Magnetic Resonance Studies of Ferroelectric Methylammonium Alum

The electron paramagnetic resonance of C${\mathrm{H}}_3$N${\mathrm{H}}_3$Al${\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$·12${\mathrm{H}}_2$O (MASD) doped with ${\mathrm{Cr}}^{3+}$ was studied from 90 to 300°K. At the first-order ferroelectric Curie point (177°K), an abrupt change in the fine-structure splitting as well as the resonance linewidths was observed. Continuous-wave and pulsed proton-resonance experiments were performed on ordinary and partially deuterated MASD. The relaxation time in the rotating coordinate system ($T_{1\rho }$) exhibited a discontinuity at the transition temperature $T_c$. The $T_{1\rho }$ data are interpreted as due to a 180° flip motion of the C${\mathrm{H}}_3$N${\mathrm{H}}_3^{+}$ ion which changes abruptly at $T_c$. The $T_1$ data show a single minimum characteristic of rotation of the C${\mathrm{H}}_3$ group about the triad axis. It is suggested that the ferroelectric transition in MASD results from a one-dimensional reorientation of the C${\mathrm{H}}_3$N${\mathrm{H}}_3^{+}$ ions. Possible reasons for the first-order nature of the transition are given.