Theory of theTe125nuclear quadrupole interaction and isomer shifts in tellurium, selenium, and sulfur

The electronic wave functions for the systems, tellurium chain and tellurium atom present as a substitutional impurity in a selenium chain and selenium and sulfur rings, are obtained by the extended Hückel procedure. Using these wave functions, the field gradients at the ${}_{}{}^{125}{}_{}{}^{}\mathrm{Te}$ nucleus in all these four systems is investigated. Very good agreement is found with Mössbauer nuclear quadrupole data in the tellurium chain, both with respect to the magnitude and sign of the quadrupole coupling constant and the asymmetry parameter. For the case ${}_{}{}^{125}{}_{}{}^{}\mathrm{Te}$ in the rings, the agreement with experiment for the quadrupole coupling constants is not as close but still quite good, the theoretical magnitudes being about 82% for both rings. The signs of the quadrupole coupling constant are not known in these systems, but the theoretical results indicate interestingly that the sign for the selenium ring is the same as in the tellurium chain but opposite in the case of sulfur. The asymmetry parameters are not available experimentally for the rings but are predicted to be very sizable, 0.87 and 0.98, respectively, for selenium and sulfur. For the selenium chain, the sign of the theoretical quadrupole coupling is found in agreement with experiment, but the magnitude is only about one-half the experimental value. The asymmetry parameter is predicted to only be 13% but there is no experimental value available for comparison. Possible reasons are discussed for the less satisfactory agreement with experiment for ${}_{}{}^{125}{}_{}{}^{}\mathrm{Te}$ in the selenium chain relative to the case of the tellurium chain and the rings. The ratio of the isomer shifts for ${}_{}{}^{125}{}_{}{}^{}\mathrm{Te}$ in selenium and sulfur ring referred to the tellurium chain is calculated and found to be in good agreement with experiment.