Paramagnetic Resonance in Chromic Methylamine Alum

The paramagnetic resonance spectrum of chromic methylamine alum has been re-examined because of an 11% discrepancy in the value of the Stark splitting between previous paramagnetic resonance and other low temperature measurements. The crystalline electric field acting on the chromic ion, which has trigonal symmetry at high temperatures, is found to undergo a change below about 160°K to rhombic symmetry. The Stark splitting is then 0.178 ± 0.002 cm -1 ; this is a little higher than the previous resonance value, reducing the discrepancy to 5%. Measurements of line width indicate that this discrepancy is probably not due to exchange interaction.