NEW LAYERED ZIRCONIUM TELLURIDES - ZR0.30ZRTE2, ZR0.29ZR2TE2AS, AND NAZR2TE2AS

The synthesis and crystal structure determinations of Zr0.30ZrTe2 and MxZr2Te2As (M = Zr, Na) compounds are reported. The structure of Zr0.3 0ZrTe2 was refined in the hexagonal space group P6(3)mc (NO. 186, Z = 2) with lattice parameters a = 3.9840(3) Angstrom and c = 13.366(3) An gstrom; Zr0.29Zr2Te2As was refined in the rhombohedral space group R ( 3) over bar m (No. 166, Z = 3) with lattice parameters a = 3.9329(4) A ngstrom and c = 29.564(5) Angstrom. Zr0.29ZrTe2 and Zr0.29Zr2Te2As hav e close structural similarities to Zr2Se3 and Ta2S2C, respectively, an d are built up by stacking hexagonal layers with [Zr-0.30-Te-Zr-Te] an d [Zr-0.29-Te-Zr-As-Zr-Te] sequences. Four-probe resistivity measureme nts (77-300 K) show both Zr0.30ZrTe2 and Zr0.29Zr2Te2As to be metallic (Zr0.29Zr2Te2As: 8.9 x 10(-5) Omega cm at 273 K). Both compounds exhi bit structures wherein Zr atoms are included between layers (ZrTe2 and Zr2Te2As) by partially filling trigonal antiprismatic holes. The repl acement of the included Zr ions in Zr0.29Zr2Te2As by Na ions has been demonstrated. Powder diffraction data showed that NaZr2Te2As is isostr uctural with Zr0.29Zr2Te2As. By use of Rietveld refinements, sodium io ns were found to reside in the trigonal antiprismatic sites between th e layers. Extended Huckel band calculations on the [Zr2Te2As](1.16-) l ayer indicate that it should be a metallic conductor and that the [Zr2 Te2As] layer can bear a greater negative charge than has so far been o bserved. We suggest that the [Zr2Te2As] layered compounds may offer ne w opportunities as electron-donating hosts.