Syntheses, structures, and characterization of the two-dimensional tellurides CsCu₂MTe₄ (M = Nb, Ta)

The new compounds CsCu₂MTe₄ (M = Nb, Ta) have been synthesized at 900°C through the reaction of Cu and Nb or Ta with a Cs₂Te₃/Te melt as a reactive flux. Single-crystal X-ray diffraction measurements show that both compounds crystallize in the space group Pn2₁a of the orthorhombic system with four formula units in cells of dimensions a = 8.198(2) Å, b = 5.988(2) Å, c = 20.527(6) Å, V = 1007.7(6) ų at 113 K for M = Nb, and a = 8.217(5) Å, b = 5.992(5) Å, c = 20.562(14) Å, V = 1012(1) ų at 113 K for M = Ta. The two compounds are isostructural. The structure of CsCu₂NbTe₄ comprises infinite, two-dimensional layers of Cu-and Nb-centered tetrahedra; Cs⁺ cations separate the layers. Each Cs⁺ cation is coordinated by nine Te atoms at the corners of a highly distorted tricapped trigonal prism. Both phases are highly resistive.