Synthesis and physical properties of the new layered ternary tellurides MIrTe₄ (M = Nb, Ta), and the structure of NbIrTe₄

Two new ternary transition-metal chalcogenides, niobium iridium tetratelluride (NbIrTe₄) and tantalum iridium tetratelluride (TaIrTe₄), have been prepared by reaction of the elemental powders at 1000°C. The structure of NbIrTe₄ has been determined by single-crystal X-ray diffraction methods. The compound crystallizes in space group C⁷_2v-Pmn2₁ of the orthorhombic system with four formula units in a cell of dimensions a = 3.768(3), b = 12.486(10), c = 13.077(9) Å at 294 K. NbIrTe₄ is a layered compound with a structure closely related to those of WTe₂ and β-MoTe₂, variants of the CdI₂ structure type. The layers comprise buckled sheets of Te atoms, with the Nb and Ir atoms residing in distorted octahedral sites. Metal-metal bonding appears to be responsible for a close association of the Nb and Ir atoms. From Weissenberg photography, the compound TaIrTe₄ is found to be isostructural to NbIrTe₄, with cell dimensions a = 3.77(3), b = 12.37(6), c = 13.17(3) Å. Electrical resistivity measurements along the a axis of both compounds show that they are metallic: ρ{variant}₂₉₈ = 8.1 × 10^-5 and 1.2 × 10^-4 Ω cm for NbIrTe₄ and TaIrTe₄, respectively. Magnetic susceptibility measurements indicate essentially temperature-independent Pauli paramagnetism for both compounds: χ_m = 1.9 × 10^-3 and 8.9 × 10^-4 emu mol^-1 for NbIrTe₄ and TaIrTe₄, respectively. The compounds NbIrTe₄ and TaIrTe₄ appear to belong to a larger class of compounds MM′Te₄ with M = Nb, Ta and M′ = Ru, Os, Rh, Ir.