Yttrium oxide is one of the scintillation material used in X-ray imaging

in order to improve the light output of doped rare earth ions

Li

+

is always codoped with the luminescent center.However the mechanism for this codoping approach is still not quite clear except the common explanation of flux effect.Further research work shows the enhance effects of Li

+

doping is related with concentration of Li

+

and the lattice constants

but the implication of this kind of relationship is still not clear. Based on the model of cluster approximation

the influence of Li

+

doped concentration on defects formation and the effect of the change of YO bond length on electronic density of states are studied with molecular dynamics and density functional simulation with software package (ADF2005).Since the clusters we chosen are relatively small in order to save simulation time

the results of former simulation are adopted for the cluster charge value.The value of cluster charge is further readjusted so that the cluster with least bonding energy is used for discussion

by this way we can eliminate partly the effect caused by dangling bond.Furthermore

the simulation result by cluster approximation is compared with that of the supercell methods.From these results we can draw conclusion that the methods we adopted in the simulation is reliable. The results shows that firstly the Li

+

tender to occupy the interstitial position around the C

2

site

which producing the stretching of the YO bond

thus lower the phonon energy of the sample

and increase the quantum efficiency of the material

by this way the light yield of the sample is improved. From the results of simulation

with the increase of Li

+

the length of YO bond for C

2

sites changes with the tendency of decrease-increase-decrease

which may explain the similar emission change via Li

+

concentration for this type of material

thus explained the two maximum peaks in the Li

+

concentration-light yield curve.The stretching of the YO bond may also cause the narrowing of the band gap

which might be the cause of small shift of the absorption spectrum.