Effects of post calcination treatment on photoluminescence and structural properties of scheelite-type LiEuW₂O₈ nanocrystals synthesized by glycothermal reaction

The scheelite-type LiEuW₂O₈ (LEW) nanoparticles of ∼50 nm in diameter were synthesized from lithium acetate, europium (III) acetate and phosphotungstic acid in 1,4-butylene glycol at 300°C for 2h by autoclave treatment. Post calcination treatment at 600°C for 2 h enhanced the photoluminescence at 465 nm due to the 4f - 4f transition excitation of Eu³⁺ by a factor of 24.4, and due to charge transfer (CT) excitation from either O^2- or WO₄^2- to Eu³⁺ by a factor of 5.8. Convergent beam electron diffractmetry and the compositional analysis revealed the non-stoichiometric structure of the scheelite-type LEW nanocrystals. Raman spectroscopy also detected the WO₄ vibrational modes due to the tetragonal scheelite-type LEW. Solid-state ⁷Li single pulse excitation MAS NMR indicated the existence of at least three Li sites in the samples. We conclude that the following three factors are improved by calcination to increase the f-f transition probability of Eu³⁺: (i) a distribution of the Eu³⁺ - Eu³⁺ distance, (ii) the symmetry of Eu³⁺ polyhedra, (iii) the symmetry of tetrahedral WO ₄ units in the vicinity of Eu³⁺ polyhedra. Furthermore, oxygen is provided for non-stoichiometric LEW during calcination to enhance the CT probability.