Tellurium-125 Mössbauer spectra of some tellurium subhalides

Abstract

The ¹²⁵Te Mössbauer spectra at 4.2 K for the crystalline tellurium subhalides Te₃Cl₂, Te₂X (X = Br or I), Te₂Br_0.75- I_0.25, β-Tel, and α-Tel show the presence of at least two different tellurium sites in each subhalide in accordance with their known crystal structures. The chemical isomer shifts δ(relative to Zn^125mTe at 4.2 K) in four different types of co-ordination have been found to be 0.6 mm s^–1 for trigonal bipyramidal tellurium atoms [Te(Te)₂(Cl)₂E], 0.8–1.0 mm s^–1 for square-planar (pseudo-octahedral) tellurium atoms [Te(Te)₂(X)₂E₂](X = Br or I), and 0.8 mm s^–1 for two-co-ordinate (pseudo-tetrahedral) tellurium atoms [Te(Te)₂E₂]. The symbols in square brackets indicate the nearest-neighbour environment of the relevant tellurium atom, the symbol E being used to represent an essentially non-bonding pair of electrons. The corresponding quadrupole splittings for the four environments were 11.0, 12.1–13.7, 3.4–6.1, and 8.1 mm s^–1 respectively. The bonding scheme for square-planar tellurium atoms in the tellurium subhalides is considered to involve two three-centre four-electron bonds and is thus similar to that in the isostructural and isoelectronic species [ICl₄]^– and XeCl₄. The Mössbauer spectra at 4.2 K for the glassy phases Te₂Br_0.75I_0.25 and TeBr₂ were different from those of the corresponding crystalline compounds but again showed the existence of at least two different tellurium sites in each compound.