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) angstrom, b = 7.239 (1) angstrom, c = 6.243 (1) angstrom, V = 357.98 (9) angstrom 3, and R = 4.11%, R(w) = 5.18%. NbCoTe2 crystallizes in the space group Cmca (No. 64) with Z = 8, a = 7.840 (1) angstrom, b = 14.431 (2) angstrom, c = 6.237 (1) angstrom, V = 705.7 (2) angstrom 3, 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 angstrom), with pairs of M atoms (M-M = 2.488 angstrom in NbFeTe2 and M-M = 2.499 angstrom 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 angstrom, 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-mu(B)/Fe. The magnetic data suggest ferromagnetic intralayer interactions but antiferromagnetic interlayer interactions leading to a T(N) almost-equal-to 20 K.