CHEMICAL BONDING IN SULFUR, SELENIUM, AND TELLURIUM FROM I-129 AND TE-125 MOSSBAUER INVESTIGATIONS

I129 Mössbauer spectra of plastic and orthorhombic TexS1-x (x=0.01), trigonal, monoclinic, and bulk amorphous TexSe1-x (x≤0.016), and trigonal Te have been investigated using Tem129 labeled sources. From these data, we obtain chemical-bonding information of the I impurity, and show that it is nearly onefold coordinated in all these twofold coordinated hosts. Further, in comparing the measured I129 quadrupole couplings to the known Te125 quadrupole couplings in the chalcogen hosts, a systematic pattern in the ratios of the two couplings emerges. First, a sign reversal of Vzz is believed to occur in going from Te to I, and is shown to be a consequence of chemical-bonding rearrangement, viz. the twofold coordination of Te involving π-bonding orbitals with two chalcogens near neighbors, transforms into a nearly onefold coordination of I involving σ-bonding orbitals with a chalcogen neighbor. Second, the ratios R of the Te125-I129 quadrupole couplings are found to be as follows: t-Te (-1.12), t-Se (-0.65), m-Se (-0.55), a-Se (-0.52), r-S (-0.48), and p-S (-0.48). The systematic reduction in the magnitude of R from Te to S is shown to be evidence of an increasingly molecular character of the host chemical bonding. In contrasting the bonding between the Se polymorphs, it is shown that a-Se is the most molecular of these solids. The bonding in a-Se is shown to be much closer to that in m-Se than in t-Se. © 1979 The American Physical Society.