Luminescence properties of Eu~(2+) and Ce~(3+) ions in calcium lithio-germanate Li_2CaGeO_4

摘要

The Eu~(2+)-doped Li_2CaGeO_4 samples have been prepared by the solid state reaction method and by thermolysis of the complex precursor. It was shown that Eu~(2+) ions in Li_2CaGeO_4 exhibit an intense emission with a maximum at 473 nm. The Stokes shift (820 cm~(-1)) and full-width at half-maximum (1415 cm~(-1)) of the emission band are close to those of the Eu~(2+) emission in Li_2CaSiO_4. Values of the crystal field splitting and the centroid shift of the Ce~(3+) 5d configuration in Li_2CaGeO_4 were also determined and compared with those of Ce~(3+) ions in Li_2CaSiO_4. Although the spectral positions of the Ce~(3+) lowest excitation band for Li_2CaRO_4 (R=Si, Ge) are practically the same, the magnitudes of the Ce~(3+) crystal field splitting appeared to be unexpectedly very different. This effect is tentatively attributed to a strong distortion of the calcium site in Li_2CaSiO_4 caused by the presence of the trivalent lanthanide ion.