Intermediate surface reactions to obtain nanocrystalline PbTe via high-energy milling

To elucidate how surface and gaseous phases interact each other to induce chemical reactions, X-ray photoelectron spectroscopy (XPS) analyses were carried out on powders as milling took place. An acute analysis of data acquired by the XPS-technique allowed us to find a series of well-defined chemical transitions from precursors to the stoichiometric PbTe phase. By coupling, theoretical and experimental data a self-consistent model was developed. Initially, the process manifested itself as an oxidation stage and secondly as a reducing process. In agreement with a thermodynamic evaluation of free energy of phases traced during milling, chemical transitions were traced as Te²⁺ to Te⁶⁺ in oxidation reactions. If high oxygen potential prevails in the milling system subsequently Pb²⁺ evolves to Pb⁴⁺. On the other way, high valence oxides like Pb⁴⁺ or Te⁴⁺ were reduced to Pb ²⁺ and Te^2-. However, the last transition an asymmetric transformations was identified as non-stoichiometric phases.