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

Abstract

Reactions of europium with copper or silver in molten alkali metal/polytelluride fluxes have produced the new polytelluride compounds, Cu _0.66 EuTe ₂ , KCu ₂ EuTe ₄ and Na _0.2 Ag _2.8 EuTe ₄ . All three compounds are stable in air and crystallize as red–brown plates in tetragonal space groups. Cu _0.66 EuTe ₂ crystallizes in the space groupP4/nmm with a = 4.481(2), c = 10.260(3) Å (at 23 °C) and Z = 2. KCu ₂ EuTe ₄ and Na _0.2 Ag _2.8 EuTe ₄ are isostructural and crystallize in the space group P4mm with a = 4.4365(6), c = 11.365(2) Å (at 23 °C) for KCu ₂ EuTe ₄ , and a = 4.4544(6), c = 11.106(2) Å (at 23 °C) for Na _0.2 Ag _2.8 EuTe ₄ . Cu _0.66 EuTe ₂ adopts the CaMnBi ₂ 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 KCu ₂ EuTe ₄ can be derived from that of Cu _0.66 EuTe ₂ by systematically replacing half of the europium atoms in the framework with potassium. Electron diffraction studies on KCu ₂ EuTe ₄ and Na _0.2 Ag _2.8 EuTe ₄ suggest a 1a × 7b superlattice, the result of a charge density wave (CDW) distortion in the square Te net of these compounds. KCu ₂ EuTe ₄ exhibits semimetallic behavior while Na _0.2 Ag _2.8 EuTe ₄ is a p-type semiconductor with a bandgap of 0.24 eV.