Emission from Mo‐O charge‐transfer state and Yb 3+ emission in Eu 3+ ‐doped and nondoped molybdates under UV excitation

Molybdates of Li + and Yb 3+ are studied to investigate the luminescence under UV excitation. LiYb(MoO 4 ) 2 and Eu 3+ ‐doped LiYb 1 ‐ x Eux(MoO 4 ) 2 (x=001–1.0) phosphors were synthesized by solid state reaction under mixing of Eu 2 O 3 , Yb 2 O 3 , Li 2 CO 3 and MoO 2 in air atmosphere. Two broad absorption bands centered at 333 and 236 nm are observed in LiYb(MoO 4 ) 2 compound. They are attributed to the 1 A 1 → 1 T 1 and 1 T 2 transitions due to the O 2− →Mo 6+ electron transfers in MoO 4 tetrahedron. An emission band with a peak at about 440 nm is found, which is attributed to the 3 T 1 → 1 A 1 transition of MoO 4 . Appearance of near‐infrared ( NIR ) Yb 3+ emission observed under UV excitation is understood by the MoO 4 →Yb 3+ Foerster‐Type energy transfer due to spectral overlap between the low‐energy tail of the broad 440 nm emission band and the high‐energy tail of the broad Yb 3+ absorption band and due to short Yb 3+ ‐MoO 4 distance. Yb 3+ emission observed in LiYb 1− x E u x (MoO 4 ) 2 by Eu 3+ excitation is understood by the Eu 3+ →Yb 3+ energy transfer by cross‐relaxation ( CR ) process between the 5 D 0 → 7 F 6 Eu 3+ transition and the 2 F 7/2 → 2 F 5/2 Yb 3+ transition. The CR efficiency shows maximum efficiency of 0.24 at x =0.15 of higher acceptor Yb 3+ concentration than donor Eu 3+ concentration. Three Yb 3+ emission bands with peaks at 994, 1002, and 1023 nm are observed, depending on the excitation wavelength. This is explained by less‐shielded 4f electrons of Yb 3+ by the 5s 2 5p 6 outermost electron shells, which are also responsible for unusual broadband Yb 3+ absorption and emission. From appearance of NIR Yb 3+ emission under excitation by not only UV light but also red light, these compounds are expected to be suitable for efficient photovoltaic application to Si‐based solar cells.