Optical refractive properties and phonon spectra of Na2SO4 single crystal

Na 2 SO 4 crystal was grown by slow evaporation method from an aqueous solution. Using the PXRD method, the crystal structure of Na 2 SO 4 was investigated at room temperature, and it was found that the crystal belongs to the orthorhombic symmetry of the space group Fddd (no. 70) with the following unit cell parameters: a = 5.8568(4) Å, b = 12.2957(7) Å, c = 9.809(1) Å, and V = 706.52(8) Å 3 . The dispersion of the refractive indices of Na 2 SO 4 crystal in the visible and ultraviolet ranges was measured at room temperature and the principle refractive indices of optical indicatrix were determined to be n X = 1.4703, n Y = 1.4847, and n Z = 1.47669 for the wavelength λ = 500 nm. The first-principle calculations of the phonon properties of the crystal were performed using the lattice dynamics method and DFPT approach. The polarized normal incidence reflection spectra of Na 2 SO 4 crystal were measured in the infrared range 50 – 1300 cm -1 by using FTIR spectrometer and synchrotron radiation. The calculated reflection spectra of Na 2 SO 4 crystal are in very good agreement with the experimental ones. In Na 2 SO 4 crystal, the Reststrahlen band in the spectral range of 1120 – 1200 cm -1 is confirmed experimentally and theoretically. • Na 2 SO 4 crystal is found to be optically positive in the range of 400 - 750 nm • Ionic input ε i to the static dielectric permittivity ε 0 of Na 2 SO 4 is twice larger than the electronic one ε ∞ • TO-LO splitting of S – O stretching vibrations in Na 2 SO 4 crystal is found to be 80 cm -1 • Triply-degenerate bending vibrations of the isolated SO 4 group are revealed in Na 2 SO 4 in the form of spectral doublets