Negative compressibility of free volumes in argon intercalated n-nonadecane

Positron annihilation lifetime spectroscopy was applied to study modification of sub-nanometer voids in one of n-alkanes subjected to high pressure. Without a contact with gaseous atmosphere the sizes of voids behaved as expected – decreased with the rise of pressure. When the pressure was applied with argon penetrating the samples – the voids expanded. After the phase transition the structure damage allowed positronium, trapped in voids, to escape out of a damaged sample.