New Layered Zirconium Tellurides:  Zr_0.30ZrTe₂, Zr_0.29Zr₂Te₂As, and NaZr₂Te₂As

The synthesis and crystal structure determinations of Zr_0.30ZrTe₂ and M_xZr₂Te₂As (M = Zr, Na) compounds are reported. The structure of Zr_0.30ZrTe₂ was refined in the hexagonal space group P6₃mc (No. 186, Z = 2) with lattice parameters a = 3.9840(3) Å and c = 13.366(3) Å; Zr_0.29Zr₂Te₂As was refined in the rhombohedral space group R3̄m (No. 166, Z = 3) with lattice parameters a = 3.9329(4) Å and c = 29.564(5) Å. Zr_0.30ZrTe₂ and Zr_0.29Zr₂Te₂As have close structural similarities to Zr₂Se₃ and Ta₂S₂C, respectively, and are built up by stacking hexagonal layers with [Zr_0.30−Te−Zr−Te] and [Zr_0.29−Te−Zr−As−Zr−Te] sequences. Four-probe resistivity measurements (77−300 K) show both Zr_0.30ZrTe₂ and Zr_0.29Zr₂Te₂As to be metallic (Zr_0.29Zr₂Te₂As:  8.9 × 10^-5 Ω cm at 273 K). Both compounds exhibit structures wherein Zr atoms are included between layers (ZrTe₂ and Zr₂Te₂As) by partially filling trigonal antiprismatic holes. The replacement of the included Zr ions in Zr_0.29Zr₂Te₂As by Na ions has been demonstrated. Powder diffraction data showed that NaZr₂Te₂As is isostructural with Zr_0.29Zr₂Te₂As. By use of Rietveld refinements, sodium ions were found to reside in the trigonal antiprismatic sites between the layers. Extended Hückel band calculations on the [Zr₂Te₂As]^1.16- layer indicate that it should be a metallic conductor and that the [Zr₂Te₂As] layer can bear a greater negative charge than has so far been observed. We suggest that the [Zr₂Te₂As] layered compounds may offer new opportunities as electron-donating hosts.