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Infrared-to-visible upconversion emission intensities are investigated in Li+/Er3+,Li+/Ho3+/Yb3+ and Li+/Tm3+/Yb3+ codoped oxide nanocrystals.By introducing Li+ ion,the upconversion emission intensity of rare-earth ions are significantly enhanced comparing with that without the Li+ ion.The local structure around Er3+ and Ho3+ ions are studied by the extended X-ray absorption fine structure spectroscopy.After doping Li+ ion,both the average bond lengths of Er-O and Ho-O are decreased.