Phase transitions of the clathrasil dodecasil 3C (MTN) for the guest molecules pyrrolidine and t-butylamine

Abstract

The clathrasil dodecasil 3C (MTN) with the guest molecules pyrrolidine and t-butylamine was studied by powder diffraction as a function of temperature and time. The symmetry of the room-and low-temperature structures of MTN depends on the symmetry of the enclathrated guest molecule in the [5 126 4] cage. Whereas the common high-temperature form (above 180°C) is cubic ( Fd3), at room-temperature symmetry, F222 or F2dd for the guest molecule pyrrolidine and I4 1 a for the guest molecule t-butylamine was found. At 80 K, both samples were indexed in space group F12 d1 , but with different settings of the unit cell. The room-temperature structure of MTN with the guest species pyrrolidine is metastable. After cooling a sample of MTN with the guest species pyrrolidine from above its phase-transition temperature to room temperature, a noncubic structure is observed. A retransition within approximately 16 h occurs from this noncubic structure to the cubic high-temperature structure at room temperature. Further investigations on this retransition revealed a first-order phase transition.