Electron Paramagnetic Resonance Investigation of the Ferroelectric Transition in N₂H₅Al(SO₄)₂·12H₂O Crystals

The EPR spectra exhibited by the doped Cr³⁺ ions is sensitive to the reported low-temperature ferroelectric transition in N₂H₅Al(SO₄)₂·12H₂O crystals. Structural informations that can be deduced from the experimental results include: (1) the appearance of merohedral twinning is concurrent with the onset of the structural phase transition; (2) the crystalline threefold symmetry, which is lost with the onset of the structural phase transition, becomes the twin law relating the orientations of the component domains; (3) the crystal symmetry below the transition temperature is orthorhombic; (4) the crystalline cubic axes of the high-temperature, cubic phase become the three crystalline twofold axes of the low-temperature, orthorhombic phase; and (5) a single Cr³⁺ species is detected in both the cubic and the orthorhombic phases, which implies that all the Al³⁺ sites are crystallographically equivalent in both phases.