Sites occupancy preference of Bi³⁺ and white light emission through co-doped Sm³⁺ in LiGd₅P₂O₈

Bi³⁺, Sm³⁺-activated LiGd₅P₂O₈ (LGPO) phosphors were prepared through high-temperature solid-state method. In LGPO host, there are 5 types of Gd crystallographic sites, named as Gd(1)/Gd(2)/Gd(3)/Gd(4), and Gd(5). Bi³⁺-activated LGPO phosphors exhibit 1 broad excitation band from 250 to 320 nm centered at 293 nm and a broad asymmetric emission band ranging from 350 to 600 nm with the maximum value approximately at 409 nm. It can be concluded from dual-emission spectra that Bi³⁺ may occupy 2 Gd sites and an obvious spectral blue-shift appeared with increasing Bi³⁺ content, which is caused by the intensity of crystal field of Bi³⁺ is decreased. Notably, through the calculation of each Gd-O chemical parameter, the environmental factor (h_e) value of each Gd site can be obtained and it can be further inferred that 2 emission bands centered at 409/461 nm are ascribed to Bi³⁺ ions which occupies Gd(3) and Gd(4) sites, respectively. Energy transfer from Bi³⁺ to Sm³⁺ ions in Bi³⁺/Sm³⁺ co-doped LGPO samples occurred and it realizes the color-tunable emission from cyan to yellow including white-light emission, through controlling Sm³⁺ content. Moreover, energy transfer mechanism between Bi³⁺ and Sm³⁺ ions is verified to be dipole-dipole interaction by analyzing the spectroscopic experimental results and the critical distance between them is calculated to be 8.22 Å by concentration quenching method. Finally, it is illustrated that Bi³⁺ and Sm³⁺ co-doped LGPO phosphors will be a promising candidate for n-UV chip pumped w-LEDs.