Structural transition of ammonium aluminium alums studied by e.p.r.

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Abstract
EPR spectroscopy results for Cr3+ in two alums A1NH4 and A1ND4 are reported. These alums undergo a phase transition at respectively 60 K and 56 K. The results for T > Tc are qualitatively explained as resulting from fluctuations of distortions associated with the structural change. As A1NH4 looks internally fractured after being cooled below Tc, only the low temperature phase of A1ND4 has been studied. The EPR spectra below T c depend on the rate of cooling. After a slow cooling, the crystal contains three types of domains, each belonging to the orthorhombic system. After a fast cooling, complicated spectra are observed and partially analysed. The analysis suggests they are due either to some form of disorder or to the existence of many domains with large walls