Preferential occupancy of Eu(3+) and energy transfer in Eu(3+) doped Sr(2)V(2)O(7), Sr(9)Gd(VO(4))(7) and Sr(2)V(2)O(7)/Sr(9)Gd(VO(4))(7) phosphors

The vanadate-based phosphors Sr(2)V(2)O(7):Eu(3+) (SV:Eu(3+)), Sr(9)Gd(VO(4))(7):Eu(3+) (SGV:Eu(3+)) and Sr(9)Gd(VO(4))(7)/Sr(2)V(2)O(7):Eu(3+) (SGV/SV:Eu(3+)) were obtained by solid-state reaction. The bond-energy method was used to investigate the site occupancy preference of Eu(3+) based on the bond valence model. By comparing the change of bond energy when the Eu(3+) ions are incorporated into the different Sr, V or Gd sites, we observed that Eu(3+) doped in SV, SGV or SV/SGV would preferentially occupy the smaller energy variation sites, i.e., Sr4, Gd and Gd sites, respectively. The crystal structures of SGV and SV, the photoluminescence properties of SGV:Eu(3+), SV, SGV/SV and SGV/SV:Eu, as well as their possible energy transfer mechanisms are proposed. Interesting tunable colours (including warm-white emission) of SGV/SV:Eu(3+) can be obtained through changing the concentration of Eu(3+) or changing the relative quantities of SGV to SV by increasing the calcination temperature. Its excitation bands consist of two types of O(2−) → V(5+) charge transfer (CT) bands with the peaks at about 325 and 350 nm respectively, as well as f–f transitions of Eu(3+). The obtained warm-white emission consists of a broad photoluminescence band centred at about 530 nm, which originates from the O(2−) → V(5+) CT of SV, and a sharp characteristic spectrum ((5)D(0)–(7)F(2)) at about 615 and 621 nm.