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 Pd 16 S 7, 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 ≈ 460 K and pressures P S2 ≈ 10 −6 mbar, Pd is already fully transformed into Pd 16 S 7 . Obtained Pd 16 S 7 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 Å, an electrical resistivity ρ = 7.2 × 10 −4 Ω·cm, and a Seebeck coefficient S = 11.8 μV/K. Their optical absorption coefficient is almost constant for photon energies in the range 1−3 eV. On increasing the temperature and sulfur pressure, Pd 16 S 7 gives rise to nanocrystalline PdS thin films (30−40 nm crystallite size) with an n-type semiconducting behavior, having an electrical resistivity ρ = 5.6 × 10 −2 Ω·cm and an a Seebeck coefficient S = −268 μV/K at RT. The optical absorption measurements indicate that the absorption edge of PdS is due to two allowed direct transitions with E G ≈ 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.