The thioantimonate anion [Sb4S8]4− acting as a tetradentate ligand: Solvothermal synthesis, crystal structure and properties of {[In(C6H14N2)2]2Sb4S8}Cl2 exhibiting unusual uniaxial negative and biaxial positive thermal expansion

The new compound {[In(C6H14N2)2]2Sb4S8}Cl2 was prepared under solvothermal conditions reacting InCl3, Sb and S using 1,2-trans-diaminocyclohexane as solvent and structure directing molecule. The compound crystallizes in the monoclinic space group C2/c with a = 29.0259(12), b = 6.7896(2), c = 24.2023(12) Å, β = 99.524(4)°, V = 4703.9(3) Å3. The central structural motif is the thioantimonate(III) anion [Sb4S8]4− acting as a tetradentate ligand thus joining two symmetry related In3+ centered complexes. This binding mode was never observed before for the [Sb4S8]4− anion. The optical band gap was determined as 2.03 eV in agreement with the red color of the compound. The thermal decomposition was monitored with in-situ X-ray diffraction experiments. After the emission of the amine molecules an amorphous intermediate is formed followed by the crystallization of InSbS3 which is stable up to about 590 °C. On further heating, InSbS3 is destroyed and reflections of γ-In2S3 appear being contaminated with some elemental Sb. Temperature dependent in-situ X-ray powder diffractometry performed between 30 and 220 °C reveals an unusual reversible negative and positive thermal expansion. The decrease of the a-axis in the temperature range is about 0.74 Å and the increase of the c-axis ca. 0.54 Å. Interestingly, the b-axis exhibits also a thermal expansion, i.e., a biaxial positive and an uniaxial negative thermal expansion coexist which is very unusual. The relative negative expansion coefficients for the a-axis of −194 × 10−6K−1 (30–120 °C) and −82 × 10−6K−1 (120–220 °C) are in the region of so-called colossal thermal expansion.

The new compound {[In(C6H14N2)2]2Sb4S8}Cl2 exhibits an unusual large reversible biaxial negative and uniaxial positive thermal expansion.Figure optionsDownload as PowerPoint slide