Dimorphism in methylammonium aluminum alum studied by X-ray diffraction and electron paramagnetic resonance

We have found a reliable method of growing single-phase methyl-ammonium alum (MASD) crystallising in the infrequently-occurring α alum structure; the presence of the perchlorate ion, ClO₄^-, in growth solutions induces the crystallisation of α MASD crystals. Seeding the growth solution with α MASD crystals also induces crystallisation of new α MASD crystals. The X-ray powder diffraction pattern and the EPR spectrum of the Cr³⁺ ions doped into α MASD are drastically different from those for the more frequently occuring β MASD. The linewidths of Cr³⁺ in α and β MASD show anomalous angular dependence. The inhomogeneous line broadening observed for β MASD could be explained by a random distribution in the orientation of the symmetry axis of the trigonal [Cr(H₂O)₆]³⁺ complex. The linewidth anomaly shows a more complicated angular dependence for α MASD; it could be that not only the orientation of the symmetry axis but also the magnitude of the axial crystal field of [Cr(H₂O)₆]³⁺ suffer random distributions.