EFG temperature dependence in iodine and zinc–the message of two forgotten experiments

The electric-field gradient (EFG) for the simple halogen solids, according to the original Bayer theory, should have a continuous, quantitatively understood, decrease with temperature due to the vibrational motion of the molecules. In fact, the data for solid chlorine reproduce this prediction extremely well. Thus, an experiment showing that the nuclear quadrupole interaction frequency in iodine initially rises with temperature has earlier been completely ignored. A semiquantitative explanation of this observation is presented. When later the temperature-dependence of the EFG had become available for a number of non-cubic metals, mostly using nuclear techniques, it was observed that they also apparently decrease in a monotonous way with temperature. A general rule of a 1-BT^3/2 dependence was consequently proposed. Again, the fact that for one simple system, zinc, the experimentally measured initial frequency rises with temperature was ignored. It has recently been demonstrated that the interplay of structural effects and lattice vibrations can quantitatively account for this unexpected behavior.