Cubic Pd16S7 as a Precursor Phase in the Formation of Tetragonal PdS by Sulfuration of Pd Thin Films

Experimental results are reported showing that sulfuration of metallic Pd thin films gives rise to Pd16S7, which finally, transforms into tetragonal PdS. The whole process has been investigated by measuring the film electrical resistance while increasing its temperature and the sulfur vapor pressure. At temperatures T approximate to 460 K and pressures P-S2 approximate to 10(-6) mbar, Pd is already fully transformed into Pd16S7. Obtained Pd16S7 nanocrystalline films (50 nm crystallite size) have been characterized (morphological, structural, optical, and transport properties at room temperature (RT)) for the first time. They show a cubic structure with a lattice parameter a = 8.921 +/- 0.009 angstrom, an electrical resistivity rho = 7.2 x 10(-4) ohm center dot cm, and a Seebeck coefficient S = 11.8 mu V/K. Their optical absorption coefficient is almost constant for photon energies in the range 1-3 eV. On increasing the temperature and sulfur pressure, Pd16S7 gives rise to nanocrystalline PdS thin films (30-40 nm crystallite size) with an n-type semiconducting behavior, having an electrical resistivity rho = 5.6 x 10(-2) ohm center dot cm and an a Seebeck coefficient S = -268 mu V/K at RT. The optical absorption measurements indicate that the absorption edge of PdS is due to two allowed direct transitions with E-G approximate to 1.45 and 1.70 eV, respectively. Finally, obtained results are discussed on the light of thermodynamic calculations, which confirm the sequence of sulfides formation described in the paper. The possible interest of PdS in thermoelectric applications is emphasized.