Controllable Synthesis, Polymorphism and Structure-Dependent Photoluminescence Properties of Europium Oxyfluorides

Abstract

Functional materials that show polymorphism are essential for comparative studies of fundamental structure–property relationships. As a pre-requisite, the establishment of phase-controllable synthetic strategies are especially significant, but still a great challenge at present. We report herein a controllable fluorination route to europium oxyfluorides having various crystal structures, namely, orthorhombic (O), monoclinic (M) and rhombohedral (R) EuOF. This facile fluorination method starts from commercial europium oxide, utilizing poly(tetrafluoroethylene) as the fluorinating agent, and the phase selectivity could be easily achieved by varying the fluorination temperature and Eu/F ratios. The phase evolution and crystal structures of O-, M- and R-EuOF were characterized by ex situ powder X-ray diffraction at different temperatures and Eu/F ratios. Thus, in this way, the crystalline structures of O- and M-EuOF were established for the first time by Rietveld refinement. The steady-state and transient photoluminescence properties of O-, M- and R-EuOF were also studied comparatively and related to their structural features. Despite their similar components, the three EuOF phases exhibit distinct excitation and emission profiles due to subtle divergences in their local structures. Exceptional transition lines were observed for M-EuOF that correspond to energy-transfer processes favoured by its local structural features. These results suggest future lines of investigations of structure–property relationships of functional materials and the rational design of better lanthanide phosphors.