New Layered Ternary Chalcogenides: Ta2PdS6, Ta2PdSe6, Nb2PdS6, Nb2PdSe6

The new ternary transition-metal chalcogenides Ta₂PdS₆, Ta₂PdSe₆, Nb₂PdS₆, and Nb₂PdSe₆ have been prepared. The phases are isostructural. The structures of the phases Ta₂PdS₆ and Ta₂PdSe₆ have been established by single-crystal X-ray measurements. [formula omitted] for 1202 observations, 30 variables, [formula omitted] for 1201 observations, 30 variables, R(F) = 0.056 for [formula omitted]. The unit cell parameters of the phases Nb₂PdSe₆ and Nb₂PdSe have been determined from single-crystal and powder X-ray measurements, respectively. Nb₂PdSe₆ crystallizes in an I-centered cell of dimensions a = 10.368 (1) Å, b = 3.362 (1) Å, c = 12.132 (2) Å, β = 113.42 (2)°, V= 388.0 ų, and Z = 2. The compound Nb₂PdS₆ crystallizes in a cell of dimensions a = 9.99 (1) Å, b = 3.30 (1) Å, c = 11.69 (1) Å, β = 114.7 (1)°, and V= 350 ų. The phases form in a new laminar structural type. Each layer consists of collocated units [formula omitted] (M = Ta or Nb; Q = S or Se) that are composed of MQ₃ face-sharing trigonal-prismatic chains bridged by Pd atoms in a square-planar environment of Q atoms. Metrical details and results of electrical conductivity measurements on Ta₂PdQ₆ indicate the simple valence description Ta^V, Pd^II, Q^-II.