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 bot h Cu2+ and Cu+ centers in Rb2ZnCl4:Cu single crystals grown from melt and their conversion by X- or gamma-irradiation. Two types of paramagn etic Cu2+ centers with different concentrations and production propert ies have been identified. The more abundant Cu2+(I) center consists of a Cu2+ ion substituting for Zn2+ at the center of a ZnC42- tetrahedro n. The less abundant Cu2+(II) center seems to be situated at a Rb site . Production experiments strongly suggest that during the crystal grow th copper enters the Rb2ZnCl4 lattice as Cu2+, mainly at Zn2+ sites, p art of it being afterwards converted to Cu+ precursor centers. The pre sence of a neighboring-charge compensating anion vacancy and its depar ture during the radiolytic Cu+(I) --> Cu2+(I) conversion seems to play an essential role in the stabilization of the Cu+(I) and Cu2+(I) cent ers, respectively.