Motional Narrowing of Electron Spin Resonance Absorption in the Plastic-Crystal Phase of [(CH3)4N]FeCl4

We report the temperature dependence of the electron spin resonance (ESR) absorption and magnetic susceptibility of plastic crystal [(CH3)4N]FeCl4. We demonstrate the extreme sensitivity of ESR to the static and dynamic structural transitions, characteristic of this material. We observed the narrowing of the ESR line in the high temperature plastic-crystal phase, which is due to fast rotation (τ ≈ 10–9 s) of the [FeCl4]− anion. This is a rare example of the effect of fast motional narrowing in crystalline solids, in which ions occupy fixed positions in the crystal lattice.