be enormous for electron-poor framework semiconductors, as manifested in frequently observed weak super-structure ordering, incommensurate modulations and diffuse scattering.

Fig.4: (a) Electronic structure and section of the crystal structure of narrow-gap semiconducting a -Zn 4Sb 3. (b) X-ray diffraction study of the a - b transition in Zn 4Sb 3 around 240 K. (c) Resistivity and thermopower along the a - b transition in Bi-doped Zn 4Sb 3. Note that the transition temperature (215 K) is shifted notably downward compared to the undoped material. Additionally r is decreased by a factor 2.

A. Tengå, F. J. García García, A. S. Mikhaylushkin, B. Espinosa-Arronte, M. Andersson, U. Häussermann “Chalcopyrite-Sphalerite Polymorphism in Semimetallic ZnSnSb 2” Chem. Mater., 17 (2005), published online.

A. S. Mikhaylushkin, J. Nylén, U. Häussermann “Structure and Bonding of Zinc Antimonides - Complex Frameworks and Narrow Band Gaps” Chem. Eur. J., 11 (2005), 4912.

J. Nylén, M. Andersson, S. Lidin , U. Häussermann “The Structure of a -Zn 4Sb 3: Ordering of the Phonon-Glass Thermoelectric Material b -Zn 4Sb 3” J. Am. Chem. Soc., 126 (2004), 1606.