Positron annihilation lifetime spectral changes on dehydration of natural beryls and cordierites

Abstract

The effect on positron annihilation lifetime spectra, measured at room temperature, of the dehydration of single crystals of beryl and cordierite was studied. In each case, the spectra were satisfactorily fitted to three lifetime components. For the beryls, the dehydration considerably enhanced the intensity of the intermediate-lifetime component (I2) and reduced the intensity of the longest-lived component (I3). I2 is attributed to positron annihilation in the empty cages in the channels of the beryl structure and I3 to annihilation by a pick-off process via unknown foreign molecules. However, for the cordierites, the main effect of the dehydration was a slight (∼10%) increase in the lifetime of the intermediate component, τ2. Here I2 was relatively high both before and after dehydration and the increase in τ2 is attributed to Si-Al ordering. No changes in the lifetime spectra were produced by γ irradiation.