Interpretation of the low temperature phase transitions in ammonium and potassium chromic alums in terms of vibrational and static crystal distortion contributions to the spin Hamiltonian zero-field splitting parameter D. III

Abstract

The spin Hamiltonian zero-field splitting parameter D obtained in ammonium and potassium chromic alums and their isomorphs is analyzed in terms of the temperature dependence of vibrational and static crystal distortion contributions. The analysis shows that the low temperature phase transitions existing in the chromic alums are due to a change in sign of the static trigonal distortion at the chromium sites. For NH4Al and KAl sulfate alums dilutely doped with Cr3+ it is found that the distortions at the Cr3+ sites change sign at temperatures higher than the known transition points in the undoped crystals. Dilutely doped NH4In sulfate alum represents a special case in which the crystal changes phase but the static distortion at the Cr3+ site does not change sign.