Abstract
Three new quaternary tellurides Ba 4M 4− x A 2Te 9, namely Ba 4Cu 3.75Si 2Te 9, Ba 4Au 3.69Ge 2Te 9, and Ba 4Ag 3.97Si 2Te 9, 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) Å, b = 13.5883(4) Å, c = 10.2252(3) Å, V = 1200.32(7) Å 3 ( Z = 2) for Ba 4Ag 3.97Si 2Te 9, and space group Pnma, with lattice parameters of a = 13.4294(4) Å, b = 20.0980(7) Å, c = 8.5699(3) Å, V = 2313.0(1) Å 3 ( Z = 4) for Ba 4Cu 3.75Si 2Te 9, and a = 13.5808(6) Å, b = 20.7033(9) Å, c = 8.6418(4) Å, V = 2429.8(2) Å 3 ( Z = 4) for Ba 4Au 3.69Ge 2Te 9. The Ag–Si telluride is isostructural with Ba 4Cu 3.71Ge 2Te 9 and Ba 4Ag 3.95Ge 2Te 9, whereas the two latter ones are isostructural members of a new structure type. Both structure types contain M 4 clusters and dimeric A 2Te 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.
Published Version
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