Color-Tunable Luminescence Properties of Bi3+ in Ca5(BO3)3F via Changing Site Occupation and Energy Transfer

Generally, 6s electron and 6p electron of Bi3+ ions are located in its outermost layer; thus, luminescence properties of Bi3+ are strongly associated with the coordination environment around Bi3+. Bi3+ ions occupy different cationic positions in hosts, which may cause the movement of the emission spectrum. In order to investigate the luminescent property of Bi3+, a series of Bi3+ doped Ca5(BO3)3F species are synthesized. There are three types of Ca2+ sites in the host, which could be substituted by Bi3+. Upon the 322 nm excitation of Bi3+, a broad emission band can be observed, which is ascribed to the 1s0 → 3p1 transition of Bi3+. Meanwhile, there is the emission shift of Ca5(BO3)3F:xBi3+, and its emission color can be altered from blue to cyan. It may result from Bi3+ occupying different positions of Ca2+ in the host, which can give rise to different degrees of a nephelauxetic effect and crystal field splitting. In order to explore the relationship between the luminescence properties of Bi3+ and the nepelauxetic effect, the value of the centroid shift (∈c) is calculated. Centroid shift (∈c) is related to the covalence and average bond length of an octahedron in which the influence of covalence is primary. The relationship between the luminescence properties of Bi3+ and the crystal field splitting is discussed. The crystal field splitting is related to the interaction between the Bi3+ species, the crystal field splitting energy (Δ), and the distortion of the crystal. Emission spectra are asymmetric; meanwhile, the emission spectra have remarkable changes at various excitation wavelengths. This proves that the broadband emission band consists of at least two emission centers. In order to assess this hypothesis, the decay curves are measured. This confirms that there are three luminescence centers in a host. On one hand, considering the effect of the centroid shift (∈c) and crystal field splitting (∈cfs), the sources of three luminescence centers are confirmed by calculating the total shift (D(A)) of the 6s6p level of Bi3+ in a host. On the other hand, the source of three luminescence centers is determined by the changing trend of the average bond length of the octahedron. In addition, the luminescence properties of Ca5(BO3)3F:Bi3+, Eu3+, are investigated as well. There is efficient energy transfer from the Bi3+ to the Eu3+ ion, and the color-tunable phosphor can be achieved by the combination of the appropriate proportion of Bi3+ and Eu3+ ions. The emission color can gradually change from cyan to red.