EPR Studies of Cr³⁺ and Fe³⁺ in Host Crystals with Langbeinite Structure

Electron paramagnetic resonance (EPR) spectra of trivalent chromium and iron ions have been investigated in host crystals of double sulfates with langbeinite structure, A₂B₂(SO₄)₃, with A = NH⁺₄, Rb⁺, Tl₊ and B = Cd²⁺, Mg²⁺. In most cases, single crystals could be grown, and the spin Hamiltonian parameters have been determined from the rotational diagrams. Powder spectra were analyzed and simulated as well. Three centres of axial symmetry with different zero field splittings have been observed for Cr³⁺, and two for Fe₃₊. The trivalent impurity ions have been shown to occupy divalent lattice sites substitutional, but the environments are different depending on the structure and the mechanism of charge compensation. A qualitative description of the incorporation of dopants has been possible on this basis, taking into account the local relaxation and the ligand field stabilization effects. Temperature dependent single crystal and powder measurements have provided information on the zero field splitting and phase transitions.