Positron annihilation and phase transitions in argon intercalated n-nonadecane

Positron annihilation method was applied to study phase transitions in argon intercalated n-nonadecane. Particular phases are easily recognizable due to large differences in ortho-positronium lifetimes. It was found that melting temperature is a non-monotonous function of pressure. In the range of temperatures 301.5-303 K with the rise of pressure the phase sequence rotator - liquid-rigid crystal was observed. After the pressure induced transition to the rigid phase the sample structure became porous due to escape of dissolved argon. The triple point in intercalated samples is located at much lower pressure and temperature than in pure ones. An admixture of alkane with other length of carbon chain increases the ortho-positronium lifetime in rigid phase, indicating Ps location in the interlamellar gaps.