Different clusters within the Ba4M4-xA2Te9 (M = Cu, Ag, Au; A = Si, Ge) series: Crystal structures and transport properties

Three new quaternary tellurides Ba(4)M(4-x)A(2)Te(9), namely Ba4Cu3.75Si2Te9, Ba4Au3.69Ge2Te9, and Ba4Ag3.97Si2Te9, were prepared in evacuated silica tubes at 1023 K. Single crystal X-ray diffraction analysis reveals that these tellurides crystallize in two different structure types: space group Pbam, with lattice parameters of a = 8.6389(3)angstrom, b = 13.5883(4)angstrom, c = 10.2252(3)angstrom, V = 1200.32(7)angstrom(3) (Z = 2) for Ba4Ag3.97Si2Te9, and space group Pnma, with lattice parameters of a = 13.4294(4)angstrom, b = 20.0980(7)angstrom, c = 8.5699(3)angstrom, V = 2313.0(1)angstrom(3) (Z = 4) for Ba4Cu3.75Si2Te9, and a = 13.5808(6)angstrom, b = 20.7033(9)angstrom, c = 8.6418(4)angstrom, V = 2429.8(2)angstrom(3) (Z = 4) for Ba4Au3.69Ge2Te9. The Ag-Si telluride is isostructural with Ba4Cu3.71Ge2Te9 and Ba4Ag3.95Ge2Te9, whereas the two latter ones are isostructural members of a new structure type. Both structure types contain M-4 Clusters and dimeric A(2)Te(6) units; however, the M-Te polyhedra are different, resulting in different structure types. The Pnma variants may be viewed as super structures to the Pbam structures, caused by different orientation of the M-4 Clusters and different M-Te coordination spheres. According to electronic structure calculations and transport property measurements, these three compounds are p-type semiconductors with calculated band gaps of 0.35 eV for the Ag compound, 0.89 eV for the Cu compound and 0.19 eV for the Au compound. (C) 2010 Elsevier B.V. All rights reserved.