Structure and physical properties of the new telluride BaAg2Te2 and its quaternary variants BaCuδAg2-δTe2

The new materials BaCuδAg2-δTe2 (0≤δ≤2) were prepared from the elements at 800 deg. C in evacuated silica tubes. BaAg2Te2 crystallizes in the α-BaCu2S2 type, space group Pnma, with lattice parameters a=10.8897(3) A, b=4.6084(1) A, c=11.8134(3) A (Z=4). The structure consists of a three-dimensional network of vertex- and edge-condensed AgTe4 tetrahedra, which includes the Ba2+ cations in linear channels running along the short b-axis. Half of the Ag atoms participate in an Ag atom zigzag chain extended parallel to the channels. BaAg2Te2 is a p-type semiconductor with large Seebeck coefficient. Within the series BaCuδAg2-δTe2, the electrical conductivity increases and the Seebeck coefficient decreases strongly with increasing Cu content. - Graphical abstract: The tellurides BaCuδAg2-δTe2 all crystallize in the α-BaCu2S2 type. The Ag atoms prefer the M2 sites with short M-M bonds (solid lines). The materials are (degenerate) p-type semiconductors. Higher Ag content reflects itself in higher Seebeck coefficient and lower electrical conductivity