Effect of Thorium on Ce Phosphors

The luminescent properties of Ce +s and Tb + ~ acti-vation in the monazite structures LaPO4 and CePO4 have been described and the energy transfer from Ce + 3 to Ce +3 and Ce +3 to Tb +3 in these and other structures have been established (1-3). Energy transfer also has been reported in which Ce +3 emission in YPO4 was sensitized by Th +4 (4). The work reported here shows the enhancement of both Ce + ~ and Tb +3 emission in the presence of Th +4 in the monazite structure, in which the Th +4 does not have a direct role in the en-ergy transfer in the ultraviolet region. Experimental Two methods of preparation were used in these tests. Phosphors were prepared from cerium and lanthanum sources of 99.9 % purity by co-precipitation of the com-position from nitrate solutions with NaH2PO4-H20. When prepared from oxides of 99.999 % purity, the oxides were mixed with (NH4)2HPO4 and fired initially at 300 ~ Final firing for both methods was 1000 ~ for several hours. Spectral distribution curves, excitation data, and decay measurements were obtained by methods which have been described previously (5). Reflectance mea-surements were made on a Cary 14, using BaSO4 as a reference standard. Results The Ce +3 emission in CePO4 and LaPO4 matrices is a double band, peaking at 320 and 340 nm, resulting from the splitting of the group state, 2F5/2 and 2F7/2 (Fig. 1). Green emisison is present when Tb +3 is in-corporated in CePO4, but is very weak in the LaPO4 matr ix unless Ce +3 is also present. * E lectrochemica l Society Act ive Member. Key words: lanthanum phosphate, cer ium phosphate, lumines-cence, energy transfer, thor ium, phosphors