EPR and dielectric properties of Pb5Ge3O11 crystals with off-center Cu2+ ions

The electron paramagnetic resonance (EPR) and dielectric properties of Pb₅Ge₃O₁₁ crystals activated by copper ion are investigated. It is shown that Cu²⁺ ions replace Pb²⁺ in trigonal symmetry positions and occupy three off-center positions displaced from a crystal lattice site in a plane perpendicular to the polar axis C. The temperature variation of EPR spectra and dielectric properties indicates the presence of thermally activated jumps of Cu²⁺ ions between off-center positions. The EPR and dielectric data are used to determine the activation energy W=0.24 eV and the eigenfrequency τ ₀ ^?1 ～ 10¹² Hz of local dynamics of Cu²⁺ ions.