All known experimental data about off-center impurities were related so far to small atoms which substitute for larger atoms in crystals. In this work we shall present the first direct observation of the off-centering of large Pb and Sn substitutional impurities in GeTe.
XAFS investigations were carried out on Ge0.9Pb0.1Te and Ge0.85Sn0.15Te samples at 77 K in transmission mode at Ge and Sn K-edges and the Pb LIII-edge. Analysis has found that impurity atoms in both isovalent solid solutions substitute for Ge atoms and are surrounded by Te atoms located at two different distances, the shorter of them being about 0.2 A shorter than the distance in corresponding "starting" binary compound (PbTe or SnTe). The data were explained by the activation of unshared s2 electron pair of Sn and Pb impurities in chemical bonding under the condition of high local stress induced by these impurities in GeTe lattice.