O2−-V5+ charge transfer band, chemical bond parameters and R/O of Eu3+ doped Ca(VO3)2 and Ca3(VO4)2: A comparable study

The dependence of photoluminescence (PL) properties of Eu3+ activated two typical vanadates Ca3(VO4)2 and Ca(VO3)2 on the crystal structure was discussed experimentally and theoretically. Compared with the broad PL excitation spectrum of Ca3(VO4)2: Eu with the peak at 339 nm, a 61-nm redshift of Ca(VO3)2: Eu (at 400 nm) can be found. Under the excitation of near-UV light, the PL spectra of Ca3(VO4)2:Eu and Ca(VO3)2:Eu dominate red emission at 624 and 615 nm due to the 5D0→7F2 transitions, respectively. By comparing their intensity ratios (R/O) of the 5D0→7F2 transition (red, its intensity is labeled using “R”) to the 5D0→7F1 transition (orange, its intensity is labeled using “O”), R/O of Ca3(VO4)2:Eu is 10 times stronger than that of Ca(VO3)2: Eu, which can be well explained by the proposed calculation model. Based on the dielectric theory of complex crystal, the important chemical bonds such as the covalency and the polarizability of the O-V or O-Ca bond volume were calculated quantitatively. It was shown that their broad excitation spectra were not the charge transfer (CT) from O to Eu, but is the CT from O to V, which also can be further demonstrated by the optical properties and the density functional theory calculations. Only O-V1 CT energy can efficiently be transferred to the activator due to the strong covalency of V1-O1 bond (0.2583) in the V-O-Eu bond. When Eu3+ ions occupy the Ca1, Ca2, Ca3, Ca4 and Ca5 sites of Ca3(VO4)2, the CT energy from O to Eu (O-Eu CT) can be predicted to be 6.22 eV (199 nm), 5.55 eV (223 nm), 5.57 eV (222 nm), 4.31 eV (287 nm) and 3.62 eV (342 nm), respectively. The predicted O-Eu CT energy in the Ca(VO3)2: Eu is 4.02 eV (296 nm). For Eu3+ doped Ca3(VO4)2 sample, the strongest red emission mainly comes from the substitution of Eu3+ to the Ca5 site through calculation of the distortion degree using the standard deviation of environmental factor of the individual bond (EFSD) σ(hei). The bandgap energies of Ca(VO3)2, Ca(VO3)2: Eu, Ca3(VO4)2 and Ca3(VO4)2: Eu were also determined.