Spiral-type heteropolyhedral coordination network based on single-crystal LiSrPO₄: Implications for luminescent materials

Novel structures of luminescent materials, which are used as light sources for next-generation illumination, are continuously being improved for use in white-light-emitting diodes. Activator-doped known structures are reported as habitual down-conversion phosphors in solid-state lightings and displays. Consequently, the intrinsic qualities of the existent compounds produce deficiencies that limit their applications. Herein we report a spiral-network single-crystal orthophosphate (LiSrPO₄) prepared in a platinum crucible with LiCl flux through crystal-growth reactions of SrCl₂ and Li₃PO₄ in air. It crystallizes in a hexagonal system with a=5.0040(2) and c=24.6320(16) Å, V=534.15(5) ų, and Z=6 in the space group P6₅. The unit cell is comprised of LiO ₄ and PO₄ tetrahedrons that form a three-dimensional LiPO₄^2- anionic framework with a helical channel structure along the c axis in which the Sr²⁺ cation is accommodated. The optical band gap of this composition is about 3.65 eV, as determined by using UV/Vis absorption and diffuse reflection spectra. We used the crystal-growth method to synthesize blue- and red-emitting crystals that exhibited pure color, low reabsorption, a large Stokes shift, and efficient conversion of ultraviolet excitation light into visible light. Emphasis was placed on the development of gratifying structure-related properties of rare-earth luminescent materials and their applications. A profound structure: LiSrPO₄ comprises LiO ₄ and PO₄ tetrahedrons that form a three-dimensional LiPO₄^2- anionic framework with a helical channel structure along the c axis in which the Sr²⁺ cation is accommodated (see figure). The LiSrPO₄:RE (RE=Eu²⁺ and Eu³⁺) phosphors have potential applications in developing near-UV white LEDs with RGB pure color, thermal stability, and high brightness.