New layered ternary niobium tellurides: Synthesis, structure, and properties of NbMTe2 (M = Fe, Co)

The layered ternary niobium tellurides NbMTe₂ (M = Fe, Co) have been synthesized by high-temperature solid-state reactions. The structures were determined by single-crystal X-ray diffraction methods, and both were found to be orthorhombic. NbFeTe₂ crystallizes in the space group Pmna (No. 53) with Z = 4, a = 7.922 (1) Å, c = 7.239 (1) Å, c = 6.243 (1) Å, V = 357.98 (9) ų, and R = 4.11%, R_w = 5.18%. NbCoTe₂ crystallizes in the space group Cmca (No. 64) with Z = 8, a = 7.840 (1) Å, b = 14.431 (2) Å, c = 6.237 (1) Å, V = 705.7 (2) ų, and R = 2.92%, R_w = 3.46%. The related NbMTe₂ compounds have a layered framework structure, consisting of a plane of distorted honeycombs of Nb (Nb-Nb = 3.187-3.213 Å), with pairs of M atoms (M-M = 2.488 Å in NbFeTe₂ and M-M = 2.499 Å in NbCoTe₂) sitting at the center of every Nb hexagon (one M above and one M below the Nb plane). The metal layers are capped above and below by tellurium atoms, leading to a tetrahedral coordination for M and a pseudooctahedral environment for Nb. The shortest interlayer Te-Te distance, 3.8 Å, is typical of van der Waals bonding between Te layers. Temperature-dependent conductivity and susceptibility measurements indicate metallic and Pauli-paramagnetic behavior of NbCoTe₂. NbFeTe₂ is metallic, and Fe has a low magnetic moment of 3.0 μ_B/Fe. The magnetic data suggest ferromagnetic intralayer interactions but antiferromagnetic interlayer interactions leading to a T_N = 20 K.