Synthesis and Crystal Structure of Binary Tantalum Polyselenides Ta₄Se₁₇ and Ta₄Se₁₈

Two crystalline tantalum polyselenides, Ta₄Se₁₇ and Ta₄Se₁₈, were synthesized in reactions of the elements at temperatures of 360-380 °C. Crystals of Ta₄Se₁₇ (1) were obtained by heating of a mixture of Ta/Se at a molar ratio of 1:6 at 380 °C, while crystals of Ta₄Se₁₈ (2) were produced from the same mixture in the presence of either ammonium chloride or ammonium bromide at 360 °C. The crystal structure of Ta₄Se₁₇ (1) (space group C2, a = 11.680(5), b = 11.870(5), c = 7.793(4) Å, β = 100.36(2)°, Z = 2) is a packing of layers which consists of chains with a repeating sequence {-Ta(Se₂)₂-Ta(Se₂)₂-Ta(Se₂)₂-Ta(Se₂)(Se₃)-}, where Ta atoms are bridged by di- and triselenide groups, with Ta···Ta distances of 3.15, 3.05, 3.15, and 3.68 Å. These chains are connected in layers by means of Se-Se bonds in the (Se₃) groups, which form infinite chains of Se atoms. The crystal structure of Ta₄Se₁₈ (2) (space group I422, a = 9.2876(2), c = 12.7133(3) Å, Z = 2) is a packing of chains {Ta(Se₂)₂}^1/2+ and anions (Se₂)^2-, the interatomic distances Ta···Ta in the chains are all 3.18 Å. The experimental observations were supported by DFT calculations of the formation enthalpies of the binary tantalum selenides. The syntheses, structures, and stability of Ta₄Se₁₇ and Ta₄Se₁₈ are discussed in this paper.