Localization and Charge Conversion of Copper in Rb₂ZnCl₄:Cu Crystals: An ESR and Optical Absorption Study

Electron spin resonance and optical studies reveal the presence of both Cu²⁺ and Cu⁺ centers in Rb₂ZnCl₄:Cu single crystals grown from melt and their conversion by X- or γ-irradiation. Two types of paramagnetic Cu²⁺ centers with different concentrations and production properties have been identified. The more abundant Cu²⁺(I) center consists of a Cu²⁺ ion substituting for Zn²⁺ at the center of a ZnCl^2—₄ tetrahedron. The less abundant Cu²⁺(II) center seems to be situated at a Rb site. Production experiments strongly suggest that during the crystal growth copper enters the Rb₂ZnCl₄ lattice as Cu²⁺, mainly at Zn²⁺ sites, part of it being afterwards converted to Cu⁺ precursor centers. The presence of a neighboring charge compensating anion vacancy and its departure during the radiolytic Cu⁺(I) → Cu²⁺(I) conversion seems to play an essential role in the stabilization of the Cu⁺(I) and Cu²⁺(I) centers, respectively.