Evidence for zero‐field fluctuations in Cr3+ near the phase transition in NH4Al(SO4)2 12 H20

The effect of the phase transition of NH4Al(SO4)2 12 H2O on the EPR of the Cr3+ ion is studied. The axial zero‐field tensor shows a strong linear temperature dependence with a change in slope occurring at −160°C which correlates to the onset of a marked broadening of the high and low field resonances as well as with an unusual increase in the intensity of the center resonance. The effects are attributed to fluctuations in the local symmetry of the H2O molecules about the Cr3+ ion. The strong temperature dependence of D is argued to be due to an unusually large coupling of the paramagnetic ion to local vibrations of the complex.