Paramagnetic resonance spectra of five chromic sulphate alums at low temperatures

Abstract
The paramagnetic resonance spectra of five chromic sulphate alums have been examined at temperatures down to 20 degrees K and wave-lengths 3 and 10 cm. Their behaviour is markedly different as follows. (a) Rubidium, caesium and methylamine alums show the normal spectrum associated with a trigonal splitting at all temperatures, but the splitting decreases as the temperature is lowered, becoming constant below 90 degrees K. The change in the splitting is most marked for rubidium, but substantially zero for methylamine alum. (b) Ammonium alum shows the normal spectrum at high temperatures, with a steadily decreasing splitting down to 80 degrees K; here a crystallographic transition takes place, and the spectrum becomes abnormal, with two splittings. (c) Potassium alum behaves like ammonium alum down to 160 degrees K, below which an anomalous spectrum appears with lines whose intensity increases as the temperature falls. The two splittings in this region are difficult to reconcile with previous measurements of the specific heat of the spin system by other methods.