Luminescence of undoped and Ce-doped hexagonal BiPO4

Undoped and Ce-doped hexagonal BiPO4 were synthesized by a precipitation method and characterized on their structure, Ce spatial distribution and chemical state, together with detailed luminescence spectroscopy investigation through systematic variation of the excitation wavelength (lambda(exc)) over the broad range 250-460 nm combined with radioluminescence measurements and extensive literature review. Up to five distinct emission bands were observed, depending on lambda(exc) and on the presence or not of the activator. The host presented three emission bands: the 350 nm (3.54 eV) band was ascribed to a defect, the 405 nm (3.06 eV) band was ascribed to metal-to-metal charge transfer (MMCT), and the 570 nm (2.18 eV) band was ascribed to the Bi3+ P-3(1) -> S-1(0) transition. The presence of Bi3+ ions perturbed by oxygen vacancies in the coordination polyhedron at varying distances to the Bi3+ ions created states with lower energies than the states of unperturbed Bi3+. Ce-incorporation led to the creation of two partially superimposing bands at about 445 nm (2.79 eV) and 490 nm (2.53 eV). Strong interaction between Ce3+ and the host was observed. These results led to enhanced understanding of the intrinsic and extrinsic luminescence of this material that presents considerable controversy in the literature.