Regular and Modulated Tantalum Seleno-Chlorides: Synthesis and Structure of Chain Compounds Ta ₄ Se ₁₆ (TaCl ₆ ) ₂ and (TaSe ₄ ) ₁₈ (TaCl ₆ ) ₁₀

The first tantalum seleno-chlorides were synthesized in two modifications: Ta 4 Se 16 (TaCl 6 ) 2 ( 1 ) and an incommensurately modulated (TaSe 4 ) 18 (TaCl 6 ) 10 ( 2 ). Both phases contain infinite cationic chains (TaSe 4 ) x+ . 1 was obtained by heating a mixture of TaSe 3, Se, and Ta 2 Cl 10 in a two-zone ampule with a 350/340 °C gradient. 2 was obtained in a similar way using TaSe 2 instead of TaSe 3 . The crystal structure of compound 1 (space group P -1, unit cell parameters a = 12.2375(3) Å, b = 12.7820(4) Å, c = 13.6349(4) Å, α = 66.633(1)°, β = 66.310(1)°, γ = 78.725(1)°, Z = 4, V = 1791.20(9) Å 3 ) includes positively charged chains {TaSe 4 } 0.5+ and molecular anionic complexes [TaCl 6 ] − . In the chains {TaSe 4 }, the Ta···Ta distances vary in the sequence: ···3.184; 3.112; 3.152; 3.335 Å···. The crystal structure of compound 2 (orthorhombic cell, a = 3.1555(3) Å, b = 16.077(2) Å, c = 19.0654(8) Å, V = 967 Å 3, modulation vector q = 0.278· a *) is incommensurately modulated. We established the X-centering of the orthorhombic cell, evident from the extinction pattern, and the helical pattern of {TaSe 4 } chains, confirmed by the reported structure solution in the 18× a supercell. The structure of 2 is characterized by an average Ta···Ta distance of 3.16 Å in the infinite chains {TaSe 4 }. In this work, we discuss the synthesis and characterization of compounds 1 and 2, their crystal structures, and the magnetic and resistivity measurements. The discussion of experimental data is supported by DFT estimation of enthalpies (T = 0).