ELECTRON-PARAMAGNETIC-RESONANCE STUDY OF GD3+ CENTERS IN CUBIC AND HEXAGONAL PEROVSKITE RBZNF3 CRYSTALS

EPR measurements have been made on Gd3+ centres in Single crystals of RbZnF3 at room temperature using an X-band spectrometer. In crystals d oped with Gd3+, both spectra with cubic and spectra with tetragonal sy mmetries are observed. These centres are ascribed to Gd3+ ions substit uted for Rb+ ions in cubic RbZnF3 (c-RbZnF3) without any local charge compensation and with a vacancy at its nearest Rb+ site, respectively. In some as-grown crystals co-doped with Gd3+, and Li+, signals from a new trigonal centre are observed in place of those from the cubic and tetragonal centres in c-RbZnF3. The trigonal centre is identified as the Gd3+-Li+ pair substituted for the Zn2+-Zn2+ bond in a Zn2F9 unit i n hexagonal RbZnF3 (h-RbZnF3), although the cubic perovskite structure is normally stabilized at room temperature. This indicates that the G d3+-Li+ pairs in Zn2F9 units may be related to the crystal growth of t he hexagonal structure in the matrix.