New rotator phase revealed in di-n-alkylammonium bromides studied by solid-state NMR, powder XRD, electrical conductivity and thermal measurements

Abstract

¹ H NMR linewidth and relaxation times, powder XRD, electrical conductivity and differential scanning calorimetry have been performed on (C _n H _2n+1 ) ₂ NH ₂ Br (n=2, 3 and 4). A new ‘rotator’ phase was revealed at 342481, 293538 and 254563 K, respectively, where rod-like cations perform uniaxial reorientation about the molecular long axis and, at the same time, translational self-diffusion at elevated temperatures. The highly disordered structure was also revealed by the transition entropies, the sum of which, at temperature above 130 K, approached the melting entropy, being as small as ca. 20 J K ^-1 mol ^-1 . These properties of the rotator phase are quite analogous to those detected in monoalkylammonium chlorides whose rotator phase can be described as a ‘low-dimensional plastic crystal’. The tetragonal crystal structure in the present rotator phase has space group I4/mmm close to the P4/nmm found for that of the monoalkylammonium salts.