Tellurium-125 Mössbauer spectra of some tellurium subhalides

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Abstract
The 125Te Mössbauer spectra at 4.2 K for the crystalline tellurium subhalides Te3Cl2, Te2X (X = Br or I), Te2Br0.75- I0.25, 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 Zn125mTe 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 Te2Br0.75I0.25 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.

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