Cu0.66EuTe2, KCu2EuTe4 and Na0.2Ag2.8EuTe4: compounds with modulated square Te nets

Reactions of europium with copper or silver in molten alkali metal/polytell uride fluxes have produced the new polytelluride compounds, Cu0.66EuTe2, KC u2EuTe4 and Na0.2Ag2.8EuTe4. All three compounds are stable in air and crys tallize as red-brown plates in tetragonal space groups. Cu0.66EuTe2 crystal lizes in the space group P4/nmm with a=4.481(2), c=10.260(3) Angstrom (at 2 3 degrees C) and Z=2. KCu2EuTe4 and Na0.2Ag2.8EuTe4 are isostructural and c rystallize in the space group P4mm with a=4.4365(6), c=11.365(2) Angstrom ( at 23 degrees C) for KCu2EuTe4, and a=4.4544(6), c=11.106(2) Angstrom (at 2 3 degrees C) for Na0.2Ag2.8EuTe4. Cu0.66EuTe2 adopts the CaMnBi2 structure- type with square antiprismatic europium atoms sandwiched between an anti-Pb O type layer of [CuTe](-) and a flat square net of tellurium. The structure of KCu2EuTe4 can be derived from that of Cu0.66EuTe2 by systematically rep lacing half of the europium atoms in the framework with potassium. Electron diffraction studies on KCu2EuTe4 and Na0.2Ag2.8EuTe4 suggest a 1a x 7b sup erlattice, the result of a charge density wave (CDW) distortion in the squa re Te net of these compounds. KCu2EuTe4 exhibits semimetallic behavior whil e Na0.2Ag2.8EuTe4 is a p-type semiconductor with a bandgap of 0.24 eV.