Synthesis, crystal structure and optical properties of the new tantalum polysulfide Rb6Ta4S25 and its thermal degradation to Rb6Ta4S22

The reaction of Rb2S3, Ta and S at 723 K yields red-orange crystals of the new ternary compound Rb6Ta4S25. It crystallizes in the monoclinic space gro up C2/c (No. 15) with a = 36.943(2), b = 8.1028(3), c = 12.6415 (8) Angstro m, beta = 98.858 (7)degrees, V = 3739.0(4) Angstrom(3) and Z = 4. In the cr ystal structure two Ta2S11 units are connected by a S-3 chain forming the [ Ta4S25](6-) anion. Each Ta5+ ion is surrounded by seven sulfur ions forming a strongly distorted pentagonal bipyramid. The coordination mode around th e Ta5+ centers may be formulated as [(Ta-2(mu(2)-eta(2),eta(1)-S-2)(3)(eta( 2)-S-2)(S)(2))(2)(mu(2)-eta(1),-eta(1)-S-5)](6-). The anions are stacked pa rallel to the crystallographic b axis and are separated by the Rb+ ions. Wi th W-Vis spectroscopic investigations the optical band gap of 2.35 eV was d etermined. Measurements using differential-scanning calorimetry show an irr eversible loss of three sulfur atoms at 681 K leading to the formation of R b6Ta4S22. (C) 2000 Editions scientifiques et medicales Elsevier SAS. All ri ghts reserved.