Spiral-Type Heteropolyhedral Coordination Network Based on Single-Crystal LiSrPO4: Implications for Luminescent Materials

• Published in: Chemistry : a European journal 2013-11, Vol.19 (45), p.15358-15365
• Main Authors: Lin, Chun Che, Shen, Chin-Chang, Liu, Ru-Shi
• Format: Article
• Language: English
• Subjects: Chemistry; crystal engineering; lithium; luminescence; phosphates; solid-state structures
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201301758

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) Å3, and Z=6 in the space group P65. The unit cell is comprised of LiO4 and PO4 tetrahedrons that form a three‐dimensional LiPO42− 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 LiO4 and PO4 tetrahedrons that form a three‐dimensional LiPO42− 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.