Magnetic Resonance Studies of Ferroelectric Methylammonium Alum

The electron paramagnetic resonance of CH₃NH₃Al(SO₄)₂·{}12H₂O (MASD) doped with Cr³⁺ 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_1ρ) exhibited a discontinuity at the transition temperature Tc. The T_1ρ data are interpreted as due to a 180^∘{} flip motion of the CH₃NH₃⁺ ion which changes abruptly at Tc. The T₁ data show a single minimum characteristic of rotation of the CH₃ group about the triad axis. It is suggested that the ferroelectric transition in MASD results from a one-dimensional reorientation of the CH₃NH₃⁺ ions. Possible reasons for the first-order nature of the transition are given.