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 group C2/c (No. 15) with a=36.943(2), b=8.1028(3), c=12.6415 (8) Å, β=98.858 (7)°, V=3739.0(4) ų and Z=4. In the crystal structure two Ta₂S₁₁ units are connected by a S₃ 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 the Ta⁵⁺ centers may be formulated as [(Ta₂(μ₂-η²,η¹-S₂)₃(η²-S₂)(S)₂)₂(μ₂-η¹,η¹-S₅)]⁶⁻. The anions are stacked parallel to the crystallographic b axis and are separated by the Rb⁺ ions. With UV–Vis spectroscopic investigations the optical band gap of 2.35 eV was determined. Measurements using differential-scanning calorimetry show an irreversible loss of three sulfur atoms at 681 K leading to the formation of Rb₆Ta₄S₂₂.