YBO3 versus Y3BO6 host on Tb3+ luminescence

Undoped and Tb3+ doped YBO3 and Y3BO6 were prepared by co-precipitation method in ethylene glycol medium at 100 degrees C followed by annealing at 800 degrees C. The spin allowed 4f(8)-> 4f(7)5d(1) (low spin) and spin forbidden 4f(8)-> 4f(7)5d(1) (high spin) transitions of Tb3+ are well resolved in YBO3 host as compared to Y3BO6. This has been attributed to difference in the distribution of local environment around Y3+/Tb3+ in these hosts. D-5(4) and D-5(3) levels of Tb3+ decays much faster in Y3BO6 host as compared to YBO3. Increased phonon energy of Y3BO6 host compared to YBO3 facilitates faster cross relaxation process between D-5(3)-D-5(4) and F-7(6)-F-7(0) energy level pairs of Tb3+ in the former leading to decrease in the corresponding excited state lifetime values. Green emission, from Tb3+ in Y3BO6 host, is found to have better colour purity compared that from YBO3 host, as revealed by corresponding CIE coordinates. From these studies it is established that factors like excited state lifetime and colour purity must be considered together while choosing suitable yttrium borate based hosts for luminescent applications.