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Yb3+/Al3+/P5+ co-doped silica glass with compositions of(95.9-x)SiO2-4A12O3-x P2O5-0.1 Yb2O3(x=0-10)were prepared by sol-gel method combined with high-temperature sintering.Relationship between the glass structure and spectroscopic properties is investigated through Raman,nuclear magnetic resonance(NMR)and pulse electron paramagnetic resonance(EPR)spectra.Results suggest that Yb3+ is mainly located in AlPO4-like but not SiO4 units at a molar ratio of P5+/Al3+ =1,and it coordinates to P=O sites at the ratio of P5+/Al3+>1.This structural change leads to a significant alteration in density,refractive index,absorption and emission cross-sections,fluorescent lifetime,and scalar crystal field NJ at a molar ratio of P5+/Al3+=1,especially the remarkable decline in absorption and emission cross section at the ratio of P5+/Al3+>1.The mechanism behind is explored through the discussion on structure-property relationship of these glasses.Above all,this work is useful for the optimization of p5+ content in Yb3+/Al3+/P5+ co-doped silica glass for high power fiber laser.