Though the long afterglow phosphors have been improved greatly since its invention in the nineteenth century

the mechanism of the occurrence of long afterglow is still unknown clearly. In this work

trivalent rare earth ions activated garnet-typed cadmium aluminium germanate Cd

3

Al

2

Ge

3

O

12

:Ln phosphors were studied to discover the relation between the ion electronegativity and afterglow performance. All samples were synthesized by a traditional solid-state reaction method. The luminescent properties were investigated. Long afterglow were observed excitation by 254nm light for all of the samples. The rare earth ions can be divided into three kinds based on their long afterglow performance. The first one is Pr

3+

Tb

3+

and Dy

3+

. Characteristic emissions with afterglow were generated after excitation by 254nm light. The second one is Eu

3+

and Sm

3+

. These ions can produce characteristic emission with excitation by 254nm light

but without afterglow. The third one is Ce

3+

La

3+

Nd

3+

and other rare earth ions. These ions can not produce characteristic emission

and have no distinctly influence on the afterglow performance of the host. By comparing their electronegativity

we found that the electronegativity was consistent with the afterglow performance. Basing on the phenomena mentioned above

we concluded that the afterglow performance of reductive ions was better than that of oxidative ions in this host. The mechanism of the long afterglow may be depicted as two steps. For example of Pr

3+

at first

Pr

3+

electrons are excited to the conduction band and are trapped at the electron traps. This process leaves Pr

4+

ions in the sample. Then the electrons at the electron traps escape from the electron traps and are captured by Pr

4+

ions to produce Pr

3+

ions in excited stated. The excited state Pr

3+

ions emit photons and return to ground state. The Pr

3+

Tb

3+

and Dy

3+

can be photoionizated to quadrivalent ions easily. It's very difficult for Eu

3+

or Sm

3+

to be photoionizated to quadrivalent ions

so afterglow can't be observed in the Eu

3+

or Sm

3+

doped samples.