The thioantimonate anion [Sb_4S_8]~(4-) acting as a tetradentate ligand: Solvothermal synthesis, crystal structure and properties of {[In(C_6H_(14)N_2)_2]_2Sb_4S_8}Cl_2 exhibiting unusual uniaxial negative and biaxial positive thermal expansion

摘要

The new compound {[In(C_6H_(14)N_2)_2]_2Sb_4S_8}Cl_2 was prepared under solvothermal conditions reacting InCl_3, 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) A, β = 99.524(4)°, V = 4703.9(3) A~3. The central structural motif is the thioantimonate(III) anion [Sb_4S_8]~(4-) acting as a tetradentate ligand thus joining two symmetry related In~(3+) centered complexes. This binding mode was never observed before for the [Sb_4S_8]~(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 InSbS_3 which is stable up to about 590 ℃. On further heating, InSbS_3 is destroyed and reflections of γ-In_2S_3 appear being contaminated with some elemental Sb. Temperature dependent in-situ X-ray powder diffractometry performed between 30 and 220 ℃ reveals an unusual reversible negative and positive thermal expansion. The decrease of the a-axis in the temperature range is about 0.74 A and the increase of the c-axis ca. 0.54 A. 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~(-6)K~(-1) (30-120 ℃) and -82 × 10~(-6)K~(-1) (120-220 ℃) are in the region of so-called colossal thermal expansion.