The origin of the crystal field and its variation with temperature in severalα-alums have been studied bynmr of27Al andepr of Cr3+ at high hydrostatic pressures and low temperatures. The results lead to an explanation of the anomalous temperature variation of the axial crystal field at the trivalent ion site. The mechanism of the phase transition in ammonium alum is also explained. A correlation between the axial crystal field as determined bynmr (e2qQ/h) and that determined byepr (D) has been obtained.
