Effects of Phase Transitions on the Positron Annihilation Characteristics in Ionic and Van Der Waals Substances.

Positron lifetime spectra were measured in the ionic solids of the ammonium halides and the van der Waals solids and liquids of perhalogenated methanes and silanes. Their dependence on solid-solid and solid-liquid phase transitions was studied. Positronium (Ps) is formed in several of the halogenated methanes and silanes as predicted by the free volume model. The analysis of the dependence of the Ps yield on the sample density shows that Ps formed whenever the substance has free volumes in excess of approximately 0.35 v sub O where v sub O is the volume inaccessible to the positron due to the repulsion by the atom cores. The pick-off lifetime of ortho positronium (oPs) decreases with temperature in some of the substances when molten. This is shown to indicate that thermal density fluctuations in these liquids rather than the thermal expansion, determine the rate of electron pick-off by oPs from the molecules.