Exciton-like and defect-related luminescence in K3WO3F3 oxyfluoride crystals
Complex metal oxyfluorides such as K3WO3F3 are attractive compounds for developing new noncentrosymmetric crystals having ferroelectric and ferroelastic properties. This is achieved due to the strong distortion of metal-(O,F) polyhedra in crystal lattice because of different ionicity of metal-O and metal- F bonds. The luminescence spectroscopy can be a sensitive method to study the character of lattice distortion of these compounds. Spectra of photoluminescence (PL) and X-ray excited luminescence, PL excitation spectra (3–22 eV), PL decay kinetics as well as thermoluminescence curves were measured for single crystals and ceramics K3WO3F3. Synchrotron radiation (HASYLAB, DESY, Hamburg) were used for low temperature PL experiments with time resolution. Crystals were grown and examined in Institute of Geology and Mineralogy SB RAS (Novosibirsk). The intrinsic luminescence of tungstates is usually ascribed to the radiative relaxation of exciton-like excitations localized on WO6 octahedra or WO4 tetrahedra. In K3WO3F3 there are anion sites with mixed oxygen/fluorine occupancy. Therefore, different octahedra form with different distortion. Two emission centers of exciton-like origin, with distinct relaxation time, different types of such octahedra were found. The time-resolved luminescence spectroscopy technique was applied to distinguish these centers, proving itself as a sensitive method to study the character of lattice distortion. The energy transfer mechanism between these PL centers is found and tentatively described by the diffusion of excitons. Apart from intrinsic luminescence, the PL of defect-related centers was found in samples irradiated by fast electrons from linear accelerator (E=10 MeV, D=120 Gy). The role of shallow charge carrier traps in the low-temperature luminescence was revealed.