EPR study of coordination of Ag and Pb cations in BaB2O4 crystals and barium borate glasses

It is shown the possibility to determine the coordination of paramagnetic ions in disordered solid structures, e.g., in barium borate glasses. For this purpose the electron paramagnetic resonance (EPR) method was used to study α-and β-BaB 2 O 4 crystals and glasses of 45·BaO × 55·B 2 O 3 and 40·BaO × 60·B 2 O 3 (mol%) composition activated by Ag + and Pb 2+ ions. After the samples were exposed to X-rays at 77 K, different EPR centers were observed in them. In α-and β-BaB 2 O 4 crystals and glasses the EPR centers Ag 2+ , Ag 0 , Pb + , Pb 3+ , and hole centers of O − type were studied. The EPR parameters of these centers and their arrangement in crystal structure were determined. It is shown that Pb 3+ ions in β-BaB 2 O 4 crystals occupy Ba 2+ position in an irregular polyhedron from the eight oxygen, whereas in α-BaB 2 O 4 crystals they occupy Bа 2 position in a sixfold coordination. Pb + ions in α-BaB 2 O 4 crystals occupy Bа 1 position in a ninefold coordination from oxygen. In barium borate glasses, Pb 3+ ions were studied in coordination polyhedron from six oxygen atoms and in a polyhedron from nine to ten oxygen atoms. It is assumed that the established difference in the structural position of Pb 3+ ions in glasses is due to their previous incorporation in associative cation–anion complexes (AC) and “free” structure-forming cations (FC). Computer simulations have been performed to analyze the stability of specific associative complexes and to compare their bond lengths with experimental data.