XAFS STUDIES OF LOCAL ATOMIC-STRUCTURE OF NIOBIUM TELLURIDES

We present XAFS (K-edge) studies of the local atomic structure around the Nb and Te ions in niobium tellurides (NbTe2,, Nb3Te4 and NbTe4) in comparison with ZrTe2. Using a multi-shell best-fit analysis procedur e, we have reconstructed local environments of the Nb and Te ions and compared them with the existing structural models. We found that at th e Nb K-edge the XAFS data of NbTe4 are extremely sensitive to metal cl ustering and to the subsequent departure from the average crystallogra phic positions. Moreover at Te K-edge the XAFS data allowed us to dist inguish the different type of Te-Te bonds, for instance: the dimer (Te -Te: 2.9 Angstrom) and the squares (Te-Te: 3.3 Angstrom) in NbTe4, the interlayer and intralayer (Te-Te: 2.9-4.0 Angstrom) in NbTe2, through their Debye-Waller characteristics. In addition, a comparative analys is of Debye-Waller factors obtained in the different coordination shel ls (Te-Te, Te-Nb, Nb-Te and Nb-Nb) shows an evidence of correlation in the vibrational motion. (C) 1997 Elsevier Science S.A.