Synthesis, crystal structure and optical properties of the new tantalum polysulfide Rb_6Ta_4S_(25) and its thermal degradation to Rb_6Ta_4S_(22)

摘要

The reaction of Rb_2S_3, Ta and S at 723 K yields red-orange crystals of the new ternary compound Rb_6Ta_4S_(25). It crystallizes in the monoclinic space group C2/c (No.15) with a = 36.943(2), b = 8.1028(3), C = 12.6415 (8) A, #beta#=98.858 (7) deg, V= 3739.0(4) A~3 and Z = 4. In the crystal structure two Ta_2S_(11) units are connected by a S_3 chain forming the (Ta_4S_(25))~(6-) anion. Each Ta~(5+) ion is surrounded by seven sulfur ions forming a strongly distorted pentagonal bipyramid. The coordination mode around the Ta~(5+) 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 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 Rb_6Ta_4S_(22).