Optical properties of Na2W4O13 crystals

Optical properties of sodium tetratungstate (Na2W4O13) crystals have been studied in a wide temperature range from 9 to 300 K with use of synchrotron radiation and D2 lamp as light sources. The reflectivity spectra exhibit appreciable anisotropy for polarized light with the electric vector parallel and perpendicular to the c axis. A sharp reflection peak due to the excitonic transition is observed at 3.65 eV for both polarizations at 9 K. A luminescence band peaking at 2.33 eV is stimulated under the excitation with photons in the fundamental absorption region. This luminescence has a decay time of 52 μs at 9 K, and is thermally quenched at around 200 K with an activation energy of 220 meV. The 2.33-eV luminescence is ascribed to radiative decay of excitons self-trapped at octahedral WO6 groups. These results are compared with those obtained in other tungstates.