Yanjie Cui

Structure change via partial Se/Te substitution: crystal structure and physical properties of the telluride Ba(2)Cu(4-x)Te(5) in contrast to the selenide-telluride Ba(2)Cu(4-x)Se(y)Te(5-y).

The chalcogenides Ba(2)Cu(4-x)Se(y)Te(5-y) were synthesized from the elements in stoichiometric ratios at 700 degrees C, followed by annealing at 600 degrees C. The ternary telluride Ba(2)Cu(4-x)Te(5) crystallizes in a new structure type, space group C2/c, with lattice dimensions of a = 9.4428(6) A, b = 9.3289(6) A, c = 13.3028(8) A, beta = 101.635(1) degrees , V = 1147.8(1) A(3), for x = 0.75(1) (Z = 4). The corresponding selenide-telluride adopts another new, but strongly related, structure type, space group P4(1)2(1)2, with a = 6.5418(3) A, c = 25.782(2) A, V = 1103.3(1) A(3), for Ba(2)Cu(3.26(2))Se(0.729(8))Te(4.271) (Z = 4). Between 0.13 and 1.0 Te per formula unit can be replaced with Se, while the Cu content appears to vary only within 0.67 <or= x <or= 0.81 for Ba(2)Cu(4-x)Se(y)Te(5-y). Despite crystallizing in different crystal systems, the telluride and the selenide-telluride exhibit topologically equivalent structure motifs, namely, chains of Cu(Se,Te)(4) tetrahedra with a Cu atom cis/trans chain as well as an almost linear Te atom chain. All these chalcogenides, as far as measured, are p-doped semiconductors, as determined by Seebeck coefficient and electrical conductivity measurements.

Synthesis and Structural and Physical Properties of New Semiconducting Quaternary Tellurides: Ba4Ag3.95Ge2Te9 and Ba4Cu3.71Ge2Te9

Two quaternary tellurides, Ba(4)Ag(3.95)Ge(2)Te(9) and Ba(4)Cu(3.71)Ge(2)Te(9), were prepared in evacuated silica tubes at 750 degrees C. Both tellurides crystallize in space group Pbam, with lattice parameters of a = 8.6835(3) A, b = 13.6421(4) A, c = 10.2612(3) A, and V = 1215.55(7) A(3) (Z = 2) for Ba(4)Ag(3.95)Ge(2)Te(9) and a = 8.6464(2) A, b = 13.5305(4) A, c = 10.0810(3) A, and V = 1179.38(6) A(3) (Z = 2) for Ba(4)Cu(3.71)Ge(2)Te(9). These structures are comprised of planar Ag(4)/Cu(4) clusters and dimeric Ge(2)Te(6) units, which are interconnected through Te atoms into a three-dimensional structure. Several split sites in the case of Ba(4)Cu(3.71)Ge(2)Te(9) are reflected in additional, different clusters, including a linear Cu(3) unit. The covalent Ag-Te/Ge-Te network surrounds a one-dimensional linear channel running along the c direction, encompassing the Ba atoms. Electronic structure calculations and transport property measurements show that these two compounds are p-type semiconductors with calculated band gaps of 0.24 eV for the Ag compound and 1.0 eV for the Cu compound.