On the Orientational Order-Disorder Transition in Crystalline Aniline Hydrobromide

Abstract

Abstract Measurements of the heat capacity (−80∼+60°C) and the thermal expansion (−40∼+60°C) of aniline hydrobromide were carried out covering the transition temperature which has been observed recently by X-ray methods. The observed heat capacity versus temperature curve was of the λ-shape, similar to that of the order-disorder phase transition of binary alloy systems. The following results were obtained ; Tc (the temperature corresponding to the heat capacity peak) 22°C, ΔH (heat of transition, −70∼+35°C) ±10cal./mol., ΔS (entropy of transition, the same temperature interval) 1.10±0.05 e. u., ΔCp (drop of heat capacity at Tc)∼9 cal./mol. deg. Also the anomalous behavior of thermal expansion was observed in the vicinity of the critical temperature. These observations showed that the transition was of higher order nature. Based on the lits of knowledge of the crystal structures above and below the critical temperature, the author has discussed the main feature of the phase transition from the sandpoint of the orientational order-disorder of the −NH3+ ionic group and obtained fairly satisfactory results.