Influence of the Structure on the Properties of Na_xEu_y(MoO₄)_z Red Phosphors

Scheelite related compounds (A′,A″) n [(B′,B″)O 4 ] m with B′, B″ = W and/or Mo are promising new materials for red phosphors in pc-WLEDs (phosphor-converted white-light-emitting-diode) and solid-state lasers. Cation substitution in CaMoO 4 of Ca 2+ by the combination of Na + and Eu 3+, with the creation of A cation vacancies, has been investigated as a factor for controlling the scheelite-type structure and the luminescent properties. Na 5 Eu(MoO 4 ) 4 and Na x Eu 3+ (2– x )/3 □ (1–2 x )/3 MoO 4 (0.138 ≤ x ≤ 0.5) phases with a scheelite-type structure were synthesized by the solid state method; their structural characteristics were investigated using transmission electron microscopy. Contrary to powder synchrotron X-ray diffraction before, the study by electron diffraction and high resolution transmission electron microscopy in this paper revealed that Na 0.286 Eu 0.571 MoO 4 has a (3 + 2)D incommensurately modulated structure and that (3 + 2)D incommensurately modulated domains are present in Na 0.200 Eu 0.600 MoO 4 . It also confirmed the (3 + 1)D incommensurately modulated character of Na 0.138 Eu 0.621 MoO 4 . The luminescent properties of all phases under near-ultraviolet (n-UV) light have been investigated. The excitation spectra of these phosphors show the strongest absorption at about 395 nm, which matches well with the commercially available n-UV-emitting GaN-based LED chip. The emission spectra indicate an intense red emission due to the 5 D 0 → 7 F 2 transition of Eu 3+, with local minima in the intensity at Na 0.286 Eu 0.571 MoO 4 and Na 0.200 Eu 0.600 MoO 4 for ∼613 nm and ∼616 nm bands. The phosphor Na 5 Eu(MoO 4 ) 4 shows the brightest red light emission among the phosphors in the Na 2 MoO 4 –Eu 2/3 MoO 4 system and the maximum luminescence intensity of Na 5 Eu(MoO 4 ) 4 (λ ex = 395 nm) in the 5 D 0 → 7 F 2 transition region is close to that of the commercially used red phosphor YVO 4:Eu 3+ (λ ex = 326 nm). Electron energy loss spectroscopy measurements revealed the influence of the structure and Na/Eu cation distribution on the number and positions of bands in the UV-optical-infrared regions of the EELS spectrum.