Preparation and luminescence characteristics of monazite Eu3+:LaPO4 nanocrystals in NH4NO3 molten salt

• The monoclinic monazite LaPO4:Eu3+ phosphors were fabricated in NH4NO3 flux at 160 °C for 4 h.
• The consultant products exhibit almost nearly monodispersity and quasihexagonal and their mean sizes are of 30 nm.
• With the enhancement of the temperature the red-shift and spectral broadening of the charge-transfer band (CTB) were observed. The optimal Eu3+ concentration is ca. 8 at.%. The higher concentration quenching was observed due to the possible nonradiative energy transfer by electric multipole–multipole interaction.
• The relationship between the FL lifetime of 5D0 energy level and Eu3+ concentration has been investigated according to Auzel’s model. The fitting results indicates the energy phonon number in the nonradiative relaxation from the lowest energy level of 5D0 and the highest energy level of 7F6 is 7 and the intrinsic lifetime τ0 of 5D0 energy level is 6.94 ms.

Molten-salt method and NH4NO3 flux were developed to fabricate monoclinic monazite Eu3+:LaPO4 nanocrystals for the first time. The products were characterized by X-ray powder diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, excitation spectra, emission spectra and luminescence decay curves. The as-obtained products were quasihexagonal Eu3+:LaPO4 nanocrystals with the mean size of 30 nm. The room temperature charge transfer bands (CTB) exhibited red-shift and spectral broadening in comparison with 10 K CTB. The optimal Eu3+ concentration was determined to be 8 mol% by a comparative study of the relative emission intensities for different Eu3+ doping concentrations. The higher concentration quenching could be caused by the possible nonradiative energy transfer (electric multipole–multipole interaction). The relationship between the FL lifetime of 5D0 energy level and Eu3+ concentration was investigated based on Auzel’s model. This work is important not only to understand the unique physical properties of Eu3+:LaPO4 nanocrystals but also to bring an opportunity for the development of the other nanocrystals via the molten salt synthesis in NH4NO3 flux.