Chemical bonding in sulfur, selenium, and tellurium fromI129andTe125Mössbauer investigations

${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ Mössbauer spectra of plastic and orthorhombic ${\mathrm{Te}}_x{\mathrm{S}}_{1-x}\mathrm{}(x=0.01\mathrm{})\mathrm{}$, trigonal, monoclinic, and bulk amorphous ${\mathrm{Te}}_x{\mathrm{Se}}_{1-x}(x\le 0.016)$, and trigonal Te have been investigated using ${}_{}{}^{129}{}_{}{}^{}\mathrm{Te}_{}^m{}_{}{}^{}$ 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 ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ quadrupole couplings to the known ${}_{}{}^{125}{}_{}{}^{}\mathrm{Te}$ quadrupole couplings in the chalcogen hosts, a systematic pattern in the ratios of the two couplings emerges. First, a sign reversal of $V_{\mathrm{zz}}$ 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 $\pi$-bonding orbitals with two chalcogens near neighbors, transforms into a nearly onefold coordination of I involving $\sigma$-bonding orbitals with a chalcogen neighbor. Second, the ratios $R$ of the ${}_{}{}^{125}{}_{}{}^{}\mathrm{Te}$-${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ 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.