title: Is BiPO4 a better luminescent host? Case study on doping and annealing effects.
abstract: Metal phosphates have been popularly regarded as  excellent luminescence hosts of lanthanide ions, while such an issue is challenged by the ignorance about the structural stability that may originate from the different chemical nature between the framework and the dopant lanthanide ions. Here, we choose BiPO(4) as a model compound to study. A detailed investigation of the effects of Eu doping and annealing on the structures and related luminescence properties of Bi(1-x)PO(4):Eu(x) (x = 0-0.199) has been carried out. A monoclinic phase (denoted as LTMP) was obtained for the undoped sample, which gradually transformed to a hexagonal phase (HP) with increasing  doping level of Eu(3+) to x = 0.068. Further, it is also found that annealing had an obvious impact on the structures of the resulted samples. With increasing the annealing temperature up to 400 °C a phase transformation from HP to LTMP happens, which is opposite to that with doping. The above phase transformation behaviors were further confirmed by performing  structural studies of doping with Dy(3+) ions and annealing undoped BiPO(4). The structural evolution had a great influence on the luminescent properties. Initially a significant decrease in Eu(3+) luminescence intensity and quantum efficiency was observed when LTMP transformed to HP. Afterward, a converse situation, increasingly enhanced luminescence performance, appeared when HP transformed to LTMP. Therefore, whether metal phosphates could be taken as better luminescence hosts must take into account their structural changes caused by the different chemical natures between framework and dopant ions, which may provide an important reference for designing new luminescent materials.
authors.affiliation: State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Graduate School of Chinese Academy of Sciences, Fuzhou 350002, PR China.
authors.forename: Minglei
authors.initials: M
authors.lastname: Zhao
authors.forename: Liping
authors.initials: L
authors.lastname: Li
authors.forename: Jing
authors.initials: J
authors.lastname: Zheng
authors.forename: Liusai
authors.initials: L
authors.lastname: Yang
authors.forename: Guangshe
authors.initials: G
authors.lastname: Li
mesh_terms: D001729:Bismuth / Q000737:chemistry*; D005063:Europium / Q000737:chemistry*; D049449:Luminescence; D008855:Microscopy, Electron, Scanning; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D005063:Europium; D001729:Bismuth; C035883:bismuth phosphate