Raman spectroscopy and optical properties of nanocrystalline NaAl(WO4)2:Cr3+ powders prepared by co-precipitation/calcination method

In this paper, we report on the pure and Cr3+-doped NaAl(WO4)2 nanopowders prepared by the co-precipitation/calcination method. X-ray diffraction, Raman/infrared spectroscopy, reflectance and luminescence studies (UV–vis optical band gap) of the obtained NaAl(WO4)2 nanoparticles are reported. Additionally, room temperature Raman and IR spectra as well as temperature-dependent Raman spectra are reported for NaAl(WO4)2 single crystal and its respective Raman-active modes are reported. Raman and IR studies showed that vibrational modes have been modified in comparison with the bulk material, indicating that with decreasing particle size some weak structural changes are induced in monoclinic NaAl(WO4)2 lattice. Optical investigations reveal that local crystal fields around Cr3+ ions in NaAl(WO4)2 nanocrystallities are of intermediate strength. A competition between the 2E and broadband 4T2 emissions is observed in these nanopowders due to small energy separation between the 2E and 4T2 energy levels of the Cr3+ ions. The splitting of the 2E level in the nanoparticles is about 84 cm−1, indicating small distortion of the octahedral environment of the chromium ions. In the promoting 2E phosphorescence vibrations of the WO4 tetrahedra are especially effective.

► Nanosized NaAl(WO4)2 can be synthesized by simple co-precipitation/calcination method.
► Assignment of phonon modes is proposed.
► Decrease of the particle size leads to weak structural changes.
► Chromium ions occupy sites of intermediate crystal field strength.
► Vibrations of the WO4 tetrahedra are especially effective in the promoting 2E phosphorescence.