Preparation and transport properties of new ternary tellurides A_xNb₆Te₈ (A = Tl, Ca, Sr, Ba, La, and Nd)

New ternary niobium tellurides A_xNb₆Te₈ (A = Ca, Sr, Ba, La, and Nd; x = 0.22-0.32) have been prepared by molten salt ion exchange reactions of Tl_xNb₆Te₈ with rare earth/alkaline earth iodides AI₃/AI₂ at relatively low temperatures, 535-795°C. Most of the niobium tellurides show metallic behavior followed by superconductivity at low temperatures, 4-6 K. Polycrystalline La_0.32Nb₆Te₈ and Nd_0.24Nb₆Te₈ show a resistivity anomaly at 46 and 43 K, respectively, while polycrystalline Tl_0.91Nb₆Te₈ shows a broad resistivity anomaly at 160-220 K. The resistivity anomaly observed in Tl_0.91Nb₆Te₈ appears to be accompanied by a minor structural distortion evidenced in differential scanning calorimetry. The resistivity anomalies observed in La_0.32Nb₆Te₈ and Nd_0.24Nb₆Te₈ are similar to those observed at ∼40 K for Nb₃Te₄ and may be related to the onset of a charge density wave type instability.