Ab-initio study of pure 77Se and 125Te systems and of the 77Se nuclear quadrupole interaction in tellurium

Using the Hartree-Fock cluster procedure, we have studied the electric-field gradient tensors at the nuclear sites of Se-77 and Te-125 in pure Te-125 systems and in tellurium crystalline system's with a Se-77 impurity. From the results for the pure systems, Se-77 in selenium and Te-125 in tellurium, using the observed quadrupole coupling constants. we have determined the values of their quadrupole moments: Q(Se-77) = 0.75 +/- 0.07 barns and Q(Te-125) = 0.35 +/- 0.04 barns. Comparison is made with earlier Values obtained by different methods. Using our calculated values of Q and the results of a study of the field-gradient tensors for Se-77 in tellurium, the theoretical values of the quadrupole coupling constants are found to agree, within about 7 percent, with experiment. The calculated asymmetry parameters are also found to be in reasonable agreement with the experiment values, although the agreement not as close as in the case of the quadrupole-coupling constants. Directions for further improvements in the theory are discussed.