Is Higher Ratio of Monoclinic to Tetragonal in LaVO4 a Better Luminescence Host? Redispersion and Polymer Film Formation

Crystalline LaVO4:Eu3+ nanophosphors (NPs) codoped with metal ions (Mn+ = Li+, Sr2+, and Bi3+) are prepared in ethylene glycol (EG) medium at temperature similar to 140 degrees C in 3 h. A mixture of monoclinic and tetragonal phases is observed. The ratio of tetragonal to monoclinic phases increases with increase of Li+ and Sr2+ concentration, but this is opposite in case of Bi3+ concentration. Lattice expansion occurs in the case of Li+ and Sr2+ codoping. Li+ ions occupy the interstitial sites instead of La3+ sites. Lattice contraction occurs in case of Bi3+ codoping indicating substitution of La3+ sites. Luminescence intensity is improved by codoping of Mn+ irrespective of crystal structure. Charges of Li+ and Sr2+ are different from that of La3+ (host lattice), whereas the charge of Bi3+ is same as that of La3+. One interesting observation is in magnetic dipole transition that the intensity of the peak at 594 nm is more than that at 587 nm in the case of charge imbalance, whereas the reverse occurs in the case of charge balance. LaVO4:Eu3+ nanophosphors prepared in water medium have more luminescence intensity when compared to those prepared in ethylene glycol, and this is related to variation of ratio of tetragonal to monoclinic phases. The luminescence intensity is also enhanced as annealing temperature increases from 600 to 800 degrees C due to the improved crystallinity. Lifetime data are analyzed on the basis of exponential and nonexponential decay equations. Samples are dispersible in polar medium due to capping of particles by EG. Polymer films are prepared by dispersion of NPs in poly(vinyl alcohol), and extra borax is added in order to make cross-link between polymer molecules. Samples of NPs in the forms of powder, dispersion in liquid medium, and film show the red emission.