Direct Crystallization of Amorphous Molecular Systems Prepared by Vacuum Deposition: X-ray Studies of Phenyl Halides

Abstract

Abstract Many amorphous molecular systems prepared by vacuum deposition onto cold substrates turn into crystals directly without exhibiting glass transition when their temperature is raised. To study the mechanism of the crystallization from amorphous states, we monitored the increase of X-ray Bragg peaks at constant temperatures for fluoro-, chloro-, and bromobenzene samples. Data were analyzed by using the Johnson–Mehl–Avrami equation. It was found that the crystallization from amorphous states of these compounds progresses through the nucleation and crystal-growth processes similar to those observed for fused materials. However, slightly complicated behaviors were observed for molecules with larger substituents. These are considered to arise from the wide distribution of molecular arrangements that are stochastically generated in samples by the vapor deposition at low temperatures.