Cu_0.66EuTe₂, KCu₂EuTe₄, and Na_0.2Ag_2.8EuTe₄: Compounds with modulated square Te nets

Reactions of europium with copper or silver in molten alkali metal/polytelluride fluxes have produced the new polytelluride compounds, Cu_0.66EuTe₂, KCu₂EuTe₄, and Na_0.2Ag_2.8EuTe₄. All three compounds are stable in air, and crystallize as red-brown plates in tetragonal space groups. Cu_0.66EuTe₂ crystallizes in the. space group P4lnmm with a=4.481(2), c= 10.260(3)Å (at 23°C), and Z=2. KCu₂EuTe₄, and Na_0.2Ag_2.8EuTe₄ are isostructural, and crystallize in the space group P4mm with 0 = 4.4365(6), c= 11.365(2) Å (at 23°C) for KCu₂EuTe₄, and a = 4.4544(6), c= 11.106(2) A (at 23 °C) for Na_0.2Ag_2.8EuTe₄. Cu_0.66EuTe₂ adopts the CaMnBi₂ structure-type with square antiprismatic europium atoms sandwiched between an anti-PbO type layer of [CuTe]^-, and a flat square net of tellurium. The structure of KCu₂EuTe₄ can be derived from that of Cu_0.66EuTe₂ by systematically replacing half of the europium atoms in the framework with potassium. Electron diffraction studies on KCu₂EuTe₄, and Na_0.2Ag_2.8EuTe₄ suggest a la × 7b superlattice, the result of a charge density wave (CDW) distortion in the square Te net of these compounds. KCu₂EuTe₄ exhibits semimetallic behavior while Na_0.2Ag_2.8EuTe₄ is a p-type semiconductor with a bandgap of 0.24 eV.