Hopping Precession of Molecules in Crystalline Carbon Dioxide Films

Abstract

We report a low-temperature transmission high-energy electron diffraction study of solid carbon dioxide films in the temperature range 15–87 K. The precise analysis of the experimental diffraction intensities shows that molecular axes noticeably deviate from the cubic space diagonals of the \(Pa\overline{3}\) structure. The molecular tips tend to be oriented toward the empty spaces between two molecules in the nearest basal planes. Nevertheless, the crystal structure is still identified as \(Pa\overline{3}\) but with 24 equivalent positions for oxygen atoms instead of 8 positions as it was thought before. We have shown that the relevant maximal angle deviations in the selected directions could be as big as \({\sim } 30^{\circ }\) at the lowest temperatures and they decrease at higher temperature. This results in hopping precession of molecules instead of simple librations.