Synthesis, structural characterization and luminescent properties of Tb 3+ -doped AgLaP 2 O 7 phosphors

Silver lanthanum diphosphates doped with terbium, AgLa1-xTbxP2O7 (x=0%, 1%, 5%, 10%, 15% and 20%), were prepared by solid state reaction at T=500°. The obtained compounds were investigated by means of a multi-methodological approach, involving the Scanning Electron Microscopy (SEM) equipped with energy dispersive X-Ray spectroscopy (EDS) for morphological investigation and semi-quantitative chemical analysis, respectively; powder X-ray diffraction (PXRD) for structural characterization and Rietveld refinement; Fourier Transform InfraRed (FTIR) and Raman spectroscopies for qualitative study. Finally, emission spectra were collected in order to detect the fluorescence properties of the compounds. The unit cell parameters and the space group of all the family members were determined by PXRD data. The compounds crystallize in the orthorhombic Pnma space group, with the AgLaP2O7 cell constants equal to: a = 8.6706(1) Å, b = 5.3218(1) Å, c = 12.8839(1) Å and cell volume V = 594.51(1) Å3. A decreasing trend for unit cell parameters was observed at increase of dopant concentration. For the pure phase, the investigation was completed with the structure solution via Direct Methods and Rietveld refinement. The crystal structure consists of compact layers of LaO9 polyhedra, down the c axis, bridged by P2O7 diphosphate groups and by zig-zag chains of AgO9 polyhedra down b. The FTIR and Raman analysis supports the chemical structure, highlighting the deformation (δPO3) and stretching (νPO3) vibration of PO3 groups, and the characteristic bands of P2O74- groups attributed to the symmetric and asymmetric stretching (νs and νas) of P-O-P bridge, respectively. The luminescence properties of Tb3+ activating ion in AgLa1-xTbxP2O7 (x=1%, 5%, 10%, 15% and 20%) were also examined. No concentration quenching of the main 5D4-→7F5 emission, responsible for the well known green emission of Tb3+ ion, was observed in the studied concentration range, whereas a concentration quenching of the emission from 5D3 level, due to 5D3-→5D4 cross relaxation process, was revealed.