Manifestation of noncentrality in the EPR spectrum of $Fe3+$ in polycrystalline substances

The EPR spectrum of an impurity $Fe3+$ ion in three organic substances is investigated in the temperature range 4.2–295 K: bromocresol green $(C21H14Br4O5S),$ crystal violet $(C25H30ClN3),$ and a compound of crystal violet with fullarene $((C25H30ClN3)+C60).$ An unusual change in the shape of the EPR spectrum as a function of temperature is observed in all substances investigated. At 4.2 K the spectrum is a superposition of three contributions: two resonance lines with effective $g$ factors $g1=4.39$ and $g2=2.03$ and a nonresonance contribution. As the temperature changes, the intensities become redistributed between these three contributions. As temperature increases, the total intensity of line 1 and the nonresonance contribution decrease and the intensity of line 2 increases. The observed properties of the EPR spectra are characteristic for noncentral ions which possess a multiple-minimum potential. The redistribution of the populations of vibronic states of a noncentral $Fe3+$ ion determines the temperature variation of the EPR spectrum.