X-ray investigation of the structure transition of methane at the λ point

Abstract

A great deal of work has been done on the crystal structure of long-chain hydrocarbons and their behaviour near transition points. These transitions are as a rule quite normal, in so far as the transition is marked by the liberation or binding of a definite heat of transformation and change in structure. The simplest of the hydrocarbons, methane, shows a change from the ordinary behaviour. At 20.4° K a so-called transition of the second kind (Clusius and Perlick 1933) takes place, of which an abnormal rise of the specific heat is typical. This begins at low temperatures where the specific heat rises at first slowly to reach a very sharp maximum at 20.4° and then falls abruptly to somewhat normal values. No latent heat appears. The effect bears a strong resemblance to the λ point of helium and other substances. The crystal structure of methane has been directly determined by two authors (McLennan and Plummer 1929; Mooy 1931) of whom only Mooy paid special attention to any possible change of structure at the transition point. He observed no change in the critical range. The structure is cubic face-centred according to both authors, but we would like to point out that Mooy’s table contains two lines which do not belong to reflexions from that lattice and which he labelled “parasitic lines”.