The temperature dependence of position lifetimes in both the brittle and plastic phases of cyclohaxane has been examined. Long-lived components in both phases are associated with the formation of positronium (Ps). Two long lifetimes attributable to ortho-Ps are resolvable in the plastic phase. The longer of these (≈ 2.5 ns), which is temperature dependent, is ascribed to ortho-Ps trapped at vacancies. The shorter lifetime (≈ 0.9 ns), shows little temperature dependence. In contrast to most other plastic crystals, no sigmoidal behaviour of the average ortho-Ps lifetime is observed. A possibility exists that some self-trapping of Ps may occur just above the phase transition. In the brittle phase only one long lifetime of ≈ 1.1 ns is resolvable. The presence of 100 ppm impurities reduces the ortho-Ps yield, especially in the brittle phase, but does not influence the lifetimes. The ratio between the apparent ortho-Ps and para-Ps, yields is close to 3 for the pure-crystals in the plastic phase, but smaller in the brittle phase. A correlation is established between the molecular packing coefficients for plastic and brittle crystals and the ortho-Ps lifetime in the crystals.
|Publication status||Published - 1985|