Stability of luminescent trivalent cerium in silica host glasses modified by boron and phosphorus

Ce-doped borosilicate (BSG), phosphosilicate (PSG), and borophosphosilicate (BPSG) glasses (B:P:Si molar ratios 8:0:92, 0:8:92, and 8:8:84; Ce:Si molar ratio 1 X 10(-4) to 1 X 10(-2)) were prepared by the sol-gel method. High-resolution transmission electron microscopy (HRTEM), P-31, Si-29, and B-11 magic angle spinning nuclear magnetic resonance (MAS NMR), electron paramagnetic resonance (EPR), and UV-vis absorption investigations demonstrated that, in PSG and BPSG, Ce3+ ions interact with phosphoryl, [O=PO3/2], metaphosphate, [O=PO2/2O](-), and pyrophosphate, [O=PO1/2O2](2-), groups, linked to a silica network. This inhibits both CeO2 segregation and oxidation of isolated Ce3+ ions to Ce4+, up to Ce:Si = 5 X 10(-3). In BSG, neither trigonal [BO3/2] nor tetrahedral [BO4/2](-) boron units coordinate cerium; thus, Ce3+, oxidation occurs even at Ce:Si = 1 X 10(-4), as in pure silica glass (SG). The homogeneous rare-earth dispersion in the host matrix and the stabilization of the Ce3+ oxidation state enhanced the intensity of the photoluminescence emission in PSG and BPSG with respect to BSG and SG. The energy of the Ce3+ emission band in PSG and BPSG matrixes agrees with the phosphate environment of the rare earth.