A.I. Lebedev, I.A. Sluchinskaya, V.N. Demin, I.H. Munro
Physics Department, Moscow State University
Moscow, 119899, Russia
Warrington, WA4 4AD, UK
The influence of impurities on the ferroelectric phase transitions was the subject of intensive studies in recent years. Various phenomenological models were proposed to explain this influence, however its microscopic description is still unknown.
The purpose of this work was to study the environment of different impurities and the Ge atoms in GeTe in order to elucidate the mechanisms of the interaction between off-center Ge atoms and impurities. EXAFS is the most direct method of studying the local environment of impurities, and GeTe is the most convenient ferroelectric due to its very simple crystal structure.
EXAFS experiments were done at Daresbury Laboratory on GeTe1-xSex (x=0.1-0.3), Ge0.9Pb0.1Te and Ge1-xMnxTe (x=0.1, 0.2) samples at 77 K in transmission mode. The doping impurities Se, Pb and Mn were chosen as the most interesting ones because of very different effect produced by them on Curie temperature.
Analysis of EXAFS data shows that the Ge atoms in GeTe1-xSex are surrounded by the Te atoms located at the distances of 2.80(1) and 3.13(1) A, and the Se atoms at 2.63(2) A; the next neighboring Ge atoms are at the mean distance of 4.18(1) A. The selenium atoms are surrounded by the Ge atoms at 2.61(1), 3.03(2) and 3.16(4) A. The last result clearly shows the displacement of the Se atom from the Te site in the GeTe structure.
Examination of the Pb environment in Ge0.9Pb0.1Te reveals the Te atoms located at two different distances of 3.04(1) and 3.26(1) A, and the second shell located at the mean distance of 4.26(1) A. Small Debye-Waller factors for both Pb-Te bonds exclude the off-centering of the Pb atoms. We think that the Pb atoms act as the dilation centers, and the appearance of unexpectedly short Pb-Te bond (3.04 A) is attributed to the removing of 6s nonbonding electron pair as a result of strong stress.
Analysis of the Mn environment in Ge1-xMnxTe shows that Mn atoms occupy the on-center positions in metal sites and are surrounded by the Te atoms at 2.91(1) A and the (Ge,Mn) atoms at 4.16(1) A.
The obtained results show that the rate of decreasing Tc in doped crystals is determined by the structural position of impurity and energy of the interaction of impurity with neighboring atoms.